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Turbulent mixing layers in the interstellar medium of galaxies
Abstract
It is proposed that turbulent mixing layers are common in the ISM of the Milky Way and selected external galaxies, with many layers per kiloparsec along typical lines of sight. All of the diffuse C I 1550-A background emission and a significant fraction of the diffuse H-alpha background at high latitude can be explained by mixing layers cooling at pressure of about 3000/cu cm K. These models also produce the correct ratio of semiforbidden C IV 15500 III 1663-A emission. Only 10 percent of the disk H-alpha is likely to arise from mixing layers. The observed Galactic absorption-line column densities of C IV, N V, Si IV, and O VI are roughly consistent with mixing-layer models with an intermediate temperature of about 10 exp 5.3 K and depleted abundances.
... Cowie & McKee 1977), cooling-condensation fronts (Shapiro & Benjamin 1991), and turbulent mixing layers (TML, e.g. Begelman & Fabian 1990;Slavin et al. 1993;Rand 1998;Collins & Rand 2001;Binette et al. 2009). Slavin et al. (1993) proposed that selfphotoionisation through EUV line emission of collisionally excited gas in mixing layers (with a temperature ∼ 10 5.0−5.5 K) between hot (∼ 10 6 K) and cold (∼ 100 K) gas can reproduce many optical/UV/EUV emission line ratios as observed in the DIG of several galaxies. ...
... Begelman & Fabian 1990;Slavin et al. 1993;Rand 1998;Collins & Rand 2001;Binette et al. 2009). Slavin et al. (1993) proposed that selfphotoionisation through EUV line emission of collisionally excited gas in mixing layers (with a temperature ∼ 10 5.0−5.5 K) between hot (∼ 10 6 K) and cold (∼ 100 K) gas can reproduce many optical/UV/EUV emission line ratios as observed in the DIG of several galaxies. However, theoretical models for TML are intricate and may lead to over-ionisation of the hot gas and Fig. 6: Showcase of the star formation + fast shocks hybrid models for the [N II] BPT. ...
... In an analogous way to TML, fast shocks induce comparable ionised conditions, characterised by an ionisation equilibrium between the hot and cold gas phases and slow mixing within the turbulent layer (Slavin et al. 1993;Rossa & Dettmar 2003b;Boettcher et al. 2019;Dirks et al. 2023). Given that shock heating is significant only if the kinetic energy is efficiently thermalised, both photoionisation and shocks can be regarded as "thermal" heating sources of the ISM. ...
The extraplanar diffuse ionised gas is a key component for understanding the feedback processes that connect galactic discs and their halos. In this paper, we present the second study of the BETIS project, which aims to explore the ionisation mechanisms of the eDIG. We use a sample of eight edge-on galaxies observed with MUSE and apply the methodology developed in the first paper of the BETIS project. We found that the vertical and radial profiles of the [NII]/Ha, [SII]/Ha, [OIII]/Hb, and [OI]/Ha ratios depict a complex ionisation structure within galactic halos, influenced by the spatial distribution of HII regions across the galactic plane as observed from our line of sigh, with photon leakage from OB associations constituting the main ionisation source. Our analysis excludes low-mass, hot, and evolved stars as viable candidates for secondary ionisation sources to elucidate the unusual behaviour of the line ratios at greater distances from the galactic midplane. In contrast, we ascertain that shocks induced in the interstellar medium by star formation related feedback mechanisms represent a promising secondary ionisation source of the eDIG. We present a suite of models integrating ionisation mechanisms arising from fast shocks and photoionisation associated with star formation. When applied to the classical BPT diagrams, these models reveal that the ionisation budget of the eDIG ranges from 20% to 50% across our sample, with local variations of up to 20% within individual galaxy halos. This correlates with the presence of filaments and other structural components observed within galaxy halos. The presence of shocks is additionally supported by the observation of high-density, high [OI]/Ha ratios, characteristic of shock-compressed ionised gas, likely induced by feedback from regions of intense SF within the disk. These results are consistent across all galaxies analysed in this sample.
... By comparing with models in Slavin et al. (1993), we estimate that the temperature is higher than 10 5.3 K if the O VI lines are emitted by a turbulent mixing layer. We also compare the brightness ratio with the shock models in the Mexican Million Models database (shock part; 3MdB s ), specifically the model results from Allen et al. (2008) and Alarie & Morisset (2019). ...
