Interstellar Medium Of Galaxies Research Articles

Star formation across cosmic time

The interstellar medium of galaxies is composed of multiple phases, including molecular, atomic, and ionized gas, as well as dust. Stars are formed within this medium from cold molecular gas clouds, which collapse due to their gravitational attraction. Throughout their life, stars emit strong radiation fields and stellar winds, and they can also explode as supernovae at the end of their life. These processes contribute to stirring the turbulent interstellar medium and regulate star formation by heating up, ionizing, and expelling part of the gas. However, star formation does not proceed uniformly throughout the history of the Universe and decrease by an order of magnitude in the last ten billion years. To understand this winding-down of star formation and assess possible variations in the efficiency of star formation, it is crucial to probe the molecular gas reservoirs from which stars are formed. In this article following my presentation at the 10th International Conference on Frontiers of Plasma Physics and Technology held in Kathmandu from 13–17 March 2023, I review some aspects of the multiphase interstellar medium and star formation, with an emphasis on the interplay between neutral and ionized phases, and present recent and ongoing observations of the molecular gas content in typical star-forming galaxies across cosmic time and in different environments. I also present some of our understanding of star-forming galaxies from theoretical models and simulations.

SALT-RSS Multiobject Spectroscopic Data Reduction to Reveal Physical Properties of [O ii]-emitting Galaxies from HETDEX and ODIN

Line flux ratios from [O ii] doublets can probe electron densities in the interstellar medium of galaxies. We employ the Southern African Large Telescope’s (SALT) Robert Stobie Spectrograph (RSS), which provides sufficient resolution (R ∼ 3000) to split the [O ii] doublets, to target galaxies from Hobby-Eberly Telescope Dark Energy Experiment and One-hundred-deg2 DECam Imaging in Narrowbands with emission line fluxes of at least 2 × 10−16 erg cm−2 s−1. Reduction is carried out using RSSMOSPipeline to reduce SALT-RSS data through wavelength calibration. Despite SALT-RSS being known for its difficulty to flux calibrate, we present spectra that have been flux calibrated using alignment stars with Sloan Digital Sky Survey spectra as standards. We combine multiple spectroscopic settings to obtain full 2D spectra across a wavelength range of 3500–9500 Å. A 1D spectrum can then be extracted to calculate flux ratios and line widths, revealing important physical properties of these bright [O ii]-emitters.

The α-element enrichment of gas in distant galaxies

Context. The chemical evolution of distant galaxies cannot be assessed from observations of individual stars, in contrast to the case of nearby galaxies. On the other hand, the study of the interstellar medium (ISM) offers an alternative way to reveal important properties of the chemical evolution of distant galaxies. The chemical enrichment of the ISM is produced by all the previous generations of stars and it is possible to precisely determine the metal abundances in the neutral ISM in galaxies. The chemical abundance patterns in the neutral ISM are determined by the gas metallicity, presence of dust (the depletion of metals into dust grains), and possible deviations due to specific nucleosynthesis, for example, α-element enhancements. Aims. We aim to derive the metallicities, dust depletion, and α-element enhancements in the neutral ISM of gas-rich mostly-metal-poor distant galaxies (Damped Lyman-α absorbers, DLAs). Furthermore, we aim to constrain the distribution of α-element enhancements with metallicity in these galaxies. Methods. We collected a literature sample of column density measurements of O, Mg, Si, S, Ti, Cr, Fe, Ni, Zn, P, and Mn in the neutral ISM of DLAs at redshifts of 0.60 < z < 3.40. We used this sample to define a golden sample of DLAs with constrained observations of Ti and at least one other α-element. By studying the abundance patterns, we determined the amount of dust depletion, solely based on the observed relative abundances of the α-elements. We then used the abundances of Fe-peak elements to determine the overall metallicity of each system, after correcting for dust depletion. In addition, we studied the deviations from the basic (linear) abundance patterns. We divided our sample into two groups of galaxies based on the widths of their absorption lines (Δv90 above or below 100 km s−1), which may be considered as a proxy for their dynamical mass. We characterised the distribution of the α-element enhancements as a function of metallicity for the galaxy population as a whole, by fitting a piecewise function (plateau, decline, plateau) to the data. Results. We observed systematic deviations from the basic abundance patterns for O, Mg, Si, S, Ti, and Mn, which we interpreted as α-element enhancements and a Mn underabundance. The distribution of the α-element enhancements with metallicity is different in the high-Δv90 and low-Δv90 groups of galaxies. We constrained the metallicity of the α-element knee for the high-Δv90 and low-Δv90 groups of galaxies to be −1.02±0.15 dex and −1.84±0.11 dex, respectively. The average α-element enhancement at the high-plateau is [α/Fe]=0.38±0.07 dex. On the other hand, Mn shows an underabundance in all DLAs in the golden sample of −0.36±0.07 dex, on average. Conclusions. We have constrained, for the first time, the distribution of the α-element enhancement with metallicity in the neutral ISM in distant galaxies. Less massive galaxies show an α-element knee at lower metallicities than more massive galaxies. This can be explained by a lower star formation rate in less massive galaxies. If this collective behaviour can be interpreted in the same way as it is for individual systems, this would suggest that more massive and metal-rich systems evolve to higher metallicities before the contribution of SN-Ia to [α/Fe] levels out that of core-collapse SNe. This finding may plausibly be supported by different SFRs in galaxies of different masses. Overall, our results offer important clues to the study of chemical evolution in distant galaxies.

