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OMEGA: OSIRIS mapping of Emission-Line Galaxies in A901/2
Abstract
This work presents the first results from an ESO Large Programme carried out
using the OSIRIS instrument on the 10m GTC telescope (La Palma). We have
observed a large sample of galaxies in the region of the Abell 901/902 system
(z ~ 0.165) which has been extensively studied as part of the STAGES project.
We have obtained spectrally and spatially resolved H-alpha and [NII] emission
maps for a very large sample of galaxies covering a broad range of
environments. The new data are combined with extensive multi-wavelength
observations which include HST, COMBO-17, Spitzer, Galex and XMM imaging to
study star formation and AGN activity as a function of environment and galaxy
properties such as luminosity, mass and morphology. The ultimate goal is to
understand, in detail, the effect of the environment on star formation and AGN
activity.
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emcee is an extensible, pure-Python implementation of Goodman &
Weare's Affine Invariant Markov chain Monte Carlo (MCMC) Ensemble
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We apply a semi-classical approach of handling waves as quasiparticle gas in a slowly varying flow - analogous to ray tracing - to calculate the Alfvén wave transmission parameters, the resulting cross-helicity of the waves and the scattering-centre compression ratio, for cases where the shock thickness is large enough for the turbulent waves in the plasma to see the transition of the background flow parameters as smooth and slowly varying. For nonrelativistic shocks the wave transmission produces similar effects on the downstream turbulence and the scattering-centre compression ratio as does the transmission through a step shock: the downstream Alfvén waves propagate predominantly towards the shock in the local plasma frame and, thus, the scattering-centre compression ratio is larger than the gas compression ratio. For thick relativistic shocks, however, we find qualitative differences with respect to the step-shock case: for low-Alfvénic-Mach-number shocks the downstream waves propagate predominantly away from the shock, and the scattering-centre compression ratio is lower than that of the gas. Thus, when taken into account, the Alfvén wave transmission can decrease the efficiency of the first-order Fermi acceleration in a thick relativistic shock.
Measurements of the low-z Halpha luminosity function have a large dispersion
in the local number density of sources, and correspondingly in the SFR density.
The possible causes for these discrepancies include limited volume sampling,
biases arising from survey sample selection, different methods of correcting
for dust obscuration and AGN contamination. The Galaxy And Mass Assembly (GAMA)
survey and Sloan Digital Sky Survey (SDSS) provide deep spectroscopic
observations over a wide sky area enabling detection of a large sample of
star-forming galaxies spanning 0.001<SFR(Halpha)<100 with which to robustly
measure the evolution of the SFR density in the low-z universe. The large
number of high SFR galaxies present in our sample allow an improved measurement
of the bright end of the luminosity function, indicating that the decrease in
number density of sources at bright luminosities is best described by a
Saunders functional form rather than the traditional Schechter function. This
result is consistent with other published luminosity functions in the FIR and
radio. For GAMA and SDSS we find the r-band apparent magnitude limit, combined
with the subsequent requirement for Halpha detection leads to an incompleteness
due to missing bright Halpha sources with faint r-band magnitudes.
The paper presents a study of the galaxy populations in 55 rich clusters
along with a discussion of the implications for the formation and/or
evolution of different morphological types. A well defined relationship
is found between local galaxy density and galaxy type, which indicates
an increasing elliptical and S0 population and a corresponding decrease
in spirals with increasing density. Attention is given to evidence which
contradicts the interpretation that these gradients in population result
from the production of S0 galaxies when spirals are swept of disk gas by
an IGM. In addition, as an alternative to the hypothesis of spiral
sweeping, it is suggested that the local density/morphological-type
relation reflects the long time scale associated with the formation of
the disk component of galaxies. Finally, the data also indicate a trend
of increasing luminosity of the spheroidal component with increasing
local density.
We analyse the relationships between galaxy morphology, colour, environment and stellar mass using data for over 10⁵ objects from Galaxy Zoo, the largest sample of visually classified morphologies yet compiled. We conclusively show that colour and morphology fractions are very different functions of environment. Both colour and morphology are sensitive to stellar mass. However, at fixed stellar mass, while colour is also highly sensitive to environment, morphology displays much weaker environmental trends. Only a small part of both the morphology–density and colour–density relations can be attributed to the variation in the stellar-mass function with environment.
Galaxies with high stellar masses are mostly red in all environments and irrespective of their morphology. Low stellar-mass galaxies are mostly blue in low-density environments, but mostly red in high-density environments, again irrespective of their morphology. While galaxies with early-type morphology do always have higher red fractions, this is subdominant compared to the dependence of red fraction on stellar mass and environment. The colour–density relation is primarily driven by variations in colour fractions at fixed morphology, in particular the fraction of spiral galaxies that have red colours, and especially at low stellar masses. We demonstrate that our red spirals primarily include galaxies with true spiral morphology, and that they constitute an additional population to the S0 galaxies considered by previous studies. We clearly show there is an environmental dependence for colour beyond that for morphology. The environmental transformation of galaxies from blue to red must occur on significantly shorter time-scales than the transformation from spiral to early-type.