We examined archival Far Ultraviolet Spectroscopic Explorer data to search for far-ultraviolet emission lines in the starburst galaxy M82. The observations were made in an outflow region that extends beyond the galactic disk. We found the O VI $\lambda\lambda$ 1032, 1038 emission lines from the galaxy's southern outflow region. The O VI lines suggest that the outflowing warm-hot gas is undergoing radiative cooling. We measured a radial velocity of $\sim$420 km s$^{-1}$ from the O VI lines, which is faster than the velocity seen in H$\alpha$ observations. The O VI $\lambda$1038 emission line seems to be blended with the C II $\lambda$1037 emission line, which has a radial velocity of $\sim$300 km s$^{-1}$, similar to what is observed in H$\alpha$ observations. The outflow medium of M82 appears to be composed of gas in multiple phases with varying temperatures and kinematics. Future spectroscopic observations in high energy regimes covering a wider spatial area are necessary to understand better the properties of the warm-hot gas medium in the outflow.
... These layers are at high enough temperatures to produce the highly ionized species we analyze here. Slavin et al. (1993) expanded upon Begelman & Fabian (1990) to produce a model of TMLs over a range of temperatures and gas velocities. These models, which we show in purple in Figure 3, correspond to entrainment velocities between 25 and 100 km s −1 as well as temperatures between 10 5.0 and 10 5.5 K. ...
The intergalactic medium (IGM) contains >50% of the baryonic mass of the Universe, yet the mechanisms responsible for keeping the IGM ionized have not been fully explained. Hence, we investigate ion abundances from the largest blind QSO absorption catalog for clouds that show C iv , N v , and O vi simultaneously. The wavelength range of present UV spectrographs, however, makes it possible to probe C iv and O vi only over a small range of redshift ( z ≈ 0.12–0.15). As a result, we only have five IGM absorbing clouds, yet these provide a powerful and representative tool to probe the IGM ionization state. We found one cloud to be in collisional ionization equilibrium while three of the five showed signs of being produced by nonequilibrium processes, specifically conductive interfaces and turbulent mixing layers. None of the models we explore here were able to reproduce the ionization state of the remaining system. Energetic processes, such as galactic feedback from star formation and active galactic nucleus winds, would be excellent candidates that can cause such widespread ionization.
We examined archival Far Ultraviolet Spectroscopic Explorer data to search for far-ultraviolet emission lines in the starburst galaxy M82. The observations were made in an outflow region that extends beyond the galactic disk. We found the O vi λ λ 1032, 1038 emission lines from the galaxy’s southern outflow region. The O vi lines suggest that the outflowing warm-hot gas is undergoing radiative cooling. We measured a radial velocity of ∼420 km s ⁻¹ from the O vi lines, which is faster than the velocity seen in H α observations. The O vi λ 1038 emission line seems to be blended with the C ii λ 1037 emission line, which has a radial velocity of ∼300 km s ⁻¹ , similar to what is observed in H α observations. The outflow medium of M82 appears to be composed of gas in multiple phases with varying temperatures and kinematics. Future spectroscopic observations in high energy regimes covering a wider spatial area are necessary to understand better the properties of the warm-hot gas medium in the outflow.
Winds from massive stars expand supersonically into their surroundings, creating dynamic and fascinating nebulae that can give us insight into physical processes in interstellar plasma, and into the evolutionary history of the stars. Around single stars, parsec-scale bubbles such as bow shocks and ring nebulae are formed, whereas in colliding-wind binary (CWB) systems the high wind density produces intense time- and space-dependent emission across the electromagnetic spectrum from radio to gamma-rays. This contribution summarizes some recent results from 3D MHD modelling of bow shocks around runaway stars such as ζ Oph, and of the wind-collision zone of the CWB systems WR140 and WR21a. A resolution study of 3D simulations of bow shocks shows that X-ray emission from the shocked wind is time-variable and that converged results can be obtained once the Kelvin-Helmholtz instability at the contact discontinuity is resolved. Simulations of the CWB system WR140 show that inverse-Compton cooling of the shocked plasma can trigger runaway cooling when the orbit is near periastron, producing strong compression and dynamical instabilities. This sharply reduces the hard-X-ray emission around periastron, in agreement with observations. Scaling tests of the simulation software pion are also presented for a model of the CWB system WR21a run on up to 8192 cores using the HPC system Karolina .
