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The effect of thermally pulsating asymptotic giant branch stars on the evolution of the rest‐frame near‐infrared galaxy luminosity function
Abstract
We address the fundamental question of matching the rest-frame K-band luminosity function (LF) of galaxies over the Hubble time using semi-analytic models after modification of the stellar population modelling. We include the Maraston evolutionary synthesis models, which feature a higher contribution by the thermally pulsating asymptotic giant branch (TP-AGB) stellar phase, into three different semi-analytic models, namely the De Lucia and Blaizot version of the Munich model, morgana and the Menci model. We leave all other input physics and parameters unchanged. We find that the modification of the stellar population emission can solve the mismatch between models and the observed rest-frame K-band luminosity from the brightest galaxies derived from UKIRT Infrared Deep Sky Survey data at high redshift. For all explored semi-analytic models, this holds at the redshifts – between 2 and 3 – where the discrepancy was recently pointed out. The reason for the success is that at these cosmic epochs the model galaxies have the right age (∼1 Gyr) to contain a well-developed TP-AGB phase, which makes them redder without the need of changing their mass or age. We have also computed a version of the Munich model using the Charlot and Bruzual models that adopt the Marigo TP-AGB prescription and find the same result as that with the Maraston models. At the same time, the known overestimation of the faint end is enhanced in the K band when including the TP-AGB contribution. At lower redshifts (z < 2) some of the explored models deviate from the data. This is due to short merging time-scales and inefficient ‘radio-mode’ active galactic nucleus feedback. Our results show that a strong evolution in mass predicted by hierarchical models is compatible with no evolution on the bright end of the K-band LF from z= 3 to the local universe. This means that, at high redshifts and contrary to what is commonly accepted, K-band emission is not necessarily a good tracer of galaxy mass.
... Also, it is in the high-redshift regime where a larger incidence of systems with enhanced TP-AGB stellar emission is known to reside (Maraston, 2005;Henriques et al., 2011). If such effect in the IR regime significantly affects IR AGN selection, than we are forced to use only the most restrictive AGN criteria (like the bright IR excess sources, e.g., Polletta et al., 2006;Dey et al., 2008) or to rely solely on the remainder spectral regimes, which sometimes is not the ideal scenario. ...
... For instance, all the IR AGN diagnostics have never been tested against the emission from TP-AGB stars (which is known to peak at 2 µm, Maraston, 2005). This is crucial to the high-redshift regime where a larger incidence of systems with enhanced TP-AGB stellar emission is known to reside (Maraston, 2005;Henriques et al., 2011). If such effect in the IR regime significantly affects IR AGN selection, than we are forced to use only the most restrictive AGN criteria (like the bright IR excess sources, e.g., Polletta et al., 2006;Dey et al., 2008) or to rely solely on the remainder spectral regimes, which sometimes is not the ideal scenario. ...
The main focus of this thesis is the IR spectral regime, which since the 70's
and 80's has revolutionised our understanding of the Universe. A
multi-wavelength analysis on Extremely Red Galaxy populations is first
presented in one of the most intensively observed patch of the sky, the Chandra
Deep Field South. By adopting a purely statistical methodology, we consider all
the photometric and spectroscopic information available on large samples of
Extremely Red Objects (EROs, 553 sources), IRAC EROs (IEROs, 259 sources), and
Distant Red Galaxies (DRGs, 289 sources). We derive general properties:
redshift distributions, AGN host fraction, star-formation rate densities, dust
content, morphology, mass functions and mass densities. The results point to
the fact that EROs, IEROs, and DRGs all belong to the same population, yet seen
at different phases of galaxy evolution. The second part of this thesis is
dedicated to the AGN selection in the IR, with particular relevance to the
James Webb Space Telescope, to be launched in 2018. We develop an improved IR
criterion (using K and IRAC bands) as an alternative to existing IR AGN
criteria for the z<2.5 regime, and develop another IR criterion which reliably
selects AGN hosts at 0<z<7 (using K, Spitzer-IRAC, and Spitzer-MIPS24um bands,
KIM). The ability to track AGN activity since the end of reionization holds
great advantages for the study of galaxy evolution. The thesis then focus on
the importance of dust. Based on deep IR data on the Cosmological Survey, we
derive rest-frame 1.6, 3.3, and 6.2um luminosity functions and their dependency
on redshift. We estimate the dust contribution to those wavelengths and show
that the hot dust luminosity density evolves since z=1-2 with a much steeper
drop than the star-formation history of the Universe. Future prospects are
finally discussed in the last chapter.
... We note that some other studies have also explored the K -band LF with semi-analytic models (e.g. Henriques et al. 2011 ;Lagos et al. 2019 ). Fig. 2 shows the cosmic star formation rate (SFR) density. ...
Measurements of the luminosity function of active galactic nuclei (AGN) at high redshift (z ≳ 6) are expected to suffer from field-to-field variance, including cosmic and Poisson variances. Future surveys, such as those from the Euclid telescope and James Webb Space Telescope (JWST), will also be affected by field variance. We use the Uchuu simulation, a state-of-the-art cosmological N-body simulation with 2.1 trillion particles in a volume of 25.7 Gpc3, combined with a semi-analytic galaxy and AGN formation model, to generate the Uchuu-ν2GC catalog, publicly available, that allows us to investigate the field-to-field variance of the luminosity function of AGN. With this Uchuu–ν2GC model, we quantify the cosmic variance as a function of survey area, AGN luminosity, and redshift. In general, cosmic variance decreases with increasing survey area and decreasing redshift. We find that at z ∼ 6 − 7, the cosmic variance depends weakly on AGN luminosity. This is because the typical mass of dark matter haloes in which AGN reside does not significantly depend on luminosity. Due to the rarity of AGN, Poisson variance dominates the total field-to-field variance, especially for bright AGN. We also examine the effect of parameters related to galaxy formation physics on the field variance. We discuss uncertainties present in the estimation of the faint-end of the AGN luminosity function from recent observations, and extend this to make predictions for the expected number of AGN and their variance for upcoming observations with Euclid, JWST, and the Legacy Survey of Space and Time (LSST).
... At z ≥ 1, we slightly overproduce the number of galaxies below the knee of the K-band luminosity function. Our results are similar to the findings of many previous SAMs (Fontanot et al. 2009;Cirasuolo et al. 2010;Henriques et al. 2011a;Somerville et al. 2012). A study by Fontanot et al. (2009) explored three independent SAMs and found that all produced lower mass galaxies too early, resulting in an overabundance of faint galaxies at high redshift. ...
We have developed a pipeline called \mentari to generate the far-ultraviolet to far-infrared spectral energy distribution (SED) of galaxies from the \dustysage semi-analytic galaxy formation model (SAM). \dustysage incorporates dust-related processes directly on top of the basic ingredients of galaxy formation like gas infall, cooling, star formation, feedback, and mergers. We derive a physically motivated attenuation model from the computed dust properties in \dustysage, so each galaxy has a self-consistent set of attenuation parameters based on the complicated dust physics that occurred across the galaxy's assembly history. Then, we explore several dust emission templates to produce infrared spectra. Our results show that a physically-motivated attenuation model is better for obtaining a consistent multi-wavelength description of galaxy formation and evolution, compared to using a constant attenuation. We compare our predictions with a compilation of observations and find that the fiducial model is in reasonable agreement with: (i) the observed $z=0$ luminosity functions from the far-ultraviolet to far-infrared simultaneously, and hence (ii) the local cosmic SED in the same range, (iii) the rest-frame K-band luminosity function across $0 < z < 3$, and (iv) the rest-frame far-ultraviolet luminosity function across $0 < z < 1$. Our model underproduces the far-ultraviolet emission at $z=2$ and $z=3$, which can be improved by altering the AGN feedback and dust processes in \dustysage. However, this combination thus worses the agreement at $z=0$, which suggests that more detailed treatment of such processes is required.
... We note that some other studies have also explored the K-band LF with semi-analytic models (e.g. Henriques et al. 2011;Lagos et al. 2019). Fig. 2 shows the cosmic star formation rate (SFR) density. ...
Measurements of the luminosity function of active galactic nuclei (AGN) at high redshift ($z\gtrsim 6$) are expected to suffer from field-to-field variance, including cosmic and Poisson variances. Future surveys, such as those from the Euclid telescope and James Webb Space Telescope (JWST), will also be affected by field variance. We use the Uchuu simulation, a state-of-the-art cosmological $N$-body simulation with 2.1 trillion particles in a volume of $25.7~\mathrm{Gpc}^3$, that has sufficient mass resolution to resolve dwarf-size systems, combined with a semi-analytic galaxy and AGN formation model, to generate the Uchuu-$\nu^2$GC catalog, publicly available, that allows us to investigate the field-to-field variance of the luminosity function of AGN. With this Uchuu-$\nu^2$GC model, we quantify the cosmic variance as a function of survey area, AGN luminosity, and redshift. In general, cosmic variance decreases with increasing survey area and decreasing redshift. We find that at $z\sim6-7$, the cosmic variance depends weakly on AGN luminosity, in particular for small survey areas (0.01 and 0.1 deg$^2$). This is because the typical mass of dark matter haloes in which AGN reside does not significantly depend on luminosity. Due to the rarity of AGN, Poisson variance dominates the total field-to-field variance, especially for bright AGN. We discuss uncertainties present in the estimation of the faint-end of the AGN luminosity function from recent observations, and extend this to make predictions for the expected number of AGN and their variance for upcoming observations with Euclid, JWST, and the Legacy Survey of Space and Time (LSST). In particular, we predict that the Euclid deep survey will find 120--240 (16--80) AGN -- depending on the model -- with rest-frame UV absolute magnitude brighter than $-20$ ($-20.5$) at $z=6.3$ ($z=7$) in the Euclid H-band deep survey (abridged).
... It has been eighty years since the publication of very first observational and 'numerical' investigations on the nature of galaxy encounters (Holmberg 1940(Holmberg , 1941. Decades later, the emergence of computers allowed researchers to conduct the first numerical experiments of idealised (non-cosmological) galaxy merging systems (Toomre & Toomre 1972;Hernquist 1989; Barnes Bower et al. 2006;Croton et al. 2006;De Lucia & Blaizot 2007;Henriques et al. 2011;Benson 2012;Guo et al. 2013;Lagos et al. 2018Lagos et al. , 2019. Often, these SAMs rely on idealised galaxy merger simulations for guidance. ...
We investigate the spatial structure and evolution of star formation and the interstellar medium (ISM) in interacting galaxies. We use an extensive suite of parsec-scale galaxy merger simulations (stellar mass ratio = 2.5:1), which employs the "Feedback In Realistic Environments-" model (fire-2). This framework resolves star formation, feedback processes, and the multi-phase structure of the ISM. We focus on the galaxy-pair stages of interaction. We find that close encounters substantially augment cool (HI) and cold-dense (H2) gas budgets, elevating the formation of new stars as a result. This enhancement is centrally-concentrated for the secondary galaxy, and more radially extended for the primary. This behaviour is weakly dependent on orbital geometry. We also find that galaxies with elevated global star formation rate (SFR) experience intense nuclear SFR enhancement, driven by high levels of either star formation efficiency (SFE) or available cold-dense gas fuel. Galaxies with suppressed global SFR also contain a nuclear cold-dense gas reservoir, but low SFE levels diminish SFR in the central region. Concretely, in the majority of cases, SFR-enhancement in the central kiloparsec is fuel-driven (55% for the secondary, 71% for the primary) -- whilst central SFR-suppression is efficiency-driven (91% for the secondary, 97% for the primary). Our numerical predictions underscore the need of substantially larger, and/or merger-dedicated, spatially-resolved galaxy surveys -- capable of examining vast and diverse samples of interacting systems -- coupled with multi-wavelength campaigns aimed to capture their internal ISM structure.
... As only upper limits were measured in all bands, DRC-4 is not shown. See Section 4 for SED fitting details. thermally pulsing asymptotic giant branch stars and/or active galactic nuclei (AGN; Chapman et al. 2009;Hainline et al. 2009;Henriques et al. 2011). We derive an average M H of - 26.06 1.40, which is in agreement (within the uncertainty) of the submillimeter galaxies (SMGs) studied in Hainline et al. (2009) and Simpson et al. (2014). ...