We also present many of our results as functions of the distance to the nearest galaxy group. This confirms that the environmental trends we present are not specific to the manner in which environment is quantified, but nevertheless provides plain evidence for an environmental process at work in groups. However, the properties of group members show little dependence on the total mass of the group they inhabit, at least for group masses .
Before using the Galaxy Zoo morphologies to produce the above results, we first quantify a luminosity-, size- and redshift-dependent classification bias that affects this data set, and probably most other studies of galaxy population morphology. A correction for this bias is derived and applied to produce a sample of galaxies with reliable morphological-type likelihoods, on which we base our analysis.
We present results of an Hα survey in the rich cluster A 1689 at z= 0.18, using the LDSS++ spectrograph on the Anglo-Australian Telescope (AAT). We obtained spectra covering redshifted Hα at z= 0.16–0.22, for 522 galaxies brighter than I= 22.5, covering a field of 8.7 × 8.7 arcmin2 (∼1.1 × 1.1 h−1 Mpc at z= 0.18). We securely detect Hα emission in 46 of these galaxies; accounting for selection effects arising from sampling and cluster membership, we determine that 24 ± 4 per cent of cluster members brighter than MR=−16.5 + 5log h are detected with Hα flux greater than 4h−2× 1038 erg s−1. This corresponds to a limiting star formation rate of 0.008h−2 M⊙ yr−1, assuming 1 mag of dust extinction. Our observations are sufficiently sensitive to detect galaxies with star formation rates comparable to that of the Milky Way (≳3 M⊙ yr−1), unless they are obscured by more than 7 mag of extinction. From a Hubble Space Telescope mosaic covering 7.5 × 10.0 arcmin, we determine morphologies for 199 galaxies brighter than I= 21, and find that ∼20 per cent of the cluster members are of type Sa or later. More than 90 per cent of cluster spirals show Hα emission, compared with less than 10 per cent of E and S0 galaxies. The cluster Hα luminosity function has a low normalization relative to the z∼ 0.2 field, by ∼50 per cent, after accounting for the different fraction of spiral galaxies in the two environments. When compared with local field galaxies, this suggests that star formation activity is suppressed in early-type cluster galaxies, relative to their field counterparts. Our sample includes 29 galaxies previously observed with ISOCAM at 6.7 and 15 μm. We detect all 15-μm sources at Hα, so there is no evidence for any star formation completely hidden at Hα. Comparing the 15-μm and Hα fluxes, we find evidence that some mid-infrared-detected galaxies could be obscured by as much as 3 mag of extinction at Hα, although this depends on the largely unknown contribution from any active galactic nucleus heated dust to the mid-infrared flux.
A numerous population of weak line galaxies (WLGs) is often left out of statistical studies on emission-line galaxies (ELGs) due to the absence of an adequate classification scheme, since classical diagnostic diagrams, such as [O iii]/Hβ versus [N ii]/Hα (the BPT diagram), require the measurement of at least four emission lines. This paper aims to remedy this situation by transposing the usual divisory lines between star-forming (SF) galaxies and active galactic nuclei (AGN) hosts and between Seyferts and LINERs to diagrams that are more economical in terms of line quality requirements. By doing this, we rescue from the classification limbo a substantial number of sources and modify the global census of ELGs. More specifically, (1) we use the Sloan Digital Sky Survey Data Release 7 to constitute a suitable sample of 280 000 ELGs, one-third of which are WLGs. (2) Galaxies with strong emission lines are classified using the widely applied criteria of Kewley et al., Kauffmann et al. and Stasińska et al. to distinguish SF galaxies and AGN hosts and Kewley et al. to distinguish Seyferts from LINERs. (3) We transpose these classification schemes to alternative diagrams keeping [N ii]/Hα as a horizontal axis, but replacing Hβ by a stronger line (Hα or [O ii]), or substituting the ionization-level sensitive [O iii]/Hβ ratio with the equivalent width of Hα(WHα). Optimized equations for the transposed divisory lines are provided. (4) We show that nothing significant is lost in the translation, but that the new diagrams allow one to classify up to 50 per cent more ELGs. (5) Introducing WLGs in the census of galaxies in the local Universe increases the proportion of metal-rich SF galaxies and especially LINERs.
In the course of this analysis, we were led to make the following points. (i) The Kewley et al. BPT line for galaxy classification is generally ill-used. (ii) Replacing [O iii]/Hβ by WHα in the classification introduces a change in the philosophy of the distinction between LINERs and Seyferts, but not in its results. Because the WHα versus [N ii]/Hα diagram can be applied to the largest sample of ELGs without loss of discriminating power between Seyferts and LINERs, we recommend its use in further studies. (iii) The dichotomy between Seyferts and LINERs is washed out by WLGs in the BPT plane, but it subsists in other diagnostic diagrams. This suggests that the right wing in the BPT diagram is indeed populated by at least two classes, tentatively identified with bona fide AGN and ‘retired’ galaxies that have stopped forming stars and are ionized by their old stellar populations.