Мы исследуем источники ионизации диффузного газа на различных галактических высотах в различных по звездной массе, светимости в H \(\alpha\) и удельному темпу звездообразования галактиках. Для этого мы привлекаем данные релиза DR16 обзора SDSS-IV MaNGA и теоретические модели фотоионизации и ударной ионизации базы данных 3MdB. Наша итоговая выборка содержит 239 галактик, наблюдаемых точно с ребра, что делает результаты статистически значимыми и позволяет с помощью процедуры сложения спектров исследовать даже большие галактические высоты. С помощью диагностических диаграмм мы показываем, что для галактик всех исследуемых типов поведение диффузного ионизованного газа адекватно описывается моделями фотоионизации молодыми OB-звездами и горячими проэволюционировавшими маломассивными звездами. Однако в галактиках с большими звездными массами или с пассивным звездообразованием ударные волны также могут вносить свой вклад в ионизацию. Для галактик всех исследуемых типов мы получаем, что поток излучения от OB-звезд и ионизационный параметр с высотой уменьшаются, а относительный вклад горячих проэволюционировавших маломассивных звезд в ионизацию увеличивается. При этом наибольшая разница вклада данных источников в ионизацию газовой среды наблюдается между галактиками с различными удельными темпами звездообразования и с различными звездными массами: проэволюционировавшие маломассивные звезды являются основным источником ионизации газа в галактиках с пассивным звездообразованием (и с большими звездными массами), тогда как в галактиках с активным звездообразованием (и с меньшими звездными массами) OB-звезды являются определяющим фактором ионизации диффузной газовой среды.
We present the Bidimensional Exploration of the warm-Temperature Ionised gaS (BETIS) project, designed for the spatial and spectral study of the diffuse ionised gas (DIG) in a selection of nearby spiral galaxies observed with the MUSE integral-field spectrograph. Our primary objective is to investigate the various ionisation mechanisms at play within the DIG. We analysed the distribution of high- and low-ionisation species in the optical spectra of the sample on a spatially resolved basis. We introduced a new methodology for spectroscopically defining the DIG, optimised for galaxies of different resolutions. Firstly, we employed an innovative adaptive binning technique on the observed datacube based on the spectroscopic signal-to-noise ratio (S/N) of the collisional line to increase the S/N of the rest of the lines including and Subsequently, we created a DIG mask by eliminating the emissions associated with both bright and faint regions. We also examined the suitability of using equivalent width ( as a proxy for defining the DIG and its associated ionisation regime. Notably, for A -- the expected emission from hot low-mass evolved stars (HOLMES) – the measured value is contingent on the chosen population synthesis technique performed. Our analysis of the showcase sample reveals a consistent cumulative DIG fraction across all galaxies in the sample, averaging around 40 -70 . The average radial distribution of the and ratios are enhanced in the DIG regimes (up to 0.2 dex). It follows similar trends between the DIG regime and the regions, as well as the surface brightness ( indicating a correlation between the ionisation of these species in both the DIG and the regions. The DIG loci in typical diagnostic diagrams are found, in general, within the line ratios that correspond to photoionisation due to the star formation. There is a noticeable offset correspondent to ionisation due to fast shocks. However, an individual diagnosis performed for each galaxy reveals that all the DIG in these galaxies can be attributed to photoionisation from star formation. The offset is primarily due to the contribution of Seyfert galaxies in our sample, which is closely aligned with models of ionisation from fast shocks and galactic outflows, thus mimicking the DIG emission. Our results indicate that galaxies exhibiting active galactic nucleus (AGN) activity should be considered separately when conducting a general analysis of the DIG ionisation mechanisms, since this emission is indistinguishable from high-excitation DIG.
Cold streams of gas with temperatures around 104 K play a crucial role in the gas accretion on to high-redshift galaxies. The current resolution of cosmological simulations is insufficient to fully capture the stability and Lyα emission characteristics of cold stream accretion, underscoring the imperative need for conducting idealized high-resolution simulations. We investigate the impact of magnetic fields at various angles and anisotropic thermal conduction (TC) on the dynamics of radiatively cooling streams through a comprehensive suite of two-dimensional high-resolution simulations. An initially small magnetic field ($\sim 10^{-3} \, \mu \rm G$), oriented non-parallel to the stream, can grow significantly, providing stability against Kelvin-Helmholtz instabilities and reducing the Lyα emission by a factor of <20 compared to the hydrodynamics case. With TC, the stream evolution can be categorised into three regimes: (1) the Diffusing Stream regime, where the stream diffuses into the surrounding hot circumgalactic medium; (2) the Intermediate regime, where TC diffuses the mixing layer, resulting in enhanced stabilization and reduced emissions; (3) the Condensing Stream regime, where the impact of magnetic field and TC on the stream’s emission and evolution becomes negligible. Extrapolating our findings to the cosmological context suggests that cold streams with a radius of $\le 1 \rm \, {\rm kpc}$ may fuel galaxies with cold, metal-enriched, magnetized gas ($B \sim 0.1\!-\!1 \, \mu \rm G$) for a longer time, leading to a broad range of Lyα luminosity signatures of $\sim 10^{37}\!-\!10^{41}\, \rm \, erg \, s^{-1}$.