The Most Distant H i Galaxies Discovered by the 500 m Dish FAST

Neutral hydrogen (H i) is the primary component of the cool interstellar medium (ISM) and is the reservoir of fuel for star formation. Owing to the sensitivity of existing radio telescopes, our understanding of the evolution of the ISM in galaxies remains limited, as it is based on only a few hundred galaxies detected in H i beyond the local Universe. With the high sensitivity of the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we carried out a blind H i search, the FAST Ultra-Deep Survey, which extends to redshifts up to 0.42 and a sensitivity of 50 μJy beam−1. Here, we report the first discovery of six galaxies in H i at z > 0.38. For these galaxies, the FAST angular resolution of ∼4′ corresponds to a mean linear size of ∼1.3h70−1 Mpc. These galaxies are among the most distant H i emission detections known, with one having the most massive H i content ( 1010.93±0.04h70−2M⊙ ). Using recent data from the DESI survey and new observations with the Hale, Big Telescope Alt-azimuth, and Keck telescopes, optical counterparts are detected for all galaxies within the 3σ positional uncertainty ( 0.5h70−1 Mpc) and 200 km s−1 in recession velocity. Assuming that the dominant source of H i is the identified optical counterpart, we find evidence of evolution in the H i content of galaxies over the last 4.2 Gyr. Our new high-redshift H i galaxy sample provides the opportunity to better investigate the evolution of cool gas in galaxies. A larger sample size in the future will allow us to refine our knowledge of the formation and evolution of galaxies.

Temperature stratification in a molecular shock: Analysis of the emission of H2 pure rotational lines in IC443G

Context. Supernovae remnants (SNRs) represent a major source of feedback from stars on the interstellar medium of galaxies. During the latest stage of supernova explosions (which lasts 10–100 kyr), shock waves produced by the initial blast modify the chemistry of gas and dust, inject kinetic energy in the surroundings, and may alter star formation characteristics. Simultaneously, γ-ray emission is generated by the interaction between the ambient medium and cosmic rays, in particular those locally accelerated in the early stages of the explosion. Aims. We aim to estimate the total molecular mass, local density, and total column density of H2 and the temperature structure in a shocked clump interacting with the supernova remnant IC443 located in a region where cosmic rays interact with the interstellar medium. Measuring the mass of the dense and neutral component of the medium is a prerequisite to understanding the chemistry, energetics, and GeV to TeV γ-ray emission. Methods. Assuming that the emission of H2 pure rotational lines is produced by a collection of gas layers with variable temperature, we compared Spitzer/IRS emission maps for the ν = 0–0 S(0) to S(7) lines with a thermal admixture model. Our description is based on a power-law distribution of thermalized components with temperatures varying between Tmin = 25 K and Tmax = 1500 K. Results. Our thermal admixture model allows the level populations of H2 to be described by a power-law distribution dN = ΛT−ΓdT, with Γ ~ 2.2−4.7. We measured a total mass MH2 = 220−80+110 M⊙ across the Spitzer/IRS field of observations. Conclusions. Our analysis shows that an estimate of the cold molecular gas temperature is paramount to accurately constraining the H2 mass, although the mass remains affected by significant uncertainties due to the assumptions on the gas temperature distribution.