Recent simulations and observations of massive galaxy cluster evolution predict that the majority of stellar mass buildup happens within cluster members by z = 2, before cluster virialization. Protoclusters rich with dusty, star-forming galaxies (DSFGs) at z > 3 are the favored candidate progenitors for these massive galaxy clusters at z ∼ 0. We present here the first study analyzing stellar emission along with cold dust and gas continuum emission in a spectroscopically confirmed z = 4.002 protocluster core rich with DSFGs, the Distant Red Core (DRC). We combine new Hubble Space Telescope and Spitzer data with existing Gemini, Herschel, and Atacama Large Millimeter/submillimeter Array observations to derive individual galaxy-level properties and compare them to coeval field and other protocluster galaxies. All of the protocluster members are massive (>10 ¹⁰ M ⊙ ), but not significantly more so than their coeval field counterparts. Within uncertainty, all are nearly indistinguishable from galaxies on the star-forming versus stellar mass main-sequence relationship and the star formation efficiency plane. Assuming no future major influx of fresh gas, we estimate that these gaseous DSFGs will deplete their gas reservoirs in ∼300 Myr, becoming the massive quiescent ellipticals dominating cluster cores by z ∼ 3. Using various methodologies, we derive a total z = 4 halo mass of ∼10 ¹⁴ M ⊙ and estimate that the DRC will evolve to become an ultramassive cluster core of mass ≳10 ¹⁵ M ⊙ by z = 0.
... For this work, we can check the SED-derived stellar masses by comparing them to estimates based on rest-frame 1.6 µm absolute magnitudes (observed-frame λ = 8.0 µm), which is taken directly from respective best fit SEDs. This wavelength traces the stellar peak while also limiting the effects of dust extinction, as well as contributions from thermally pulsing asymptotic giant branch stars and/or AGN (Hainline et al. 2009;Chapman et al. 2009;Henriques et al. 2011). We derive an average M H of −26.06 ± 1.40, which is in agreement (within uncertainty) of the SMGs studied in Hainline et al. (2009) and Simpson et al. (2014). ...
Recent simulations and observations of massive galaxy cluster evolution predict that the majority of stellar mass build up happens within cluster members by $z=2$, before cluster virialization. Protoclusters rich with dusty, star-forming galaxies (DSFGs) at $z>3$ are the favored candidate progenitors for these massive galaxy clusters at $z\sim0$. We present here the first study analyzing stellar emission along with cold dust and gas continuum emission in a spectroscopically confirmed $z=4.002$ protocluster core rich with DSFGs, the Distant Red Core (DRC). We combine new \textit{HST} and \textit{Spitzer} data with existing Gemini, \textit{Herschel}, and ALMA observations to derive individual galaxy-level properties, and compare them to coeval field and other protocluster galaxies. All of the protocluster members are massive ($>10^{10}$ M$_\odot$), but not significantly more so than their coeval field counterparts. Within uncertainty, all are nearly indistinguishable from galaxies on the star-forming vs. stellar mass main-sequence relationship. However, when placed on the star formation efficiency plane, DRC components exhibit starburst-like characteristics with SFRs 10-100$\times$ greater than the expected field value at a given molecular gas mass. Assuming no future major influx of fresh gas, we estimate that these gas poor (f$_\mathrm{gas}<25\%$) yet bursty DSFGs will deplete their gas reservoirs in $<30$ Myr. Using various methodologies, we derive a total $z=4$ halo mass of $\sim10^{14}$ M$_\odot$, and estimate that the DRC will evolve to become an ultra-massive cluster core of mass $\gtrsim10^{15}$ M$_\odot$ by $z=0$.
... A main feature of this model lies in its treatment of the post-main-sequence stellar evolution stages, such as TP-AGB, based on the fuel consumption theorem. The contribution of TP-AGB stars is expected to be crucial for modeling the SEDs of young and intermediate-age (0.1-2 Gyr) stellar populations, which predominate the z 1.5 3 redshift range (Maraston 2005;Maraston et al. 2006;Henriques et al. 2011). Except for the different treatment of TP-AGB stars, the M05 model has employed the input stellar evolution tracks/isochrones of Cassisi et al. (1997aCassisi et al. ( , 1997bCassisi et al. ( , 2000, which are different from those used in the BC03 and CB07 models. ...
When modeling and interpreting the spectral energy distributions (SEDs) of galaxies, the simple stellar population (SSP) model, star formation history (SFH), and dust attenuation law (DAL) are three of the most important components. However, each of them carries significant uncertainties that have seriously limited our ability to reliably recover the physical properties of galaxies from the analysis of their SEDs. In this paper, we present a Bayesian framework to deal with these uncertain components simultaneously. Based on the Bayesian evidence, a quantitative implement of the principle of Occam's razor, the method allows a more objective and quantitative discrimination among the different assumptions about these uncertain components. With a K s -selected sample of 5467 low-redshift (mostly with z ≲ 1) galaxies in the COSMOS/UltraVISTA field and classified into passively evolving galaxies (PEGs) and star-forming galaxies (SFGs) with the UVJ diagram, we present a Bayesian discrimination of a set of 16 SSP models from five research groups (BC03 and CB07, M05, GALEV, Yunnan-II, BPASS V2.0), five forms of SFH (Burst, Constant, Exp-dec, Exp-inc, Delayed-τ), and four kinds of DAL (Calzetti law, MW, LMC, SMC). We show that the results obtained with the method are either obvious or understandable in the context of stellar/galaxy physics. We conclude that the Bayesian model comparison method, especially that for a sample of galaxies, is very useful for discriminating the different assumptions in the SED modeling of galaxies. The new version of the BayeSED code, which is used in this work, is publicly available at https://bitbucket.org/hanyk/bayesed/. © 2018. The American Astronomical Society. All rights reserved.
... The capability of an ANN is mainly determined by the structure of its hidden layers. According to the universal approximation theorem (Cybenko 1989;Kurt & Hornik 1991;Haykin 1999), a multilayer feed-forward network with only one hidden layer can approximate any continuous function to arbitrary precision. However, the neurons in the hidden layer must have a continuous, bounded and nonconstant activation function. ...
We present a newly developed version of BayeSED, a general Bayesian approach
to the spectral energy distribution (SED) fitting of galaxies. The new BayeSED
code has been systematically tested on a mock sample of galaxies. The
comparison between estimated and inputted value of the parameters show that
BayeSED can recover the physical parameters of galaxies reasonably well. We
then applied BayeSED to interpret the SEDs of a large Ks-selected sample of
galaxies in the COSMOS/UltraVISTA field with stellar population synthesis
models. With the new BayeSED code, a Bayesian model comparison of stellar
population synthesis models has been done for the first time. We found that the
model by Bruzual & Charlot (2003), statistically speaking, has larger Bayesian
evidence than the model by Maraston (2005) for the Ks-selected sample. Besides,
while setting the stellar metallicity as a free parameter obviously increases
the Bayesian evidence of both models, varying the IMF has a notable effect only
on the Maraston (2005) model. Meanwhile, the physical parameters estimated with
BayeSED are found to be generally consistent with those obtained with the
popular grid-based FAST code, while the former exhibits more natural
distributions. Based on the estimated physical parameters of galaxies in the
sample, we qualitatively classified the galaxies in the sample into five
populations that may represent galaxies at different evolution stages or in
different environments. We conclude that BayeSED could be a reliable and
powerful tool for investigating the formation and evolution of galaxies from
the rich multi-wavelength observations currently available. A binary version of
MPI parallelized BayeSED code is publicly available at
https://bitbucket.org/hanyk/bayesed.
... The overall blackbody emission from the stellar population combined with the minimum in the opacity of the H − ion in stellar photospheres peaks at ∼1.6 μm, which is generally observed, as is the CO absorption at 2.35-2.5 μm from red supergiants. Emission from hot dust surrounding stars in the asymptotic giant branch phase is expected in young systems ( 1 Gyr) and is expected to peak at 2-5 μm (Sajina et al. 2005;Maraston 2005;Henriques et al. 2011). Furthermore, the strength of the polycyclic aromatic hydrocarbon (PAH) features, seen mostly beyond 6 μm, increases with SF activity and metallicity (Calzetti et al. 2007;Engelbracht et al. 2008;Hunt et al. 2010). ...
[Abridged] It is widely accepted that observations at mid-infrared (mid-IR)
wavelengths enable the selection of galaxies with nuclear activity, which may
not be revealed even in the deepest X-ray surveys. In this work new near- and
mid-IR color diagnostics are explored, aiming for improved efficiency - better
completeness and less contamination - in selecting AGN out to very high
redshifts. We restrict our study to the James Webb Space Telescope wavelength
range (0.6-27um). The criteria are created based on the predictions by
state-of-the-art galaxy and AGN templates covering a wide variety of galaxy
properties, and tested against control samples with deep multi-wavelength
coverage (ranging from the X-rays to radio frequencies). We show that the
colors Ks-[4.5], [4.5]-[8.0], and [8.0]-[24] are ideal as AGN/non-AGN
diagnostics at, respectively, z<~1, 1<~z<~2.5, and z>~2.5-3. However, when the
source redshift is unknown, these colors should be combined. We thus develop an
improved IR criterion (using Ks and IRAC bands, KI) as a new alternative at
z<~2.5. KI does not show improved completeness (50-60% overall) in comparison
to commonly used IRAC-based AGN criteria, but is less affected by non-AGN
contamination (revealing a >50-90% level of successful AGN selection). We also
propose KIM (using Ks, IRAC, and MIPS-24um bands, KIM), which aims to select
AGN hosts from local distances to as far back as the end of reionization
(0<z<~7) with reduced non-AGN contamination. However, the necessary
testing-constraints and the small control-sample sizes prevent the confirmation
of its improved efficiency at z<~2.5. Overall, KIM shows a ~30-40% completeness
and a >70-90% level of successful AGN selection. KI and KIM are built to be
reliable against a ~10-20% error in flux, are based on existing filters, and
are suitable for immediate use.
We have developed a pipeline called mentari to generate the far-ultraviolet to far-infrared spectral energy distribution (SED) of galaxies from the Dusty SAGE semi-analytic galaxy formation model (SAM). Dusty SAGE incorporates dust-related processes directly on top of the basic ingredients of galaxy formation like gas infall, cooling, star formation, feedback, and mergers. We derive a physically motivated attenuation model from the computed dust properties in Dusty SAGE, so each galaxy has a self-consistent set of attenuation parameters based on the complicated dust physics that occurred across the galaxy’s assembly history. Then, we explore several dust emission templates to produce infrared spectra. Our results show that a physically-motivated attenuation model is better for obtaining a consistent multi-wavelength description of galaxy formation and evolution, compared to using a constant attenuation. We compare our predictions with a compilation of observations and find that the fiducial model is in reasonable agreement with: (i) the observed z = 0 luminosity functions from the far-ultraviolet to far-infrared simultaneously, and hence (ii) the local cosmic SED in the same range, (iii) the rest-frame K-band luminosity function across 0 < z < 3, and (iv) the rest-frame far-ultraviolet luminosity function across 0 < z < 1. Our model underproduces the far-ultraviolet emission at z = 2 and z = 3, which can be improved by altering the AGN feedback and dust processes in Dusty SAGE. However, this combination thus worses the agreement at z = 0, which suggests that more detailed treatment of such processes is required.
The jackknife method gives an internal covariance estimate for large-scale structure surveys and allows model-independent errors on cosmological parameters. Using the SDSS-III BOSS CMASS sample, we study how the jackknife size and number of resamplings impact the precision of the covariance estimate on the correlation function multipoles and the error on the inferred baryon acoustic scale. We compare the measurement with the MultiDark Patchy mock galaxy catalogues, and we also validate it against a set of lognormal mocks with the same survey geometry. We build several jackknife configurations that vary in size and number of resamplings. We introduce the Hartlap factor in the covariance estimate that depends on the number of jackknife resamplings. We also find that it is useful to apply the tapering scheme to estimate the precision matrix from a limited number of resamplings. The results from CMASS and mock catalogues show that the error estimate of the baryon acoustic scale does not depend on the jackknife scale. For the shift parameter α, we find an average error of 1.6 per cent, 2.2 per cent and 1.2 per cent, respectively from CMASS, Patchy and lognormal jackknife covariances. Despite these uncertainties fluctuate significantly due to some structural limitations of the jackknife method, our α estimates are in reasonable agreement with published pre-reconstruction analyses. Jackknife methods will provide valuable and complementary covariance estimates for future large-scale structure surveys.
We investigate the spatial structure and evolution of star formation and the interstellar medium (ISM) in interacting galaxies. We use an extensive suite of parsec-scale galaxy merger simulations (stellar mass ratio = 2.5:1), which employs the ’Feedback In Realistic Environments-2’ model (fire-2). This framework resolves star formation, feedback processes, and the multi-phase structure of the ISM. We focus on the galaxy-pair stages of interaction. We find that close encounters substantially augment cool (HI) and cold-dense (H2) gas budgets, elevating the formation of new stars as a result. This enhancement is centrally-concentrated for the secondary galaxy, and more radially extended for the primary. This behaviour is weakly dependent on orbital geometry. We also find that galaxies with elevated global star formation rate (SFR) experience intense nuclear SFR enhancement, driven by high levels of either star formation efficiency (SFE) or available cold-dense gas fuel. Galaxies with suppressed global SFR also contain a nuclear cold-dense gas reservoir, but low SFE levels diminish SFR in the central region. Concretely, in the majority of cases, SFR-enhancement in the central kiloparsec is fuel-driven (55% for the secondary, 71% for the primary) - whilst central SFR-suppression is efficiency-driven (91% for the secondary, 97% for the primary). Our numerical predictions underscore the need of substantially larger, and/or merger-dedicated, spatially-resolved galaxy surveys - capable of examining vast and diverse samples of interacting systems - coupled with multi-wavelength campaigns aimed to capture their internal ISM structure.