We present an overview of the Space Telescope A901/2 Galaxy Evolution Survey (STAGES). STAGES is a multiwavelength project designed to probe physical drivers of galaxy evolution across a wide range of environments and luminosity. A complex multicluster system at z ~ 0.165 has been the subject of an 80-orbit F606W Hubble Space Telescope (HST)/Advanced Camera for Surveys (ACS) mosaic covering the full span of the supercluster. Extensive multiwavelength observations with XMM-Newton, GALEX, Spitzer, 2dF, Giant Metrewave Radio Telescope and the 17-band COMBO-17 photometric redshift survey complement the HST imaging. Our survey goals include simultaneously linking galaxy morphology with other observables such as age, star formation rate, nuclear activity and stellar mass. In addition, with the multiwavelength data set and new high-resolution mass maps from gravitational lensing, we are able to disentangle the large-scale structure of the system. By examining all aspects of an environment we will be able to evaluate the relative importance of the dark matter haloes, the local galaxy density and the hot X-ray gas in driving galaxy transformation. This paper describes the HST imaging, data reduction and creation of a master catalogue. We perform the Sérsic fitting on the HST images and conduct associated simulations to quantify completeness. In addition, we present the COMBO-17 photometric redshift catalogue and estimates of stellar masses and star formation rates for this field. We define galaxy and cluster sample selection criteria, which will be the basis for forthcoming science analyses, and present a compilation of notable objects in the field. Finally, we describe the further multiwavelength observations and announce public access to the data and catalogues.
Observations of star formation rates (SFRs) in galaxies provide vital clues
to the physical nature of the Hubble sequence, and are key probes of the
evolutionary properties of galaxies. The focus of this review is on the broad
patterns in the star formation properties of galaxies along the Hubble
sequence, and their implications for understanding galaxy evolution and the
physical processes that drive the evolution. Star formation in the disks and
nuclear regions of galaxies are reviewed separately, then discussed within a
common interpretive framework. The diagnostic methods used to measure SFRs are
also reviewed, and a self-consistent set of SFR calibrations is presented as an
aid to workers in the field.
We use a complete sample of galaxies drawn from the SDSS to study how structure, star formation and nuclear activity depend on local density and on stellar mass. Local density is estimated by counting galaxies above a fixed absolute magnitude limit within cylinders 2 Mpc in projected radius and +-500 km/s in depth. The stellar mass distribution of galaxies shifts by nearly a factor of two towards higher masses between low and high density regions. At fixed stellar mass, both star formation and nuclear activity depend strongly on local density, while structural parameters such as size and concentration are almost independent of it. The galaxy property most sensitive to environment is specific star formation rate. For galaxies with stellar masses less than 3 x 10^10 M_sun, the median SFR/M* decreases by more than a factor of 10 from low to high densities. This decrease is less marked for massive galaxies. At fixed stellar mass, twice as many galaxies host AGN with strong [OIII] emission in low density regions as in high. Massive galaxies in low-density environments also contain more dust. We have analyzed correlations between spectroscopic indicators that probe SFH on different timescales (D4000, Hdelta_A and SFR/M*). The correlations do not depend on environment, suggesting that the decrease in star formation has ocurred over long (>1 Gyr) timescales. Since structure does not depend on environment for more massive galaxies, trends in recent SFH, dust and AGN for these systems cannot be driven by processes that alter structure, e.g. mergers. The SFH-density correlation is strongest for small scale (< 1 Mpc) estimates of local density. Finally, we highlight a striking similarity between changes in the galaxy population as a function of density and as a function of redshift and we interpret this using N-body simulations.
Some previous investigations have found that the fraction (f_AGN) of active galactic nuclei (AGNs) is lower in clusters than in the field. This can result from the suppression of galaxy-galaxy mergers in high-velocity dispersion (sigma_v) clusters, if the formation and/or fueling of AGNs is directly related to the merging process. We investigate the existence of a relation between f_AGN and sigma_v in galaxy clusters in order to shed light on the formation and evolution processes of AGNs and cluster galaxies. Using data from the Sloan Digital Sky Survey we determine f_AGN and sigma_v for the clusters in two samples, extracted from the catalogs of Popesso et al. (2006a) and Miller et al. (2005), and excluding clusters with significant evidence for substructures. We find a significant f_AGN-sigma_v anti-correlation. Clusters with sigma_v lower and, respectively, higher than 500 km/s have AGN fractions of $0.21 \pm 0.01$ and $0.15 \pm 0.01$, on average. The f_AGN-sigma_v relation can be described by a model that assumes f_AGN is proportional to the galaxies merging rate, plus a constant. Since f_AGN increases with decreasing sigma_v, AGNs are likely to have played a significant role in heating the intra-cluster medium and driving galaxy evolution in cluster precursors and groups. Comment: 4 pages, 3 figures, A&A Letter accepted for publication
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