Both multiphase gas and magnetic fields are ubiquitous in astrophysics. However, the influence of magnetic fields on mixing of the different phases is still largely unexplored. In this study, we use both turbulent radiative mixing layer (TRML) and turbulent box simulations to examine the effects of magnetic fields on cold gas growth rates, survival, and the morphology of the multiphase gas. Our findings indicate that, in general, magnetic fields suppress mixing in TRMLs while turbulent box simulations show comparatively marginal differences in growth rates and survival of the cold gas. We reconcile these two seemingly contrasting results by demonstrating that similar turbulent properties result in comparable mixing – regardless of the presence or absence of magnetic fields. We, furthermore, find the cold gas clump size distribution to be independent of the magnetic fields but the clumps are more filamentary in the MHD case. Synthetic MgII absorption lines support this picture being marginally different with and without magnetic fields; both cases aligning well with observations. We also examine the magnetic field strength and structure in turbulent boxes. We generally observe a higher mean magnetic field in the cold gas phase due to flux freezing and reveal fractal-like magnetic field lines in a turbulent environment.
This book deals with the nature of interstellar matter, treating both
the rarefied hydrogen and helium gas pervading the universe and the
heavier atoms present in both the gas and dust grains. Observational
results on all phases of the interstellar medium are summarized, various
local phenomena occurring in the gas and in dust grains are discussed,
and elastic collisions are studied in detail. Those interactions of the
gas with radiation that can be understood without any detailed analysis
of excitation conditions are examined and then treated on the basis of
the steady-state equation, which is also used to analyze ionization,
dissociation, and gas heating. The optical properties of grains and
corresponding results for grain radii, composition, and mean density are
presented; results of optical polarization studies and an alignment
analysis are also reported. The theory of such grain properties as
electric charge, temperature, radiative acceleration, and likely
evolution is considered. The overall equilibrium and dynamics of the
interstellar gas are investigated, including explosions of H II regions
and supernova shells as well as processes of star formation.
The evolution of plane-parallel magnetized thermal conduction fronts in the interstellar medium (ISM) was studied. Separating the coronal ISM phase and interstellar clouds, these fronts have been thought to be the site of the intermediate-temperature regions whose presence was inferred from O VI absorption-line studies. The front evolution was followed numerically, starting from the initial discontinuous temperature distribution between the hot and cold medium, and ending in the final cooling stage of the hot medium. It was found that, for the typical ISM pressure of 4000 K/cu cm and the hot medium temperature of 10 to the 6th K, the transition from evaporation to condensation in a nonmagnetized front occurs when the front thickness is 15 pc. This thickness is a factor of 5 smaller than previously estimated. The O VI column densities in both evaporative and condensation stages agree with observations if the initial hot medium temperature Th exceeds 750,000 K. Condensing conduction fronts give better agreement with observed O VI line profiles because of lower gas temperatures. 66 refs.
Additional evidence is presented for the existence of an extended (about 10 kpc; about 135 arcsec), diffuse ionized medium (DIM) recently detected in the nearby, luminous Seyfert galaxy NGC 1068. The DIM phase was recognized from spectrophotometric H-alpha and N II forbidden-line 6548, 6584 line emission maps obtained with the Hawaii Imaging Fabry-Perot Interferometer (HIFI). It is demonstrated that the H-alpha equivalent-width distribution, when combined with the N II forbidden-line 6584/H-alpha line ratio distribution, permits a clean separation of the DIM from the adjacent H II region population. The main reasons that lead to a belief that the DIM is photoexcited predominantly by scattered light from the active nucleus rather than the extended stellar population are summarized.
The maintenance of the diffuse, ionized gas responsible for the pulsar dispersion measures requires at least 4 x 10 to the -5th ergs/s to 4 x 10 to the -3rd ergs/s per sq cm of Galactic disk, depending upon its electron temperature. Approximately half of the power is for gas at absolute value of z greater than 500 pc. The minimum power requirements are comparable to the total power injected into the diffuse interstellar medium by luminous stars and supernovae, which implies that most of the ionized gas, even at high absolute value of z, is at a temperature near 10,000 K. This gas accounts for at least one-third of the Galactic H-alpha background and must have a volume filling fraction f greater that about 0.2. 21 refs.