We use three semi-analytical models (SAMs) of galaxy formation and evolution run on the same 1 h−1 Gpc MultiDark Planck2 cosmological simulation to investigate the properties of [O ii] emission line galaxies at redshift z ∼ 1. We compare model predictions with different observational data sets, including DEEP2–firefly galaxies with absolute magnitudes. We estimate the [O ii] luminosity ($L{\left[\rm{O\,{\small II}}\right]}$) of our model galaxies using the public code get_ emlines , which ideally assumes as input the instantaneous star formation rates (SFRs). This property is only available in one of the SAMs under consideration, while the others provide average SFRs, as most models do. We study the feasibility of inferring galaxies’ $L{\left[\rm{O\,{\small II}}\right]}$ from average SFRs in post-processing. We find that the result is accurate for model galaxies with dust attenuated $L{\left[\rm{O\,{\small II}}\right]}$ ≲ 1042.2 erg s−1 ($\lt 5{{\ \rm per\ cent}}$ discrepancy). The galaxy properties that correlate the most with the model $L{\left[\rm{O\,{\small II}}\right]}$ are the SFR and the observed-frame u and g broad-band magnitudes. Such correlations have r-values above 0.64 and a dispersion that varies with $L{\left[\rm{O\,{\small II}}\right]}$ . We fit these correlations with simple linear relations and use them as proxies for $L{\left[\rm{O\,{\small II}}\right]}$ , together with an observational conversion that depends on SFR and metallicity. These proxies result in [O ii] luminosity functions and halo occupation distributions with shapes that vary depending on both the model and the method used to derive $L{\left[\rm{O\,{\small II}}\right]}$ . The amplitude of the clustering of model galaxies with $L{\left[\rm{O\,{\small II}}\right]}$ >1040.4 erg s−1 remains overall unchanged on scales above 1 $\, h^{-1}$ Mpc, independently of the $L{\left[\rm{O\,{\small II}}\right]}$ computation.
We study the quenching of star formation as a function of redshift, environment and stellar mass in the galaxy formation simulations of Henriques et al. (2015), which implement an updated version of the Munich semi-analytic model (L-GALAXIES) on the two Millennium Simulations after scaling to a Planck cosmology. In this model massive galaxies are quenched by AGN feedback depending on both black hole and hot gas mass, and hence indirectly on stellar mass. In addition, satellite galaxies of any mass can be quenched by ram-pressure or tidal stripping of gas and through the suppression of gaseous infall. This combination of processes produces quenching efficiencies which depend on stellar mass, host halo mass, environment density, distance to group centre and group central galaxy properties in ways which agree qualitatively with observation. Some discrepancies remain in dense regions and close to group centres, where quenching still seems too efficient. In addition, although the mean stellar age of massive galaxies agrees with observation, the assumed AGN feedback model allows too much ongoing star formation at late times. The fact that both AGN feedback and environmental effects are stronger in higher density environments leads to a correlation between the quenching of central and satellite galaxies which roughly reproduces observed conformity trends inside haloes.
This is the first of a series of papers presenting the Modules for Experiments in Stellar Astrophysics (MESA) Isochrones and Stellar Tracks (MIST) project, a new comprehensive set of stellar evolutionary tracks and isochrones computed using MESA, a state-of-the-art open-source 1D stellar evolution package. In this work, we present models with solar-scaled abundance ratios covering a wide range of ages ($5 \leq \rm \log(Age)\;[yr] \leq 10.3$), masses ($0.1 \leq M/M_{\odot} \leq 300$), and metallicities ($-2.0 \leq \rm [Z/H] \leq 0.5$). The models are self-consistently and continuously evolved from the pre-main sequence to the end of hydrogen burning, the white dwarf cooling sequence, or the end of carbon burning, depending on the initial mass. We also provide a grid of models evolved from the pre-main sequence to the end of core helium burning for $-4.0 \leq \rm [Z/H] < -2.0$. We showcase extensive comparisons with observational constraints as well as with some of the most widely used existing models in the literature. The evolutionary tracks and isochrones can be downloaded from the project website at http://waps.cfa.harvard.edu/MIST/.
We study the debated contribution from thermally pulsing asymptotic-giant-branch (TP-AGB) stars in evolutionary population
synthesis models. We investigate the spectral energy distributions (SEDs) of a sample of 51 spectroscopically confirmed, high-z (1.3 < zspec < 2.7), galaxies using three evolutionary population synthesis models with strong, mild and light TP-AGB. Our sample is the
largest of spectroscopically confirmed galaxies on which such models are tested so far. Galaxies were selected as passive,
but we model them using a variety of star formation histories in order not to be dependent on this pre-selection. We find
that the observed SEDs are best fitted with a significant contribution of TP-AGB stars or with substantial dust attenuation.
Without including reddening, TP-AGB-strong models perform better and deliver solutions consistent within 1σ from the best-fitted
ones in the vast majority of cases. Including reddening, all models perform similarly. Using independent constraints from
observations in the mid- and far-IR, we show that low/negligible dust attenuation, i.e. E(B − V) ≲ 0.05, should be preferred for the SEDs of passively selected galaxies. Given that TP-AGB-light models give systematically
older ages for passive galaxies, we suggest number counts of passive galaxies at higher redshifts as a further test to discriminate
among stellar population models.
We present a new version of the GALFORM semi-analytical model of galaxy
formation. This brings together several previous developments of GALFORM into a
single unified model, including a different initial mass function (IMF) in
quiescent star formation and in starbursts, feedback from active galactic
nuclei supressing gas cooling in massive halos, and a new empirical star
formation law in galaxy disks based on their molecular gas content. In
addition, we have updated the cosmology, introduced a more accurate treatment
of dynamical friction acting on satellite galaxies, and updated the stellar
population model. The new model is able to simultaneously explain both the
observed evolution of the K-band luminosity function and stellar mass function,
and the number counts and redshift distribution of sub-mm galaxies selected at
850 mu. This was not previously achieved by a single physical model within the
LambdaCDM framework, but requires having an IMF in starbursts that is somewhat
top-heavy. The new model is tested against a wide variety of observational data
covering wavelengths from the far-UV to sub-mm, and redshifts from z=0 to z=6,
and is found to be generally successful. These observations include the optical
and near-IR luminosity functions, HI mass function, Tully-Fisher relation,
fraction of early type galaxies, metallicity-luminosity relation and
size-luminosity relation at z=0, as well as far-IR number counts, and far-UV
luminosity functions at z ~ 3-6. [abridged]
We report the discovery of the closest collisional ring galaxy to the Milky Way. Such rare systems occur due to ‘bulls-eye’
encounters between two reasonably matched galaxies. The recessional velocity of about 840 km s−1 is low enough that it was detected in the Anglo-Australian Observatory/UK Schmidt Telescope Survey for Galactic H α emission.
The distance is only ∼10 Mpc and the main galaxy shows a full ring of star-forming knots, 6.1 kpc in diameter surrounding
a quiescent disc. The smaller assumed ‘bullet’ galaxy also shows vigorous star formation. The spectacular nature of the object
had been overlooked because of its location in the Galactic plane and proximity to a bright star and even though it is the
60th brightest galaxy in the HI Parkes All Sky Survey (HIPASS) H i survey. The overall system has a physical size of ∼15 kpc, a total mass of M* = 6.6 × 109 M⊙ (stars + H i), a metallicity of [O/H]∼ −0.4, and a star formation rate of 0.2–0.5 M⊙ yr−1, making it a Magellanic-type system. Collisional ring galaxies therefore extend to much lower galaxy masses than commonly
assumed. We derive a space density for such systems of 7 × 10− 5 Mpc− 3, an order of magnitude higher than previously estimated. This space density suggests Kathryn's Wheel is the nearest such
system. We present discovery images, CTIO 4-m telescope narrow-band follow-up images and spectroscopy for selected emission
components. Given its proximity and modest extinction along the line of sight, this spectacular system provides an ideal target
for future high spatial resolution studies of such systems and for direct detection of its stellar populations.
We have studied the panchromatic broad-band properties from the FUV to the MIR of a sample of 808 post-starburst galaxies. We find that in the optical and near-IR bands post-starburst galaxies (PSGs) form a remarkably uniform class of objects and that, on average, simple populations synthesis models (SSP) reproduce very well the SEDs of PSGs over a broad wavelength range, but not in the UV. We also find that, while the photometric variance in the optical and near-IR properties of the sample is small and comparable to the observational errors, both in the UV and the mid-IR the observed variance is much larger than the errors. We find a strong correlation between the UV fluxes and those in the mid-IR, indicating that the large variance in UV properties of PSGs could be related to a non-uniform distribution of dust covering the intermediate age populations. The disagreement between models and observations in the UV could be due to inadequate modelling; to the contribution of AGB and post-AGB stars; or to a non-uniform distribution of dust; possibly all three. Further progress in understanding this important class of galaxies, therefore, requires at the same time better modelling and better observations in the UV and mid-IR.
Galaxy spectral energy distribution (SED) modelling is a powerful tool, but constraining how well it is able to infer the true values for galaxy properties (e.g. the star formation rate) is difficult because independent determinations are often not available. However, galaxy simulations can provide a means of testing SED modelling techniques. Here, we present a numerical experiment in which we apply the SED modelling code MAGPHYS to ultraviolet-millimetre synthetic photometry generated from hydrodynamical simulations of an isolated disc galaxy and a major galaxy merger by performing three-dimensional dust radiative transfer. We compare the properties inferred from the SED modelling with the true values and find that MAGPHYS recovers most physical parameters of the simulated galaxies well. In particular, it recovers consistent parameters irrespective of the viewing angle, with smoothly varying results for neighbouring time steps of the simulation, even though each viewing angle and time step is modelled independently. The notable exception to this rule occurs when we use a Small Magellanic Cloud-type intrinsic dust extinction curve in the radiative transfer calculations. In this case, the two-component dust model used by MAGPHYS is unable to effectively correct for the attenuation of the simulated galaxies, which leads to potentially significant errors (although we obtain only marginally acceptable fits in this case). Overall, our results give confidence in the ability of SED modelling to infer physical properties of galaxies, albeit with some caveats.
We present predictions for the two-point correlation function of galaxy clustering as a function of stellar mass, computed using two new versions of the GALFORM semi-analytic galaxy formation model. These models make use of a high resolution, large volume N-body simulation, set in the WMAP7 cosmology. One model uses a universal stellar initial mass function (IMF), while the other assumes different IMFs for quiescent star formation and bursts. Particular consideration is given to how the assumptions required to estimate the stellar masses of observed galaxies (such as the choice of IMF, stellar population synthesis model and dust extinction) influence the perceived dependence of galaxy clustering on stellar mass. Broad-band spectral energy distribution fitting is carried out to estimate stellar masses for the model galaxies in the same manner as in observational studies. We show clear differences between the clustering signals computed using the true and estimated model stellar masses. As such, we highlight the importance of applying our methodology to compare theoretical models to observations. We introduce an alternative scheme for the calculation of the merger timescales for satellite galaxies in GALFORM, which takes into account the dark matter subhalo information from the simulation. This reduces the amplitude of small-scale clustering. The new merger scheme offers improved or similar agreement with observational clustering measurements, over the redshift range 0 < z < 0.7. We find reasonable agreement with clustering measurements from GAMA, but find larger discrepancies for some stellar mass ranges and separation scales with respect to measurements from SDSS and VIPERS, depending on the GALFORM model used.