Multiphase Gas in Elliptical Galaxies: The Role of Type Ia Supernovae

Massive elliptical galaxies harbor large amounts of hot gas (T ≳ 106 K) in their interstellar medium (ISM) but are typically quiescent in star formation. The jets of active galactic nuclei (AGNs) and Type Ia supernovae (SNe Ia) inject energy into the ISM, which offsets its radiative losses and keeps it hot. SNe Ia deposit their energy locally within the galaxy compared to the larger few ×10 kiloparsec-scale AGN jets. In this study, we perform high-resolution (5123) hydrodynamic simulations of a local (1 kpc3) density-stratified patch of the ISM of massive galaxies. We include radiative cooling and shell-averaged volume heating, as well as randomly exploding SN Ia. We study the effect of different fractions of supernova (SN) heating (with respect to the net cooling rate), different initial ISM density/entropy (which controls the growth time t ti of the thermal instability), and different degrees of stratification (which affect the freefall time t ff). We find that SNe Ia drive predominantly compressive turbulence in the ISM with a velocity dispersion of σ v up to 40 km s−1 and logarithmic density dispersion of σ s ∼ 0.2–0.4. These fluctuations trigger multiphase condensation in regions of the ISM, where min(tti)/tff≲0.6exp(6σs) , in agreement with theoretical expectations that large density fluctuations efficiently trigger multiphase gas formation. Since the SN Ia rate is not self-adjusting, when the net cooling drops below the net heating rate, SNe Ia drive a hot wind which sweeps out most of the mass in our local model. Global simulations are required to assess the ultimate fate of this gas.

PDRs4All. VII. The 3.3 μm aromatic infrared band as a tracer of physical properties of the interstellar medium in galaxies

Aromatic infrared bands (AIBs) are a set of broad emission bands at 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 mu m, seen in the infrared spectra of most galaxies. With the James Webb space telescope (JWST), the 3.3 mu m AIB can in principle be detected up to a redshift of $ 7$. Relating the evolution of the 3.3 mu m AIB to local physical properties of the interstellar medium (ISM) is thus of paramount importance. By applying a dedicated machine learning algorithm to JWST NIRSpec observations of the Orion Bar photodissociation region obtained as part of the PDRs4All Early Release Science (ERS) program, we extracted two template spectra capturing the evolution of the AIB-related emission in the 3.2-3.6 range, which includes the AIB at 3.3 and its main satellite band at 3.4 um. In the Orion Bar we analyzed the spatial distribution of the templates and their relationship with the ro-vibrational H$_2$ line at 2.12 the pure rotational line of H$_2$ at 4.69 and the Pfund delta line at 3.29 um. We find that one template (AIB$_ Irrad $) traces regions of neutral atomic gas with strong far-UV fields, while the other template (AIB$_ Shielded $) corresponds to shielded regions with lower FUV fields and a higher molecular gas fraction. We then show that these two templates can be used to fit the NIRSpec AIB-related spectra of nearby galaxies. The relative weight of the two templates (AIB$_ Irrad/Shielded $) is a tracer of the radiative feedback from massive stars on the ISM. We derive an estimate of AIB$_ Irrad/Shielded $ in a $z=4.22$ lensed galaxy and find that it has a lower value than for local galaxies. This pilot study illustrates how a detailed analysis of AIB emission in nearby regions can be used to probe the physical conditions of the extragalactic ISM.

Disentangling Multiple Emitting Components in Molecular Observations with Nonnegative Matrix Factorization

Molecular emission from the galactic and extragalactic interstellar medium (ISM) is often used to determine the physical conditions of the dense gas. However, even from spatially resolved regions, the observed molecules do not necessarily arise from a single component. Disentangling multiple gas components is often a degenerate problem in radiative transfer studies. In this paper, we investigate the use of the nonnegative matrix factorization (NMF) approach as a means to recover gas components from a set of blended line intensity maps of molecular transitions that may trace different physical conditions. We run a series of experiments on synthetic data sets designed to replicate conditions in two very different environments: galactic pre-stellar cores and the ISM in high-redshift galaxies. We find that the NMF algorithm often recovers the multiple components resembling those used in the data-generating process, provided that the different components have similar column densities. When NMF fails to recover all the individual components it does however group together the most similarly emitting ones. We further found that initialization and regularisation are key factors in the efficiency of the NMF algorithm.