Analytic solutions are obtained for the rate of evaporative mass loss
from an isolated spherical cloud embedded in a hot tenuous gas,
neglecting effects of radiation and magnetic fields. Consideration is
given only to the case of spherical clouds, where the evaporative
mass-loss rate is an eigenvalue of the time-independent
energy-conservation equation. The upper bound on the heat flux which may
be carried by electrons is found to be smaller than previously expected,
it is assumed that the exact heat flux should closely approximate this
maximum value in collisionless gases, and such a theory is shown to give
good numerical agreement with solar-wind observations. A straightforward
solution is obtained for those cases where the classical
thermal-conduction formula is applicable throughout the interface, and
limitations are given under which analytic solutions can be found when
the classical formula does not apply throughout the interface. The
results of the present theory are compared with those of a previous and
less accurate analysis which considered conducting fronts analogous to
ionization fronts. Cloud evaporation within supernova remnants is
discussed, and it is concluded that the present results should provide a
reasonable approximation even in such an extreme case.
A chimney model for the Galaxy is presented and its underlying physical
principles are explained. The crucial point of the model is that the
Galactic disk and halo are connected by chimneys which are a consequence
of superbubbles bursting out of the disk forming these collimated
structures through which the global mass and energy exchange flows from
disk to halo. For canonical Galaxy parameters, the chimney phase is
found to be associated with a mass flow rate of 0.3-3 solar masses/yr
and a global power input of 10 to the 40th to 10 to the 42nd ergs/s.
The cloud-intercloud phase transition of interstellar gas in pressure
equilibrium is investigated, including effects of metal abundance
variations, grain heating, and X-ray fluxes from the Galactic halo.
Equilibrium solutions for the gas pressure P and hydrogen density nH
obey fairly universal scaling laws, plotted as (PD/zeta) versus
(nHD/zeta), where zeta is the cosmic-ray ionization rate and D is the
metal-abundance enhancement factor. Both grain and X-ray heating enhance
the susceptibility of intercloud gas to thermal instability, possibly
resulting in the conversion of intercloud gas to clouds in the inner
disks of spiral galaxies. X-ray ionization may be responsible for a
class of C IV - Si IV absorption lines seen above the Galactic disk and
in quasar absorption systems.
A search for the forbidden O I emission line from the diffuse
interstellar medium was carried out with the Wisconsin 15 cm dual etalon
Fabry-Perot interferometer. The scans place an upper limit of 0.02 on
the forbidden O I 6300 A/H-alpha line intensity which appears to be
inconsistent with the standard McKee and Ostriker (1977) photoionization
model of the gas if the temperature is as high as 8000 K. These
observations indicate that either the electron temperature is near
5500-6000 K or the origin and distribution of the warm, ionized gas is
very different from that in the McKee and Ostriker model.
The radiative recombination of hydrogenic ions is calculated in the
limit of low density by the 'nl-method', extending the results of
Pengelly (1964) to much lower temperatures (both case A and case B). A
strong motivation is the detection of the emission-line spectrum of the
old shell of Nova DQ Her 1934 reported by Williams et al. (1978), which
revealed an electron temperature T(e) of about 500 K. The results are
compared with the 1959 result of Seaton using the 'n-method', with those
of Hummer and Storey (1987), which allow for the effects of collisions
at finite density, and with the data for three cold old nova shells.
Another motivation is the detection of relatively strong optical
radiative recombination lines of C, N, and O in these shells. The
abundances of these elements (recombining ions of charge Z) can be
derived using a hydrogenic approximation to the effective radiative
recombination coefficients, requiring the recombination coefficients of
H(+) at even lower temperatures.
The observational evidence for chimney models of the interstellar medium
is reviewed. The variation of the state of the interstellar medium of an
external galaxy between the three-phase mode, the chimney mode and the
two-phase mode as a function of: galaxy type, galactocentric radius
within is given galaxy, and the time varying star formation rate is
discussed. In the context of the chimney models it is shown how the
photoionization of the halo can be achieved by utilizing both direct
ionizing rays from the central OB association and diffuse reradiation
from the chimney walls that will create an extended low ionization halo
around chimneys with SII/H-alpha about 0.5.