The galaxy stellar mass function (GSMF) at high-z provides key information on
star-formation history and mass assembly in the young Universe. We aimed to use
the unique combination of deep optical/NIR/MIR imaging provided by HST, Spitzer
and the VLT in the CANDELS-UDS, GOODS-South, and HUDF fields to determine the
GSMF over the redshift range 3.5<z<7.5. We utilised the HST WFC3/IR NIR imaging
from CANDELS and HUDF09, reaching H~27-28.5 over a total area of 369 arcmin2,
in combination with associated deep HST ACS optical data, deep Spitzer IRAC
imaging from the SEDS programme, and deep Y and K-band VLT Hawk-I images from
the HUGS programme, to select a galaxy sample with high-quality photometric
redshifts. These have been calibrated with more than 150 spectroscopic
redshifts in the range 3.5<z<7.5, resulting in an overall precision of
sigma_z/(1+z)~0.037. We have determined the low-mass end of the high-z GSMF
with unprecedented precision, reaching down to masses as low as M*~10^9 Msun at
z=4 and ~6x10^9 Msun at z=7. We find that the GSMF at 3.5<z<7.5 depends only
slightly on the recipes adopted to measure the stellar masses, namely the
photo-z, the SFHs, the nebular contribution or the presence of AGN on the
parent sample. The low-mass end of the GSMF is steeper than has been found at
lower redshifts, but appears to be unchanged over the redshift range probed
here. Our results are very different from previous GSMF estimates based on
converting UV galaxy luminosity functions into mass functions via tight M/L
relations. Integrating our evolving GSMF over mass, we find that the growth of
stellar mass density is barely consistent with the time-integral of the SFR
density over cosmic time at z>4. These results confirm the unique synergy of
the CANDELS+HUDF, HUGS, and SEDS surveys for the discovery and study of
moderate/low-mass galaxies at high redshifts.
We present our methods for generating a catalogue of 7000 synthetic images and 40 000 integrated spectra of redshift z = 0 galaxies from the Illustris simulation. The mock data products are produced by using stellar population synthesis models
to assign spectral energy distributions (SED) to each star particle in the galaxies. The resulting synthetic images and integrated
SEDs therefore properly reflect the spatial distribution, stellar metallicity distribution, and star formation history of
the galaxies. From the synthetic data products, it is possible to produce monochromatic or colour-composite images, perform
SED fitting, classify morphology, determine galaxy structural properties, and evaluate the impacts of galaxy viewing angle.
The main contribution of this paper is to describe the production, format, and composition of the image catalogue that makes
up the Illustris Simulation Observatory. As a demonstration of this resource, we derive galactic stellar mass estimates by
applying the SED fitting code fast to the synthetic galaxy products, and compare the derived stellar masses against the true stellar masses from the simulation.
We find from this idealized experiment that systematic biases exist in the photometrically derived stellar mass values that
can be reduced by using a fixed metallicity in conjunction with a minimum galaxy age restriction.
We have updated the Munich galaxy formation model to the Planck first-year cosmology, while modifying the treatment of baryonic processes to reproduce recent data on the abundance and passive
fractions of galaxies from z = 3 down to z = 0. Matching these more extensive and more precise observational results requires us to delay the reincorporation of wind
ejecta, to lower the surface density threshold for turning cold gas into stars, to eliminate ram-pressure stripping in haloes
less massive than ${\sim }10^{14}{\rm \, M_{{\odot }}}$, and to modify our model for radio mode feedback. These changes cure the most obvious failings of our previous models, namely
the overly early formation of low-mass galaxies and the overly large fraction of them that are passive at late times. The
new model is calibrated to reproduce the observed evolution both of the stellar mass function and of the distribution of star
formation rate at each stellar mass. Massive galaxies (log M⋆/M⊙ ≥ 11.0) assemble most of their mass before z = 1 and are predominantly old and passive at z = 0, while lower mass galaxies assemble later and, for log M⋆/M⊙ ≤ 9.5, are still predominantly blue and star forming at z = 0. This phenomenological but physically based model allows the observations to be interpreted in terms of the efficiency
of the various processes that control the formation and evolution of galaxies as a function of their stellar mass, gas content,
environment and time.
In a previous paper we compared the SEDs of a sample of 808 K+A galaxies from
the FUV to the MIR to the predictions of the spectrum synthesis models
explicitly using AGB components. Here we use the new AGB-light models from C.
Maraston (including less fuel for the later stages of stellar evolution and
improved calibrations) to address the discrepancies between our observations
and the AGB-heavy models used in our previous paper, which over-predict the
infrared fluxes of post-starburst galaxies by an order of magnitude. The new
models yield a much better fit to the data, especially in the near-IR, compared
to previous realizations where AGB stars caused a large excess in the H and K
bands. We { also compare the predictions of the M2013 models to those with BC03
and find that both reproduce the observations equally well. } We still find a
significant discrepancy with { both sets of models} in the Y and J bands, which
however is probably due to the spectral features of AGB stars. We also find
that { both the M2013 and the BC03 models} still over-predict the observed
fluxes in the UV bands, even invoking extinction laws that are stronger in
these bands. While there may be some simple explanations for this discrepancy,
we find that further progress requires new observations and better modelling.
Excess mid-infrared emission longward of 5$\mu$m is well modelled by a
$T_{dust}=300^oK$ Black-Body, which may arise from dust emission from the
circumstellar envelopes of Oxygen rich M stars (expected for a metal-rich
population of AGB stars).
We measure the angular clustering of 33 415 extremely red objects (EROs) in the Elais-N1 field covering 5.33 deg2, which cover the redshift range z = 0.8 to 2. This sample was made by merging the UKIDSS Deep eXtragalactic Survey (DXS) with the optical Subaru and Pan-STARRS
PS1 data sets. We confirm the existence of a clear break in the angular correlation function at ∼0.02° corresponding to 1 h−1 Mpc at z ∼ 1. We find that redder or brighter EROs are more clustered than bluer or fainter ones. Halo occupation distribution (HOD)
model fits imply that the average mass of dark matter haloes which host EROs is over 1013 h−1 M⊙ and that EROs have a bias ranging from 2.7 to 3.5. Compared to EROs at z ∼ 1.1, at z ∼ 1.5 EROs have a higher bias and fewer are expected to be satellite galaxies. Furthermore, EROs reside in similar dark matter
haloes to those that host 1011.0 M⊙ < M* < 1011.5 M⊙ galaxies. We compare our new measurement and HOD fits with the predictions of the galform semi-analytical galaxy formation model. Overall, the clustering predicted by galform gives an encouraging match to our results. However, compared to our deductions from the measurements, galform puts EROs into lower mass haloes and predicts that a larger fraction of EROs are satellite galaxies. This suggests that the
treatment of gas cooling may need to be revised in the model. Our analysis illustrates the potential of clustering analyses
to provide observational constraints on theoretical models of galaxy formation.
We introduce the Theoretical Astrophysical Observatory (TAO), an online
virtual laboratory that houses mock observations of galaxy survey data. Such
mocks have become an integral part of the modern analysis pipeline. However,
building them requires an expert knowledge of galaxy modelling and simulation
techniques, significant investment in software development, and access to high
performance computing. These requirements make it difficult for a small
research team or individual to quickly build a mock catalogue suited to their
needs. To address this TAO offers access to multiple cosmological simulations
and semi-analytic galaxy formation models from an intuitive and clean web
interface. Results can be funnelled through science modules and sent to a
dedicated supercomputer for further processing and manipulation. These modules
include the ability to (1) construct custom observer light-cones from the
simulation data cubes; (2) generate the stellar emission from star formation
histories, apply dust extinction, and compute absolute and/or apparent
magnitudes; and (3) produce mock images of the sky. All of TAO's features can
be accessed without any programming requirements. The modular nature of TAO
opens it up for further expansion in the future.
We present a new release of the galform semi-analytical model of galaxy formation and evolution, which exploits a Millennium Simulation-class N-body run performed with the Wilkinson Microwave Anisotropy Probe 7 cosmology. We use this new model to study the impact of the choice of stellar population synthesis (SPS) model on the predicted
evolution of the galaxy luminosity function. The semi-analytical model is run using seven different SPS models. In each case,
we obtain the rest-frame luminosity function in the far-ultraviolet, optical and near-infrared (NIR) wavelength ranges. We
find that both the predicted rest-frame ultraviolet and optical luminosity function are insensitive to the choice of SPS model.
However, we find that the predicted evolution of the rest-frame NIR luminosity function depends strongly on the treatment
of the thermally pulsating asymptotic giant branch (TP-AGB) stellar phase in the SPS models, with differences larger than
a factor of 2 for model galaxies brighter than MAB(K) − 5 log h < −22 (∼L* for 0 ≤ z ≤ 1.5). We have also explored the predicted number counts of galaxies, finding remarkable agreement between the results with
different choices of SPS model, except when selecting galaxies with very red optical–NIR colours. The predicted number counts
of these extremely red galaxies appear to be more affected by the treatment of star formation in discs than by the treatment
of TP-AGB stars in the SPS models.
Galaxy formation is at the forefront of observation and theory in cosmology.
An improved understanding is essential for improving our knowledge both of the
cosmological parameters, of the contents of the universe, and of our origins.
In these lectures intended for graduate students, galaxy formation theory is
reviewed and confronted with recent observational issues. In Lecture 1, the
following topics are presented: star formation considerations, including IMF,
star formation efficiency and star formation rate, the origin of the galaxy
luminosity function, and feedback in dwarf galaxies. In Lecture 2, we describe
formation of disks and massive spheroids, including the growth of supermassive
black holes, negative feedback in spheroids, the AGN-star formation connection,
star formation rates at high redshift and the baryon fraction in galaxies.
Identification of dark matter is one of the most urgent problems in cosmology.
I describe the astrophysical case for dark matter, from both an observational
and a theoretical perspective. I also review the current status of direct and
in direct detection of dark matter, and review the prospects for future
advances.
We investigate for the first time the effects of a warm dark matter
(WDM) power spectrum on the statistical properties of galaxies using a
semi-analytic model of galaxy formation. The WDM spectrum we adopt as a
reference case is suppressed - compared to the standard cold dark matter
(CDM) case - below a cut-off scale ≈1 Mpc corresponding (for thermal
relic WDM particles) to a mass mX= 0.75 keV. This ensures
consistency with present bounds provided by the microwave background
Wilkinson Microwave Anisotropy Probe data and by the comparison of
hydrodynamical N-body simulations with observed Lyman-α forest. We
run our fiducial semi-analytic model with such a WDM spectrum to derive
galaxy luminosity functions (in B, UV and K bands) and the stellar mass
distributions over a wide range of cosmic epochs, to compare with recent
observations and with the results in the CDM case. The predicted colour
distribution of galaxies in the WDM model is also checked against the
data. When compared with the standard CDM case, the luminosity and
stellar mass distributions we obtain assuming a WDM spectrum are
characterized by (i) flattening of the faint-end slope and (ii)
sharpening of the cut-off at the bright end for z≲ 0.8. We discuss
how the former result is directly related to the smaller number of
low-mass haloes collapsing in the WDM scenario, while the latter is
related to the smaller number of satellite galaxies accumulating in
massive haloes at a low redshift, thus suppressing the accretion of
small lumps on the central, massive galaxies. These results shows how
adopting a WDM power spectrum may contribute to solving two major
problems of CDM galaxy formation scenarios, namely, the excess of
predicted faint (low-mass) galaxies at low and - most of all - high
redshifts, and the excess of bright (massive) galaxies at low redshifts.
We study the mid-infrared color space of 30 galaxies from the Spitzer Infrared Nearby Galaxies Survey (SINGS) survey for which Sloan Digital Sky Survey data are also available. We construct two-color maps for each galaxy and compare them to results obtained from combining Maraston evolutionary synthesis models, galactic thermally pulsating asymptotic giant branch (TP-AGB) colors, and smooth star formation histories. For most of the SINGS sample, the spatially extended mid-IR emission seen by Spitzer in normal galaxies is consistent with our simple model in which circumstellar dust from TP-AGB stars dominates at 8 and 24 μm. There is a handful of exceptions that we identify as galaxies that have high star formation rates presumably with star formation histories that cannot be assumed to be smooth, or anemic galaxies, which were depleted of their H I at some point during their evolution and have very low ongoing star formation rates.
We present the first photometric redshift distribution for a large sample of 870 μm submillimeter galaxies (SMGs) with robust identifications based on observations with ALMA. In our analysis we consider 96 SMGs in the Extended Chandra Deep Field South, 77 of which have 4-19 band photometry. We model the SEDs for these 77 SMGs, deriving a median photometric redshift of z
phot = 2.3 ± 0.1. The remaining 19 SMGs have insufficient photometry to derive photometric redshifts, but a stacking analysis of Herschel observations confirms they are not spurious. Assuming that these SMGs have an absolute H-band magnitude distribution comparable to that of a complete sample of z ~ 1-2 SMGs, we demonstrate that they lie at slightly higher redshifts, raising the median redshift for SMGs to z
phot = 2.5 ± 0.2. Critically we show that the proportion of galaxies undergoing an SMG-like phase at z ≥ 3 is at most 35% ± 5% of the total population. We derive a median stellar mass of M
= (8 ± 1) × 1010M
☉, although there are systematic uncertainties of up to 5 × for individual sources. Assuming that the star formation activity in SMGs has a timescale of ~100 Myr, we show that their descendants at z ~ 0 would have a space density and MH distribution that are in good agreement with those of local ellipticals. In addition, the inferred mass-weighted ages of the local ellipticals broadly agree with the look-back times of the SMG events. Taken together, these results are consistent with a simple model that identifies SMGs as events that form most of the stars seen in the majority of luminous elliptical galaxies at the present day.