The dynamic centres of infrared-dark clouds and the formation of cores

ABSTRACT High-mass stars have an enormous influence on the evolution of the interstellar medium in galaxies, so it is important that we understand how they form. We examine the central clumps within a sample of seven infrared-dark clouds (IRDCs) with a range of masses and morphologies. We use 1-pc-scale observations from the Northern Extended Millimeter Array (NOEMA) and the IRAM 30m telescope to trace dense cores with 2.8-mm continuum, and gas kinematics in C18O, HCO+, HNC, and N2H+ (J = 1–0). We supplement our continuum sample with six IRDCs observed at 2.9 mm with the Atacama Large Millimeter/submillimeter Array (ALMA), and examine the relationships between core- and clump-scale properties. We have developed a fully automated multiple-velocity component hyperfine line-fitting code called mwydyn which we employ to trace the dense gas kinematics in N2H+ (1–0), revealing highly complex and dynamic clump interiors. We find that parsec-scale clump mass is the most important factor driving the evolution; more massive clumps are able to concentrate more mass into their most massive cores – with a log-normally distributed efficiency of around 9 per cent – in addition to containing the most dynamic gas. Distributions of linewidths within the most massive cores are similar to the ambient gas, suggesting that they are not dynamically decoupled, but are similarly chaotic. A number of studies have previously suggested that clumps are globally collapsing; in such a scenario, the observed kinematics of clump centres would be the direct result of gravity-driven mass inflows that become ever more complex as the clumps evolve, which in turn leads to the chaotic mass growth of their core populations.

Circumgalactic Enrichment in Local Dwarf Galaxies as Probed by Star-Formation Driven Outflows

Galactic feedback driven by massive stars and active galactic nuclei plays a fundamen-tal role in regulating galaxy formation and evolution. In particular, intense starburst episodes can generate strong outflows able to suppress star formation by expelling large amount of dust and metals out of the galaxies, possibly enriching their circum (or even inter) galactic media. Local dwarf galaxies are particularly sensitive to feedback processes and offer a unique opportunity to study these phenomena in great details. We search for outflow signatures in a sample of ∼ 30 lo-cal dwarf galaxies drawn from the Dwarf Galaxy Survey. We make use of Herschel/PACS archival data to detect atomic outflows in the broad wings of observed [CII] 158 µm line profiles. We find that atomic outflows in these galaxies have typical mass-loading factors consistent with unity and velocities high enough to bring gas and dust outward. Our results can be used as input for chemical models, posing new constraints on the processes of dust growth and destruction in the interstellar medium of galaxies.

Gauging the mass of metals in the gas phase of galaxies from the Local Universe to the Epoch of Reionization

The chemical enrichment of dust and metals are vital processes in constraining the star formation history of the universe. These are important ingredients in the formation and evolution of galaxies overall. Previously, the dust masses of high-redshift star-forming galaxies have been determined through their far-infrared continuum, however, equivalent, and potentially simpler, approaches to determining the metal masses have yet to be explored at z ≳ 2. Here, we present a new method of inferring the metal mass in the interstellar medium (ISM) of galaxies out to z ≈ 8, using the far-infrared [C II]−158 μm emission line as a proxy. We calibrated the [C II]-to-MZ, ISM conversion factor based on a benchmark observational sample at z ≈ 0, in addition to gamma-ray burst sightlines at z > 2 and cosmological hydrodynamical simulations of galaxies at z ≈ 0 and z ≈ 6. We found a universal scaling across redshifts of log(MZ, ISM/M⊙) = log(L[CII]/L⊙)−0.45, with a 0.4 dex scatter, which is constant over more than two orders of magnitude in metallicity. We applied this scaling to recent surveys for [C II] in galaxies at z ≳ 2 and compared their inferred MZ, ISM to their stellar mass (M⋆). In particular, we determined the fraction of metals retained in the gas-phase ISM, MZ, ISM/M⋆, as a function of redshift and we showed that an increasing fraction of metals reside in the ISM of galaxies at higher redshifts. We place further constraints on the cosmic metal mass density in the ISM (ΩZ, ISM) at z ≈ 5 and ≈7 based on recent estimates of the [C II]−158 μm luminosity functions at these epochs, yielding ΩZ,ISM = 6.6−4.3+13 × 10−7 M⊙ Mpc−3 (z ≈ 5) and ΩZ,ISM = 2.0−1.3+3.5 × 10−7 M⊙ Mpc−3 (z ≈ 7), respectively. These results are consistent with the expected metal yields from the integrated star formation history at the respective redshifts. This suggests that the majority of metals produced at z ≳ 5 are confined to the ISM, with strong implications that disfavor efficient outflow processes at these redshifts. Instead, these results suggest that the extended [C II] halos predominantly trace the extended neutral gas reservoirs of high-z galaxies.