Identification of dark matter is one of the most urgent problems in cosmology. I describe the astrophysical case for dark matter, from both an observational and a theoretical perspective.RésuméLʼidentification de la matière noire est lʼun des problèmes les plus urgents de lʼastrophysique. Je décris lʼaspect astrophysique du problème, dans une perspective observationnelle autant que théorique.
We present a study of Ks band luminosity evolution of the asymptotic giant
branch (AGB) population in simple stellar systems using star clusters in the
Large Magellanic Cloud (LMC). We determine physical parameters of LMC star
clusters including center coordinates, radii, and foreground reddenings. Ages
of 83 star clusters are derived from isochrone fitting with the Padova models,
and those of 19 star clusters are taken from the literature. The AGB stars in
102 star clusters with log(age) = 7.3 - 9.5 are selected using near-infrared
color magnitude diagrams based on 2MASS photometry. Then we obtain the Ks band
luminosity fraction of AGB stars in these star clusters as a function of ages.
The Ks band luminosity fraction of AGB stars increases, on average, as age
increases from log(age) ~ 8.0, reaching a maximum at log(age) ~ 8.5, and it
decreases thereafter. There is a large scatter in the AGB luminosity fraction
for given ages, which is mainly due to stochastic effects. We discuss this
result in comparison with five simple stellar population models. The maximum Ks
band AGB luminosity fraction for bright clusters is reproduced by the models
that expect the value of 0.7 - 0.8 at log(age) = 8.5 - 8.7. We discuss the
implication of our results with regard to the study of size and mass evolution
of galaxies.
Pressure-regulated star formation is a simple variant on the usual
supernova-regulated star formation efficiency that controls the global star
formation rate as a function of cold gas content in star-forming galaxies, and
accounts for the Schmidt-Kennicutt law in both nearby and distant galaxies.
Inclusion of AGN-induced pressure, by jets and/or winds that flow back onto a
gas-rich disk, can lead under some circumstances to significantly enhanced star
formation rates, especially at high redshift and most likely followed by the
more widely accepted phase of star formation quenching. Simple expressions are
derived that relate supermassive black hole growth, star formation and outflow
rates. The ratios of black hole to spheroid mass and of both black hole
accretion and outflow rates to star formation rate are predicted as a function
of time. I suggest various tests of the AGN-triggered star formation
hypothesis.
We use the Galaxy And Mass Assembly survey (GAMA) I data set combined with GALEX, Sloan Digital Sky Survey (SDSS) and UKIRT Infrared Deep Sky Survey (UKIDSS) imaging to construct the low-redshift (z < 0.1) galaxy luminosity functions in FUV, NUV, ugriz and YJHK bands from within a single well-constrained volume of 3.4 x 10(5) (Mpc h(-1))(3). The derived luminosity distributions are normalized to the SDSS data release 7 (DR7) main survey to reduce the estimated cosmic variance to the 5 per cent level. The data are used to construct the cosmic spectral energy distribution (CSED) from 0.1 to 2.1 mu m free from any wavelength-dependent cosmic variance for both the elliptical and non-elliptical populations. The two populations exhibit dramatically different CSEDs as expected for a predominantly old and young population, respectively. Using the Driver et al. prescription for the azimuthally averaged photon escape fraction, the non-ellipticals are corrected for the impact of dust attenuation and the combined CSED constructed. The final results show that the Universe is currently generating (1.8 +/- 0.3) x 10(35) h W Mpc(-3) of which (1.2 +/- 0.1) x 10(35) h W Mpc-3 is directly released into the inter-galactic medium and (0.6 +/- 0.1) x 10(35) h W Mpc(-3) is reprocessed and reradiated by dust in the far-IR. Using the GAMA data and our dust model we predict the mid- and far-IR emission which agrees remarkably well with available data. We therefore provide a robust description of the pre- and post-dust attenuated energy output of the nearby Universe from 0.1 mu m to 0.6 mm. The largest uncertainty in this measurement lies in the mid- and far-IR bands stemming from the dust attenuation correction and its currently poorly constrained dependence on environment, stellar mass and morphology.
Recent measurements of the abundance of AGN with low-luminosities (L_X< 10^44
erg/s in the 2-10 keV energy band) at high redshifts z>4 provide a serious
challenge for Cold Dark Matter (CDM) models based on interaction-driven fueling
of AGN. Using a semi-analytic model of galaxy formation we investigate how such
observations fit in a Warm Dark Matter (WDM) scenario of galaxy formation, and
compare the results with those obtained in the standard CDM scenario with
different efficiencies for the stellar feedback. Taking on our previous
exploration of galaxy formation in WDM cosmology, we assume as a reference case
a spectrum which is suppressed - compared to the standard CDM case - below a
cut-off scale ~ 0.2$ Mpc corresponding (for thermal relic WDM particles) to a
mass m_X=0.75 keV. We run our fiducial semi-analytic model with such a WDM
spectrum to derive AGN luminosity functions from z~6 to the present over a wide
range of luminosities (10^43< L_X/erg/s < 10^46 in the 2-10 keV X-ray band), to
compare with recent observations and with the results in the CDM case. When
compared with the standard CDM case, the luminosity distributions we obtain
assuming a WDM spectrum are characterized by a similar behaviour at low
redshift, and by a flatter slope at faint magnitudes for z>3, which provide an
excellent fit to present observations. We discuss how such a result compares
with CDM models with maximized feedback efficiency, and how future deep AGN
surveys will allow for a better discrimination between feedback and
cosmological effects on the evolution of AGN in interaction-driven models for
AGN fueling.
We apply Monte Carlo Markov Chain (MCMC) methods to large-scale simulations
of galaxy formation in a LambdaCDM cosmology in order to explore how star
formation and feedback are constrained by the observed luminosity and stellar
mass functions of galaxies. We build models jointly on the Millennium and
Millennium-II simulations, applying fast sampling techniques which allow
observed galaxy abundances over the ranges 7<log(M*/Msun)<12 and z=0 to z=3 to
be used simultaneously as constraints in the MCMC analysis. When z=0
constraints alone are imposed, we reproduce the results of previous modelling
by Guo et al. (2012), but no single set of parameters can reproduce observed
galaxy abundances at all redshifts simultaneously, reflecting the fact that
low-mass galaxies form too early and thus are overabundant at high redshift in
this model. The data require the efficiency with which galactic wind ejecta are
reaccreted to vary with redshift and halo mass quite differently than
previously assumed, but in a similar way as in some recent hydrodynamic
simulations of galaxy formation. We propose a specific model in which
reincorporation timescales vary inversely with halo mass and are independent of
redshift. This produces an evolving galaxy population which fits observed
abundances as a function of stellar mass, B- and K-band luminosity at all
redshifts simultaneously. It also produces a significant improvement in two
other areas where previous models were deficient. It leads to present day dwarf
galaxy populations which are younger, bluer, more strongly star-forming and
more weakly clustered on small scales than before, although the passive
fraction of faint dwarfs remains too high.
We build a theoretical model to study the origin of the globular cluster metallicity bimodality in the hierarchical galaxy assembly scenario. The model is based on empirical relations such as the galaxy mass-metallicity relation [O/H]-M
star as a function of redshift, and on the observed galaxy stellar mass function up to redshift z ~ 4. We make use of the theoretical merger rates as a function of mass and redshift from the Millennium simulation to build galaxy merger trees. We derive a new galaxy [Fe/H]-M
star relation as a function of redshift, and by assuming that globular clusters share the metallicity of their original parent galaxy at the time of their formation, we populate the merger tree with globular clusters. We perform a series of Monte Carlo simulations of the galaxy hierarchical assembly, and study the properties of the final globular cluster population as a function of galaxy mass, assembly and star formation history, and under different assumptions for the evolution of the galaxy mass-metallicity relation. The main results and predictions of the model are the following. (1) The hierarchical clustering scenario naturally predicts a metallicity bimodality in the galaxy globular cluster population, where the metal-rich subpopulation is composed of globular clusters formed in the galaxy main progenitor around redshift z ~ 2, and the metal-poor subpopulation is composed of clusters accreted from satellites, and formed at redshifts z ~ 3-4. (2) The model reproduces the observed relations by Peng et al. for the metallicities of the metal-rich and metal-poor globular cluster subpopulations as a function of galaxy mass; the positions of the metal-poor and metal-rich peaks depend exclusively on the evolution of the galaxy mass-metallicity relation and the [O/Fe], both of which can be constrained by this method. In particular, we find that the galaxy [O/Fe] evolves linearly with redshift from a value of ~0.5 at redshift z ~ 4 to a value of ~0.1 at z = 0. (3) For a given galaxy mass, the relative strength of the metal-rich and metal-poor peaks depends exclusively on the galaxy assembly and star formation history, where galaxies living in denser environments and/or early-type galaxies show a larger fraction of metal-poor clusters, while galaxies with a sparse merger history and/or late-type galaxies are dominated by metal-rich clusters. (4) The globular cluster metallicity bimodality disappears for galaxy masses around and below M
star ~ 109M
☉, and for redshifts z > 2.
We present near-infrared (NIR) color-magnitude diagrams (CMDs) for the resolved stellar populations within 26 fields of 23 nearby galaxies ( 4 Mpc), based on images in the F110W and F160W filters taken with the Wide-Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). The CMDs are measured in regions spanning a wide range of star formation histories, including both old dormant and young star-forming populations. We match key NIR CMD features with their counterparts in more familiar optical CMDs, and identify the red core helium-burning (RHeB) sequence as a significant contributor to the NIR flux in stellar populations younger than a few 100 Myr old. The strength of this feature suggests that the NIR mass-to-light ratio can vary significantly on short timescales in star-forming systems. The NIR luminosity of star-forming galaxies is therefore not necessarily proportional to the stellar mass. We note that these individual RHeB stars may also be misidentified as old stellar clusters in images of nearby galaxies. For older stellar populations, we discuss the CMD location of asymptotic giant branch (AGB) stars in the HST filter set and explore the separation of AGB subpopulations using a combination of optical and NIR colors. We empirically calibrate the magnitude of the NIR tip of the red giant branch in F160W as a function of color, allowing future observations in this widely adopted filter set to be used for distance measurements. We also analyze the properties of the NIR red giant branch (RGB) as a function of metallicity, showing a clear trend between NIR RGB color and metallicity. However, based on the current study, it appears unlikely that the slope of the NIR RGB can be used as an effective metallicity indicator in extragalactic systems with comparable data. Finally, we highlight issues with scattered light in the WFC3, which becomes significant for exposures taken close to a bright Earth limb.
We investigate the evolution of Brightest Cluster Galaxies (BCGs) from
redshift z~1.6 to z~0. We use the semi-analytic model of Croton et al. (2006)
with a new spectro-photometric model based on the Maraston (2005) stellar
populations and a new recipe for the dust extinction. We compare the model
predictions of the K-band luminosity evolution and the J-K, V-I and I-K colour
evolution with a series of datasets, including Collins et al. (Nature, 2009)
who argued that semi-analytic models based on the Millennium simulation cannot
reproduce the red colours and high luminosity of BCGs at z>1. We show instead
that the model is well in range of the observed luminosity and correctly
reproduces the colour evolution of BCGs in the whole redshift range up to
z~1.6. We argue that the success of the semi-analytic model is in large part
due to the implementation of a more sophisticated spectro-photometric model. An
analysis of the model BCGs shows an increase in mass by a factor ~2 since z~1,
and star formation activity down to low redshifts. While the consensus
regarding BCGs is that they are passively evolving, we argue that this
conclusion is affected by the degeneracy between star formation history and
stellar population models used in SED-fitting, and by the inefficacy of
toy-models of passive evolution to capture the complexity of real galaxies,
expecially those with rich merger histories like BCGs. Following this argument,
we also show that in the semi-analytic model the BCGs show a realistic mix of
stellar populations, and that these stellar populations are mostly old. In
addition, the age-redshift relation of the model BCGs follows that of the
universe, meaning that given their merger history and star formation history,
the ageing of BCGs is always dominated by the ageing of their stellar
populations. In a LambdaCDM universe, we define such evolution as "passive in
the hierarchical sense".
We calculate stellar masses for massive luminous galaxies at redshift 0.2-0.7
using the first two years of data from the Baryon Oscillation Spectroscopic
Survey (BOSS). Stellar masses are obtained by fitting model spectral energy
distributions to u,g,r,i,z magnitudes, and simulations with mock galaxies are
used to understand how well the templates recover the stellar mass. Accurate
BOSS spectroscopic redshifts are used to constrain the fits. We find that the
distribution of stellar masses in BOSS is narrow (Delta log M~0.5 dex) and
peaks at about logM ~ 11.3 (for a Kroupa initial stellar mass function), and
that the mass sampling is uniform over the redshift range 0.2 to 0.6, in
agreement with the intended BOSS target selection. The galaxy masses probed by
BOSS extend over ~10^{12} M, providing unprecedented measurements of the
high-mass end of the galaxy mass function. We find that the galaxy number
density above ~ 2.5 10^{11} M agrees with previous determinations. We perform a
comparison with semi-analytic galaxy formation models tailored to the BOSS
target selection and volume, in order to contain incompleteness. The abundance
of massive galaxies in the models compare fairly well with the BOSS data, but
the models lack galaxies at the massive end. Moreover, no evolution with
redshift is detected from ~0.6 to 0.4 in the data, whereas the abundance of
massive galaxies in the models increases to redshift zero. Additionally, BOSS
data display colour-magnitude (mass) relations similar to those found in the
local Universe, where the most massive galaxies are the reddest. On the other
hand, the model colours do not display a dependence on stellar mass, span a
narrower range and are typically bluer than the observations. We argue that the
lack of a colour-mass relation for massive galaxies in the models is mostly due
to metallicity, which is too low in the models.
Understanding galaxy formation is one of the most pressing issues in
cosmology. We review the current status of galaxy formation from both an
observational and a theoretical perspective, and summarise the prospects for
future advances.
Simulations of galaxy evolution aim to capture our current understanding as
well as to make predictions for testing by future experiments. Simulations and
observations are often compared in an indirect fashion: physical quantities are
estimated from the data and compared to models. However, many applications can
benefit from a more direct approach, where the observing process is also
simulated and the models are seen fully from the observer's perspective. To
facilitate this, we have developed the Millennium Run Observatory (MRObs), a
theoretical virtual observatory which uses virtual telescopes to `observe'
semi-analytic galaxy formation models based on the suite of Millennium Run dark
matter simulations. The MRObs produces data that can be processed and analyzed
using the standard software packages developed for real observations. At
present, we produce images in forty filters from the rest-frame UV to IR for
two stellar population synthesis models, three different models of IGM
absorption, and two cosmologies (WMAP1/7). Galaxy distributions for a large
number of mock lightcones can be `observed' using models of major ground- and
space-based telescopes. The data include lightcone catalogues linked to
structural properties of galaxies, pre-observation model images, mock telescope
images, and Source Extractor products that can all be traced back to the higher
level dark matter, semi-analytic galaxy, and lightcone catalogues available in
the Millennium database. Here, we describe our methods and announce a first
public release of simulated surveys (e.g., SDSS, CFHT-LS, GOODS, GOODS/ERS,
CANDELS, and HUDF). The MRObs browser, an online tool, further facilitates
exploration of the simulated data. We demonstrate the benefits of a direct
approach through a number of example applications (galaxy number counts in
CANDELS, clusters, morphologies, and dropout selections).
We present the Spitzer Extragalactic Representative Volume Survey (SERVS), an
18 square degrees medium-deep survey at 3.6 and 4.5 microns with the
post-cryogenic Spitzer Space Telescope to ~2 microJy (AB=23.1) depth of five
highly observed astronomical fields (ELAIS-N1, ELAIS-S1, Lockman Hole, Chandra
Deep Field South and XMM-LSS). SERVS is designed to enable the study of galaxy
evolution as a function of environment from z~5 to the present day, and is the
first extragalactic survey both large enough and deep enough to put rare
objects such as luminous quasars and galaxy clusters at z>1 into their
cosmological context. SERVS is designed to overlap with several key surveys at
optical, near- through far-infrared, submillimeter and radio wavelengths to
provide an unprecedented view of the formation and evolution of massive
galaxies. In this paper, we discuss the SERVS survey design, the data
processing flow from image reduction and mosaicing to catalogs, as well as
coverage of ancillary data from other surveys in the SERVS fields. We also
highlight a variety of early science results from the survey.
We used Early Release Science (ERS) observations taken with the Wide Field
Camera 3 (WFC3) in the GOODS-S field to study the galaxy stellar mass function
(GSMF) at 0.6<=z<4.5. Deep WFC3 near-IR data (for Y as faint as 27.3, J and H
as faint as 27.4 AB mag at 5 sigma), as well as deep Ks (as faint as 25.5 at 5
sigma) Hawk-I band data, provide an exquisite data set with which determine in
an unprecedented way the low-mass end of the GSMF, allowing an accurate probe
of masses as low as M~7.6 10^9 Msun at z~3. Although the area used is
relatively small (~33 arcmin^2), we found generally good agreement with
previous studies on the entire mass range. Our results show that the slope of
the faint-end increases with redshift, from alpha=-1.44+/-0.03 at z~0.8 to
alpha=-1.86+/-0.16 at z~3, although indications exist that it does not steepen
further between z~3 and z~4. This result is insensitive to any uncertainty in
the M* parameter. The steepness of the GSMF faint-end solves the well-known
disagreement between the stellar mass density (SMD) and the integrated star
formation history at z>2. However, we confirm the that there appears to be an
excess of integrated star formation with respect to the SMD at z<2, by a factor
of ~2-3. Our comparison of the observations with theoretical predictions shows
that the models forecast a greater abundance of low mass galaxies, at least up
to z~3, as well as a dearth of massive galaxies at z~4 with respect to the
data, and that the predicted SMD is generally overestimated at z<~2.
Quasars are useful tracers of the cosmological evolution of the black hole mass–galaxy relation. We compare the expectations of semi-analytical models (SAMs) of galaxy evolution to the largest available data sets of quasar host galaxies out to z≃ 3.
Observed quasar hosts are consistent with no evolution from the local MBH−Lhost relation and suggest a significant increase of the mass ratio from z= 0 to 3. Taken at face value, this is totally at odds with the predictions of SAMs, where the intrinsic Γ shows little evolution and quasar host galaxies at high redshift are systematically overluminous (and/or have an undermassive BH). However, since quasars preferentially trace very massive black holes (109–1010 M⊙) at the steep end of the luminosity and mass function, the ensuing selection biases can reconcile the present SAMs with the observations. A proper interpretation of quasar host data thus requires the global approach of SAMs so as to account for statistical biases.
The density distribution arising at the nonlinear stage of gravitational instability is similar to intermittency phenomena in acoustic turbulence. Initially small-amplitude density fluctuations of Gaussian type transform into thin dense pancakes, filaments, and compact clumps of matter. It is perhaps surprising that the motion of self-gravitating matter in the expanding universe is like that of noninteracting matter moving by inertia. A similar process is the distribution of light reflected or refracted from rippled water. The similarity of gravitational instability to acoustic turbulence is highlighted by the fact that late nonlinear stages of density perturbation growth can be described by the Burgers equation, which is well known in the theory of turbulence. The phenomena discussed in this article are closely related to the problem of the formation of large-scale structure of the universe, which is also discussed.
A method of calculation of the large-scale structure of the universe
based on the adiabatic theory (A-theory) of its formation is proposed.
The initial spectrum of perturbation is related to some observable
parameters of the structure, which are objectively defined as a set of
regions enclosed by the border of constant density rho = rhoc
(rhoc is a free parameter of the theory). The parameters are:
(1) W is the fraction of matter within the regions of high density rho
greater than rhoc; (2) L is the mean size of a region defined
as the diameter of the circle circumscribed around the region; (3) D is
a mean separation of dense regions taken along a straight line and (4) n
is a mean number of dense regions in a unit area. Equations relate these
parameters to the fundamental length which is associated with the
initial spectrum. The conclusions of the theory are checked by several
numerical models and are applied to observational parameters.
In the gravitational instability picture of galaxy formation, the precursor of a galaxy or a cluster of galaxies is a region
with a significantly perturbed mean density or velocity field. This region may, and generally does, have considerable substructure.
To relate the early perturbations to the currently observable structures it is necessary to distinguish between perturbations
which merge with other perturbations and those, more encompassing ones, which form galaxies by themselves. The latter can
be thought of as ‘isolated’ fluctuations; they conserve mass during their evolution so that the present distribution of over-dense
regions provides a direct record of these fluctuations at earlier epochs. The relationship between isolated fluctuations and
current observations is discussed in general terms in this paper and is explored in more detail elsewhere. One of the main
focuses of the present work is the computation of the mass spectra of ‘extremal mass’ fluctuations for a Poisson distribution
of point masses. The spectra of isolated fluctuations can be derived from the spectra of extremal fluctuations when the dynamics
of small perturbations in a uniform background is determined. It is found that the space density of extremal perturbations
is nearly constant per logarithmic mass interval at low masses and then falls off sharply.
A series of N-body experiments have been performed in order to
investigate gravitational clustering from scale-free initial conditions
in an Einstein-de Sitter universe. The three-dimensional shapes, angular
momenta, and internal velocity distributions of clumps are studied. It
is shown that in general, although nonlinear clustering destroys the
hierarchical arrangement of the initial conditions, their scaling
properties still determine the scaling properties of the mass
distribution at high density contrast.
In this paper we seek the best way of identifying 'objects to be' in the
primordial density field, and of assigning a logical size distribution
to them. We identify the objects in terms of a locally adaptive window
function whose scale is determined by the neighbourhoods of the maxima.
This largely takes account of the problem of hierarchical nesting of
maxima and gives a mass spectrum having a form which is very similar to
the classical Press-Schechter form. We present a critique of and an
alternative to the standard Press- Schechter derivation of the mass
function, and then go on to present an alternative view of the mass
function in biased galaxy formation scenarios. Notwithstanding its
apparent success, the Press-Schechter method for deriving the mass
function is correct. We offer an improvement on the method that at least
removes some of the major areas of doubt from their derivation. The
improved result turns out to be very close to the Press- Schechter
function, yet it has a quite different mathematical form. The method we
propose is only strictly valid when the density contrast threshold for
galaxy formation is large compared with the variance of the density
fluctuations. We can remove the small-amplitude limitation by making a
further departure from the Press-Schechter method and considering peaks
in the density distribution having densities close to the threshold
limit. This allows us to derive a 'local' window function for each
incipient object and a mass function that is independent of an arbitrary
choice of window function. The distribution of locally windowed peaks
will of course coincide with our modification of the Press-Schechter
method at high thresholds.
The collapse theories for the mass distribution of cosmic structures -
of which the theory of Press and Schechter (1974) constitutes the
archetype - are reformulated in terms of a rate equation with finite
collapse and survival times. The mass distribution and the optical
luminosity function expected for groups and clusters of galaxies in
these hierarchical clustering scenarios are derived, considering both
the collapse of high peaks in the initial overdensity field and the
subsequent mass infall. The transition from direct collapse to merging
and aggregation theories are discussed.
The Press-Schechter (1974) approximation is used to determine the
abundance and large-scale clustering of dark haloes in the cold dark
matter cosmogony. It is found that there exists a wide range of haloes
which are both sufficiently massive and sufficiently numerous to host
quasars, and that if the diffuse X-ray background is generated by
discrete sources associated with dark haloes, then the halo masses are
constrained to be less than 10 to the 12th/h solar masses. Two simple
models for galaxy formation are considered.
The Zel'dovich approximation is often used to mimic the effects of
nonlinear gravitational time evolution. The authors compare the
predictions of the Zel'dovich approximation with those of the true
nonlinear time evolution, for the probability distribution of mass
density fluctuations averaged over a Gaussian ball of large radius, and
for the large-scale streaming velocities of objects that do not trace
the mass.
We derive the mass function of spiral galaxy haloes using the luminosity
function and the inferred variation in mass-to-light ratio with spiral
galaxy luminosity. We show that the resulting mass function is
consistent with hierarchical clustering models of galaxy formation. This
contrasts with previous results based on the assumption of a constant
mass-to-light ratio for all spirals, which predict too many
low-luminosity galaxies. By reversing the argument, we show that, if
spirals form through hierarchical clustering, then the observed
luminosity function requires an increase in the dark matter fraction
with decreasing luminosity.
The mass function of galaxy systems in a cold dark matter cosmogony is
considered with the peak constraint that proto-objects form around local
density maxima. Different possible definitions of peak mass are analyzed
to test their reliability down to the low-mass tail. The effects of
non-Gaussian features on the distribution of primordial fluctuations are
also discussed. A prediction of the model is the time evolution of the
luminosity function and the characteristic mass: a self-similar
evolution provides a fairly accurate description even at moderate
redshifts. The resulting mass multiplicity, within a biased cold dark
matter model with Gaussian, scale-invariant (n = 1), adiabatic
primordial perturbations, provides a fit of the existing data on the
optical luminosity function for groups and clusters of galaxies.