Two-screen scattering in CRAFT FRBs

ABSTRACT Temporal broadening is a commonly observed property of fast radio bursts (FRBs), associated with turbulent media which cause radiowave scattering. Similarly to dispersion, scattering is an important probe of the media along the line of sight to an FRB source, such as the circumburst or circumgalactic media (CGM). Measurements of characteristic scattering times alone are insufficient to constrain the position of the dominant scattering media along the line of sight. However, where more than one scattering screen exists, Galactic scintillation can be leveraged to form strong constraints. We quantify the scattering and scintillation in 10 FRBs with (1) known host galaxies and redshifts and (2) captured voltage data enabling high-time resolution analysis. We find strong evidence for two screens in three cases. For FRBs 20190608B and 20210320C, we find evidence for scattering screens less than approximately 16.7 and 3000 kpc, respectively, from their sources, consistent with the scattering occurring in the circumburst environment, the host interstellar medium (ISM) or the CGM. For FRB 20201124A, we find a low modulation index that evolves over the burst’s scattering tail, indicating the presence of a scattering screen ≈9 kpc from the host, and excluding the circumburst environment from potential scattering sites. By assuming that pulse broadening is contributed by the host galaxy ISM or circumburst environment, the lack of observed scintillation in four FRBs in our sample suggests that existing models may be poor estimators of scattering times associated with the Milky Way’s ISM, similar to the anomalously low scattering observed for FRB 20201124A.

The Physical Conditions of Emission-line Galaxies at Cosmic Dawn from JWST/NIRSpec Spectroscopy in the SMACS 0723 Early Release Observations

We present rest-frame optical emission-line flux ratio measurements for five z > 5 galaxies observed by the James Webb Space Telescope Near-Infared Spectrograph (NIRSpec) in the SMACS 0723 Early Release Observations. We add several quality-control and post-processing steps to the NIRSpec pipeline reduction products in order to ensure reliable relative flux calibration of emission lines that are closely separated in wavelength, despite the uncertain absolute spectrophotometry of the current version of the reductions. Compared to z ∼ 3 galaxies in the literature, the z > 5 galaxies have similar [O iii]λ5008/Hβ ratios, similar [O iii]λ4364/Hγ ratios, and higher (∼0.5 dex) [Ne III]λ3870/[O II]λ3728 ratios. We compare the observations to MAPPINGS V photoionization models and find that the measured [Ne III]λ3870/[O II]λ3728, [O iii]λ4364/Hγ, and [O iii]λ5008/Hβ emission-line ratios are consistent with an interstellar medium (ISM) that has very high ionization (, units of cm s−1), low metallicity (Z/Z ⊙ ≲ 0.2), and very high pressure (, units of cm−3). The combination of [O iii]λ4364/Hγ and [O iii]λ(4960 + 5008)/Hβ line ratios indicate very high electron temperatures of , further implying metallicities of Z/Z ⊙ ≲ 0.2 with the application of low-redshift calibrations for “T e -based” metallicities. These observations represent a tantalizing new view of the physical conditions of the ISM in galaxies at cosmic dawn.

NOEMA spatially resolved view of the multiphase outflow in IRAS17020+4544: a shocked wind in action?

ABSTRACT The Narrow Line Seyfert 1 Galaxy IRAS17020+4544 is one of the few active galactic nuclei (AGNs) where a galaxy-scale energy-conserving outflow was revealed. This paper reports on NOEMA observations addressed to constrain the spatial scale of the CO emission in outflow. The molecular outflowing gas is resolved in five components tracing approaching and receding gas, all located at a distance of 2–3 kpc on the west and east sides of the active nucleus. This high-velocity gas (up to vout = ±1900 km s−1) is not coincident with the rotation pattern of the CO gas in the host galaxy disc. The estimated mass outflow rate shows that with a global mass output of $\dot{M}_{H_2}$ = 139$\pm 20~\, \mathrm{M}_\odot$ yr−1, this powerful galaxy-scale outflow is consistent with the wind conserving its energy, and with a momentum rate boost of a factor of ∼30 compared to the momentum rate of the nuclear X-ray wind. Preliminary results from ancillary X-ray (Chandra) and radio images (e-MERLIN) are reported. While the nature of the radio source is not conclusive, the Chandra image may tentatively trace extended emission, as expected by an expanding bubble of hot X-ray gas. The outcome of the NOEMA analysis and of the past and ongoing publications dedicated to the description of the outflow multiband phenomenology in IRAS17020+4544 concur to provide compelling reasons to postulate that an outflow shocking with the galaxy interstellar medium is driving the multiphase wind in this peculiar AGN.