We develop a picture of cosmic structure formation that identifies
virialized cosmological objects with a point process of "peak patches"
in the initial (Lagrangian) space. The strongly nonlinear internal
dynamics of the collapsing patch is largely decoupled from the weakly
nonlinear dynamics describing peak-patch flow. A modest set of local
field measurements on the patch region suffices to approximately
determine the fate of the peak. The candidate peak points are identified
using a hierarchy of smoothing operations on the density field. Patch
size and mass are found by requiring complete collapse of a homogeneous
ellipsoid model for internal patch dynamics which includes the important
influence of the external tidal field. Exclusion algorithms that prevent
peak-patch overlaps trim the candidate list into the final "hierarchical
peak" list. The Zeldovich approximation with locally adaptive filtering
of the displacement field is used for external dynamics to move the
surviving patches to their final state (Eulerian) positions, augmented
by a quadratic nonlinear correction where necessary. Thus both
statistical and dynamical clustering of the peaks are included. Our
technique is the natural generalization of both the Press-Schechter
method (to include nonlocal effects) and the BBKS single-filter peaks
theory (to allow a mass spectrum and solve the cloud-in-cloud problem).
It allows efficient Monte Carlo constructions of three-dimensional
catalogs of objects such as groups and clusters of galaxies, and, with
more selection criteria, of galaxies in their formation phases.
The mass function of cosmic structures is computed in the framework of
the hierarchical clustering picture for a general statistics of density
perturbations. 'Hierarchical' distributions are extensively analyzed; it
is found that the multiplicity function preserves the Press-Schechter
functional form with enhanced power on large scales compared to the
Gaussian case. A class of scale-invariant non-Gaussian statistics, among
which are a model due to Peebles and the lognormal distribution, are
also analyzed. All these predict a mass function which is a decreasing
power law at low mass followed by an exponential decay at high mass;
none of them, however, yields a mass function of the Press-Schechter
type. The effect of a statistical bias on the origin of condensations is
also discussed. The comparison of these theoretical formulae with the
observed mass multiplicity of galaxies, groups, and clusters may
represent a powerful tool to test the statistics of cosmological
perturbations.
Consideration is given to the validity of the Press-Schechter (1974)
formalism for the mass distribution function of bound objects which
condense out of a primordial density perturbation field. Several
modifications to the Press-Schechter formalism are discussed, including
correctly accounting for the fate of material in underdense regions,
incorporating recent results on density peaks, and using a simple model
for the effects of cooling to estimate the mass function for luminous
objects. As an example of the modified formalism, the mass function for
galaxies is calculated, producing a qualitative explanation for the form
of the galaxy luminosity function.
An analytic description is proposed of the direct mergings that involve
galaxies in groups. Shape and evolution predictions of the distribution
of the galactic masses in these finite systems, including the formation
of a cD-like merger that analytically appears as a critical phenomenon.
These findings constitute a close counterpart of many N-body results.
Similar aggregation phenomena conceivably take place in other cosmic
structures.
We combine the photometric model of isochrone synthesis recently
published by Chariot and Bruzual (1991) with an updated library of
stellar spectra to predict the spectral evolution of stellar populations
with solar metallicity. The library of spectra assembled here supersedes
other existing libraries by its spectral range, its complete coverage of
the color-magnitude diagram, and its inclusion of observed near-infrared
spectra out to 2.56 micron. Also, the spectra are distributed on the
stellar evolutionary tracks using optical/near-infrared color
calibrations, as an improvement over models that used a single color of
the effective temperature of the stars alone. The spectroscopic results
obtained here confirm and extend the previous photometric predictions of
the isochrone synthesis models while including the recent revision of
evolutionary tracks for stars between 1.3 and 2.5 solar masses by their
authors.
This work develops models for the spectral evolution of early-type
galaxies using population synthesis techniques. Different choices of the
initial mass function and the star formation rate allow the construction
of models which at the present epoch cover the range of observed galaxy
spectra. Special emphasis is given to the ultraviolet region of the
spectrum. Ultraviolet spectra of late-type stars and early-type galaxies
obtained with the International Ultraviolet Explorer satellite are used
in this work. The evolving spectral energy distributions are used to
compute galaxy colors, magnitudes, K-corrections, and evolutionary
corrections as functions of redshift for different photometric systems.
The Lagrangian perturbation theory of Friedman-Lemaitre cosmologies,
which was investigated and solved up to the second order in earlier
papers, is evaluated up to the third order. Based on this, a model for
non-linear clustering applicable to the modelling of large-scale
structure in the Universe for generic initial conditions is formulated.
A truncated model is proposed which represents the `main body' of the
perturbation sequence in the early non-linear regime by neglecting all
gravitational sources that describe interaction of the perturbations. I
also, however, give the irrotational solutions generated by the
interaction terms to the third order, which induce vorticity in
Lagrangian space. The consequences and applicability of the solutions
are put into perspective. In particular, the model presented enables the
study of pre-virialization effects in gravitational clustering and the
onset of non-dissipative gravitational turbulence within the cluster
environment.
We report on a study of the richness dependence of the spatial
correlations of clusters of galaxies. We employ the method devised by
Bond & Couchman, which combines the theory of the statistics of
peaks in Gaussian random fields with the evolution of the cosmological
density field by the Zeldovich Approximation, to calculate analytically
both the statistical and dynamical contributions to the clustering. We
compute the cluster correlation function for a variety of popular
cosmological models and compare our results with data from four recent
cluster samples. We find no model able to account for all of the
observations, although, conversely, the observational data are of
insufficient quality to rule out firmly any of the models. The model
that fares best is one which has been advocated as accounting for the
angular correlation function of APM galaxies: that the same power
spectrum gives the best fit to both the galaxy and cluster correlation
data may be taken as support for the standard picture, due to Kaiser, in
which objects form at the sites of peaks in an initial cosmological
density field which obeyed Gaussian statistics. No model is able to
reproduce the correlation length of a sample of Abell R >= 2
clusters. This result may just indicate the inadequacy of the
correlation length when taken alone as a diagnostic statistic, or that
this cluster sample is seriously corrupted by projection effects. We
consider, however, alternative explanations, including the possibility
that non-Gaussian initial conditions are required and that the
identification of peaks in the linear density field as sites of nascent
clusters may break down for the highest peaks, corresponding to the
richest clusters.
A dissipative N-body code is used to follow the evolution of a cold dark
matter spectrum in an Omega = 1, h = 0.5 background. The star and galaxy
formation in boxes of three different sizes is shown, and the relative
degrees of clustering of dark matter, halos, and galaxies is discussed
and used to normalize to the current epoch. The streaming velocities of
the galaxies and dark matter are measured and compared with linear
theory. The pairwise velocities of the galaxies and dark matter are
studied and used with estimates of galactic merger rates to discuss the
role that mergers play in galaxy formation. The number density-redshift
relation of galaxies is shown to be well described by the statistical
description of density perturbation peaks by Bardeen et al. (1986),
whereas halos are better fitted by the volume-averaging, hierarchical
model of Press and Schechter (1974). The model galaxies exhibit only a
modest enhancement of their two-point correlation function over that of
the dark matter.
The evolution of density inhomogeneities and the velocity field in an
expanding continuous medium has been explored using the model equation
of nonlinear diffusion together with an equation of continuity
incorporating mass density. The method provides an approximate
description of density inhomogeneity growth at the advanced nonlinear
stage of gravitational instability. The statistical characteristics of
an ensemble of density clumps, such as their masses, velocities, and
spatial distribution, can be determined.
A new grid of 450 theoretical models for old simple stellar populations
is presented. Evolutionary synthesis explores the influence of relevant
parameters such as age, chemical composition, initial mass function, and
stellar mass loss, on the integrated spectral energy distribution. Ages
range between 4 and 18 Gyr, and the metallicity Z ranges from 0.0001 to
0.03, with helium content Y = 0.23 and 0.25, respectively. Three values
are considered for the initial mass function, assumed as a power law,
N(M) proportional to M exp-s; s = 1.35, 2.35, and 3.35. The
computational code takes into account in a quantitative way the
contributions from all the relevant stellar evolutionary phases
according to the theory of the stellar evolution. Thus, late stage in
the life of low- mass stars, such as the horizontal branch and the
asymptotic and postasymptotic giant branches are accounted for.
Furthermore, a simplified treatment for the evolution of the horizontal
branch is developed, and the influence of different morphologies are
investigated.
The solution of a stochastic differential equation with a given initial value defines a Markov process. Moreover, the mapping assigning to each initial value the solution at time t defines a random diffeomorphism, and the family of these diffeomorphisms for t ∈ IR forms a cocycle (flow) over the Wiener space. This observation, which is surprisingly recent, has important consequences: It allows to view and treat a stochastic differential equation as the generator of a smooth dynamical system in the classical sense. We need a second ingredient to do all the things done by the local theory of nonlinear systems: eigenvalues and eigenspaces. These objects are provided by Oseledec’s multiplicative ergodic theorem and its generalizations. Now the door is open for stochastic stability, stochastic invariant manifolds (in particular stochastic center manifolds), stochastic bifurcation theory, stochastic normal forms, etc.
A set of new mathematical results on the theory of Gaussian random
fields is presented, and the application of such calculations in
cosmology to treat questions of structure formation from small-amplitude
initial density fluctuations is addressed. The point process equation is
discussed, giving the general formula for the average number density of
peaks. The problem of the proper conditional probability constraints
appropriate to maxima are examined using a one-dimensional illustration.
The average density of maxima of a general three-dimensional Gaussian
field is calculated as a function of heights of the maxima, and the
average density of 'upcrossing' points on density contour surfaces is
computed. The number density of peaks subject to the constraint that the
large-scale density field be fixed is determined and used to discuss the
segregation of high peaks from the underlying mass distribution. The
machinery to calculate n-point peak-peak correlation functions is
determined, as are the shapes of the profiles about maxima.
We apply updated semi-analytic galaxy formation models simultaneously to the
stored halo/subhalo merger trees of the Millennium and Millennium-II
simulations. These differ by a factor of 125 in mass resolution, allowing
explicit testing of resolution effects on predicted galaxy properties. We have
revised the treatments of the transition between the rapid infall and cooling
flow regimes of gas accretion, of the sizes of bulges and of gaseous and
stellar disks, of supernova feedback, of the transition between central and
satellite status as galaxies fall into larger systems, and of gas and star
stripping once they become satellites. Plausible values of efficiency and
scaling parameters yield an excellent fit not only to the observed abundance of
low-redshift galaxies over 5 orders of magnitude in stellar mass and 9
magnitudes in luminosity, but also to the observed abundance of Milky Way
satellites. This suggests that reionisation effects may not be needed to solve
the "missing satellite" problem except, perhaps, for the faintest objects. The
same model matches the observed large-scale clustering of galaxies as a
function of stellar mass and colour. The fit remains excellent down to ~30kpc
for massive galaxies. For M* < 6 x 10^10Msun, however, the model overpredicts
clustering at scales below 1 Mpc, suggesting that the sigma_8 adopted in the
simulations (0.9) is too high. Galaxy distributions within rich clusters agree
between the simulations and match those observed, but only if galaxies without
dark matter subhalos (so-called orphans) are included. Our model predicts a
larger passive fraction among low-mass galaxies than is observed, as well as an
overabundance of ~10^10Msun galaxies beyond z~0.6, reflecting deficiencies in
the way star-formation rates are modelled.
The approach to formation of high-contrast structures by Press and Schechter (1974), representative of the clustering theories by direct hierarchical collapses, leads to distributions of masses, of velocity dispersions and of X-ray temperatures which are at variance with data and with simulations as for shape and evolution. Aggregations constitute a complementary route to hierarchical clustering, especially relevant at the high mass end. The authors describe the full emergence of structure from initial fluctuations by combining the two routes into one stochastic process represented with a branching Cayley tree.
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The stellar masses, mean ages, metallicities, and star formation histories of galaxies are now commonly estimated via stellar population synthesis (SPS) techniques. SPS relies on stellar evolution calculations from the main sequence to stellar death, stellar spectral libraries, phenomenological dust models, and stellar initial mass functions (IMFs) to translate the evolution of a multimetallicity, multi-age set of stars into a prediction for the time-evolution of the integrated light from that set of stars. Each of these necessary inputs carries significant uncertainties that have until now received little systematic attention. The present work is the first in a series that explores the impact of uncertainties in key phases of stellar evolution and the IMF on the derived physical properties of galaxies and the expected luminosity evolution for a passively evolving set of stars. A Monte Carlo Markov Chain approach is taken to fit near-UV through near-IR photometry of a representative sample of low- and high-redshift galaxies with this new SPS model. Significant results include the following. (1) Including uncertainties in stellar evolution, stellar masses at z ~ 0 carry errors of ~0.3 dex at 95% CL with little dependence on luminosity or color, while at z ~ 2, the masses of bright red galaxies are uncertain at the ~0.6 dex level. (2) Either current stellar evolution models, current observational stellar libraries, or both, do not adequately characterize the metallicity-dependence of the thermally pulsating AGB phase. (3) Conservative estimates on the uncertainty of the slope of the IMF in the solar neighborhood imply that luminosity evolution per unit redshift is uncertain at the ~0.4 mag level in the K band, which is a substantial source of uncertainty for interpreting the evolution of galaxy populations across time. Any possible evolution in the IMF, as suggested by several independent lines of evidence, will only exacerbate this problem. (4) Assuming a distribution of stellar metallicities within a galaxy, rather than a fixed value as is usually assumed, can yield important differences when considering bands blueward of V, but is not a concern for redder bands. Spectroscopic information may alleviate some of these concerns, though uncertainties in the stellar spectral libraries and the importance of nonsolar abundance ratios have not yet been systematically investigated in the SPS context.
The high end of the stellar mass function of galaxies is observed to have little evolution since z~1. This represents a stringent constraint for merger-based models, aimed at explaining the evolution of the most massive galaxies in the concordance LambdaCDM cosmology. In this Letter we show that it is possible to remove the tension between the above observations and model predictions by allowing a fraction of stars to be scattered to the diffuse stellar component (DSC) of galaxy clusters at each galaxy merger, as recently suggested by the analysis of N-body hydrodynamical simulations. To this purpose, we use the MORGANA model of galaxy formation in a minimal version, in which gas cooling and star formation are switched off after z=1. In this way, any predicted evolution of the galaxy stellar mass function is purely driven by mergers. We show that, even in this extreme case, the predicted degree of evolution of the high end of the stellar mass function is larger than that suggested by data. Instead, the assumption that a significant fraction, ~30%, of stars are scattered in the DSC at each merger event leads to a significant suppression of the predicted evolution, in better agreement with observational constraints, while providing a total amount of DSC in clusters, which is consistent with recent observational determinations.
We combine N-body simulations of structure growth with physical modelling of galaxy evolution to investigate whether the shift in cosmological
parameters between the first- and third-year results from the Wilkinson Microwave Anisotropy Probe (WMAP) affects predictions for the galaxy population. Structure formation is significantly delayed in the WMAP3 cosmology, because the initial matter fluctuation amplitude is lower on the relevant scales. The decrease in dark matter
clustering strength is, however, almost entirely offset by an increase in halo bias, so predictions for galaxy clustering
are barely altered. In both cosmologies, several combinations of physical parameters can reproduce observed, low-redshift
galaxy properties; the star formation, supernova feedback and active galactic nucleus feedback efficiencies can be played
off against each other to give similar results. Models which fit observed luminosity functions predict projected two-point
correlation functions which scatter by about 10–20 per cent on large scale and by larger factors on small scale, depending
both on cosmology and on details of galaxy formation. Measurements of the pairwise velocity distribution prefer the WMAP1 cosmology, but careful treatment of the systematics is needed. Given present modelling uncertainties, it is not easy to distinguish
between the WMAP1 and WMAP3 cosmologies on the basis of low-redshift galaxy properties. Model predictions diverge more dramatically at high redshift.
Better observational data at z > 2 will better constrain galaxy formation and perhaps also cosmological parameters.
A new approximation to the evolution of large-scale structures in the Universe is proposed which is based on neglecting the
role of particle inertia compared to the damping implied by the Hubble drag. We call this approximation frozen flow because
particles move by updating at each step their velocity to the local value of the peculiar velocity field, here approximated
by its growing linear mode: stream-lines are then frozen to their initial shape. The situation is quite different from that
of the Zel'dovich algorithm, where the velocity is kept constant along each particle trajectory. The advantage of this approximation
is that the emerging density field is free of singularities: no caustics appear at finite time. This property allows an extrapolation
into the non-linear regime beyond the time at which shell crossings would have appeared according to the Zel'dovich approximation.
We test the validity of this method by applying it to suitable toy models and by following the evolution of large-scale structures,
starting from the standard cold dark matter initial power spectrum of Gaussian perturbations. A formal connection between
this approximation and the adhesion model is pointed out.
The dynamics of superclusters is studied by using the Zel'dovich
quasi-linear formalism and by assuming the initial density perturbation
field to be Gaussian. The primordial velocity field and its dependence
on the primordial density contrast is analyzed and is used to set the
typical initial conditions of superclusters. A detailed comparison with
the spherical nonlinear model is made, and its applicability in
obtaining Omega0 is investigated. A global shear is an
inevitable component of the velocity field of any object that is formed
out of a random density field. The shear affects the dynamics of
collapsing objects, and it leads to infall velocities which are larger
than in the case of nonshearing ones. An ensemble of density
perturbations has been constructed, and an apparent value of
Omega0 has been derived by the use of the spherical nonlinear
infall model.
Few recent generations of cosmologists have solved non-local newtonian equations of the gravitational instability in an expanding universe. In this approach pancaking is the predominant form of first collapsing objects. Relativistic counterparts of these equations contain the electric and magnetic parts of the Weyl tensor. In the linear theory the magnetic part is associated with gravitational waves. If the magnetic part is ignored, then the newtonian limit of the relativistic equations is reduced to the closed set of the local Lagrangian equations. Recently this fact drew much attention since the gravitational instability in that form would greatly simplify the study of cosmic structure formation. In particular, the filamentary structure of collapsing is predicted. In this paper we resolve the contradiction between the newtonian theory and relativistic version adopted in some recent papers. We show that dropping the magnetic part from the basic relativistic equations is {\it incorrect}. The correct newtonian limit is derived by the $1/c$-expansion of the GR equations and the Bianchi identities for the Weyl tensor. The last ones begin with $\sim 1/c^3$ order, therefore one {\it must} take into account the magnetic part in the post newtonian order $\sim 1/c^3$, which contains non-local terms, related to the non-local gravitational interaction. For the first time we rigorously show that the GR equations with the magnetic part are reduced precisely to the canonic newtonian non-local equations. Thus, the correct treatment of the relativistic version of the gravitational instability resurrects the canonic picture of the structure formation.
The Press–Schechter theory provides a simple analytical description for the evolution of gravitational structure in a hierarchical
universe. In this paper, we extend the original theory in order to focus our attention on the subset of regions that will
eventually collapse to form clusters of a set mass. The first part of the paper is concerned with obtaining the conditional multiplicity function of groups at an epoch with redshift z, given that they are bound into an object of a particular mass at the present epoch. This is combined with the present distribution
of group masses in order to obtain the joint multiplicity function. The difficulty in obtaining these functions lies in determining the cross-corelation between a set of small volumes and
the larger volume that contains them. We show that this correlation has a simple form. The formulae we derive are checked
for self-consistency, and are compared with the N-body simulations of Efstathiou et al. The numerical results are found to be in extremely good agreement.
These multiplicity functions are applied to study the evolutionary histories of groups and clusters. We obtain a number of
results relating to the infall of galaxies in small groups into large clusters, and to the density of the more massive groups
at early times. In particular, we illustrate our analysis by making an in-depth comparison of the theoretical evolution with
the Butcher-Oemler effect observed in rich clusters at moderate redshifts. We find that the growth of the infall rate over
these look-back times is not (by itself) sufficient to explain the rapidly increasing fraction of blue galaxies in these clusters,
but that a good quantitative fit to the available data can be obtained if allowance is made for the increased star burst activity
that is seen in the spectra of the distant blue galaxies.
A survey is presented of the suite of constraints needed to test the
ability of the biased cold dark matter (CDM) theory to account for the
mass concentrations observed in rich clusters of galaxies. An attempt is
made to place limits on the uncertainties in the galaxy clustering
length r(0) and the bias parameter b. The limits are favorable but not
overly generous for the CDM theory. It is found that no values for r(0)
and b are concordant with all available constraints.
The problem of estimating the mass distribution of groups of galaxies is
addressed by analyzing the extent to which the observational estimate of
the group mass distribution is sensitive to the features of the catalogs
and, in particular, to the mass estimators. The observational mass
distribution of the catalogs GH83, T87, and MDL89 is compared, and a
significant inhomogeneity is found. The same result is obtained by
considering other dynamical parameters such as the velocity dispersion,
crossing time, and dynamical state. Owing to the sensitivity of the
physical parameters of groups to the group identification algorithms,
two different families of observed groups are defined: Cfof
contains groups obtained by using the friends-of-friends identification
algorithm with a low number-density contrast, while Chc
contains groups obtained by using the hierarchical clustering algorithm.
The relevance of the bias affecting the observational estimate of the
mass distribution is clarified.
Using a full nonlinear numerical gravitational clustering simulation
with Ω = 1 cold dark matter and Zel'dovich initial conditions, we
show that the gravitational potential evolves very little up to the
present on length scales ≥ 1.25 h-1 Mpc. We present a new
approximation for the nonlinear evolution of large-scale structure, in
which the gravitational potential field is assumed to remain constant up
to the present, but the matter obeys the usual nonlinear equations of
motion in this potential field. We calculate evolved density fields
using this approximation and compare them to the Zel'dovich
approximation and a full nonlinear evolution. At late times, the
accuracy of our results lies between the Zel'dovich approximation and a
full nonlinear evolution.
The mass distribution of cosmic structures has been connected in Paper I
to the number of random trajectories (overdensities as a function of
mass scale) in a Cayley tree. These may diffuse and/or branch with
decreasing mass, and the two processes were related to the Press &
Schechter mass distribution or to the Smoluchowski aggregation equation,
respectively.
Here we start from the dynamical equations for the cosmic matter field,
specifically in the adhesion approximation, and we show that the
distribution of the masses condensed into isotropic "schocks" is given
by a specific instance of the above branching diffusion process, with
the branching rate provided by the adhesion approximation. Such
equivalence establishes a bridging between the dynamics of the cosmic
fluid under gravity and the phenomenological mass distributions
generated (as limiting cases) by our model based on branching diffusion.
We discuss how this approach can be extended beyond the adhesion
approximation, to describe the emergence of cosmic structure as a random
process of the branching diffusion kind.
A large N-body simulation is used to compare the galaxies found by
tagging peaks in the linear density field with the haloes that actually
form. A variety of filters on the density field are tried in order to
improve this correspondence, but none seems to do particularly well. The
correlation function and velocity dispersion of the tagged peaks and the
actual haloes also do not correspond very well. These comparisons bring
into question the results of any study of galaxy formation that assumes
that galaxies form at peaks in the initial density field, or simulations
of large-scale structure that use the high-peak model to determine the
galaxy distribution.
The collapse of density peaks is investigated in Gaussian random density
fields with power spectra P(k) proportional to k exp n, in the cases n =
-2, -1, 0, and + 1. The density, velocity dispersion, and angular
momentum are computed, taking into account density perturbations
superposed on a spherical central peak. In the absence of dissipation,
flat rotation curves require n roughly -1. If a fraction f roughly 0.1
of the mass dissipates its energy and falls toward the center of the
mass distribution, then n roughly -2 leads to flat rotation curves. The
dimensionless spin parameter Lambda is insensitive to the value of n;
the spherical infall approximation yields Lambda roughly 0.09 for 3
sigma peaks.
The relation between the virialized mass concentrations produced in a
hierarchical gravitational instability picture and the amplitude of the
primeval mass fluctuations is estimated. The analysis starts from an
explicit construction of a developing protocluster and deduces the
particle orbits that lead to the development of this system out of a
homogeneous mass distribution. The solution yields a fairly direct
measure of the primeval mass contrast and eliminates the possibility of
unrealistic two-body relaxation during formation of the protocluster
because early orbit crossing is suppressed.
It is argued that the evolution of the universe proceeded from a nearly
uniform initial state to a progressively more irregular and clumpy
universe. The discussion centers on the clusters of galaxies, the
empirical evidence of the nature of clustering, and theories of how the
clustering evolves in an expanding universe. A historical introduction
to the subject is given; and a survey of methods used to deal with the
Newtonian approximation to the theory of the evolution of the mass
distribution is presented. Recent progress in the use of statistical
measures of the clustering is described; and techniques for dealing with
cosmic evolution, in the statistical measures of clustering and under
general relativity theory, are considered. An assessment is made of
attempts to link theory and observation to arrive at a well-established
physical picture of the nature and evolution of the universe.
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A detailed review of progress in understanding the last stages of evolution of low and intermediate mass stars is presented. The thermal pulse phase of asymptotic giant branch (AGB) evolution is addressed first, and estimates and implications of noncatastrophic mass loss and ejection from AGB stars are reviewed. The concept of synthetic AGB evolution is presented and the algorithms that follow from this concept are used to review estimates of the ages of stellar aggregates that contain AGB stars. Evolution to the white dwarf state as well as the nature of type I 1/2 supernovae are discussed. Theoretical predictions are compared with observations, emphasizing predictions about the surface abundances given by 'canonical' stellar evolution versus abundances estimated from spectroscopic observations.