Ionized Outflows in Nearby Quasars Are Poorly Coupled to Their Host Galaxies

We analyze Multi-Unit Spectroscopic Explorer observations of nine low-redshift (z < 0.1) Palomar-Green quasar host galaxies to investigate the spatial distribution and kinematics of the warm, ionized interstellar medium, with the goal of searching for and constraining the efficiency of active galactic nucleus (AGN) feedback. After separating the bright AGN from the starlight and nebular emission, we use pixel-wise, kpc-scale diagnostics to determine the underlying excitation mechanism of the line emission, and we measure the kinematics of the narrow-line region (NLR) to estimate the physical properties of the ionized outflows. The radial size of the NLR correlates with the AGN luminosity, reaching scales of ∼5 kpc and beyond. The geometry of the NLR is well-represented by a projected biconical structure, suggesting that the AGN radiation preferably escapes through the ionization cone. We find enhanced velocity dispersions (≳100 km s−1) traced by the Hα emission line in localized zones within the ionization cones. Interpreting these kinematic features as signatures of interaction between an AGN-driven ionized gas outflow and the host galaxy interstellar medium, we derive mass-outflow rates of ∼0.008–1.6 M ⊙ yr−1 and kinetic injection rates of ∼1039–1042 erg s−1, which yield extremely low coupling efficiencies of ≲10−3. These findings add to the growing body of recent observational evidence that AGN feedback is highly ineffective in the host galaxies of nearby AGNs.

Planetary Nebulae and the Ionization of the Interstellar Medium in Galaxies

We discuss the role of planetary nebulae and their progeny in galaxy context in terms of ionization of the galaxy interstellar medium. This regards ionized gas outside the disk of spiral galaxies, the diffuse ionized medium in spiral galaxies, and the weak line emission of elliptical galaxies.

Pulsar wind nebulae of runaway massive stars

ABSTRACT A significant fraction of massive stars move at speed through the interstellar medium of galaxies. After their death as core-collapse supernovae, a possible final evolutionary state is that of a fast-rotating magnetized neutron star, shaping its circumstellar medium into a pulsar wind nebula. Understanding the properties of pulsar wind nebulae requires knowledge of the evolutionary history of their massive progenitors. Using two-dimensional magnetohydrodynamical simulations, we demonstrate that, in the context of a runaway high-mass red-supergiant supernova progenitor, the morphology of its subsequent pulsar wind nebula is strongly affected by the wind of the defunct progenitor star pre-shaping the stellar surroundings throughout its entire past life. In particular, pulsar wind nebulae of obscured runaway massive stars harbour asymmetries as a function of the morphology of the progenitor’s wind-blown cavity, inducing projected asymmetric up–down synchrotron emission.

Cool circumgalactic gas in galaxy clusters: connecting the DESI legacy imaging survey and SDSS DR16 Mg ii absorbers

ABSTRACT We investigate the cool gas absorption in galaxy clusters by cross-correlating Mg ii absorbers detected in quasar spectra from data release 16 of the Sloan Digital Sky Survey (SDSS) with galaxy clusters identified in the Dark Energy Spectroscopic Instrument (DESI) survey. We find significant covering fractions ($1\!-\!5\, \mathrm{per\,cent}$ within r500, depending on the chosen redshift interval), ∼4–5 times higher than around random sightlines. While the covering fraction of cool gas in clusters decreases with increasing mass of the central galaxy, the total Mg ii mass within r500 is none the less ∼10 times higher than for SDSS luminous red galaxies. The Mg ii covering fraction versus impact parameter is well described by a power law in the inner regions and an exponential function at larger distances. The characteristic scale of the transition between these two regimes is smaller for large equivalent width absorbers. Cross-correlating Mg ii absorption with photo−$z$ selected cluster member galaxies from DESI reveals a statistically significant connection. The median projected distance between Mg ii absorbers and the nearest cluster member is ∼200 kpc, compared to ∼500 kpc in random mocks with the same galaxy density profiles. We do not find a correlation between Mg ii strength and the star formation rate of the closest cluster neighbour. This suggests that cool gas in clusters, as traced by Mg ii absorption, is: (i) associated with satellite galaxies, (ii) dominated by cold gas clouds in the intracluster medium, rather than by the interstellar medium of galaxies, and (iii) may originate in part from gas stripped from these cluster satellites in the past.

An Empirical Representation of a Physical Model for the ISM [C ii], CO, and [C i] Emission at Redshift 1 ≤ z ≤ 9

Submillimeter emission lines produced by the interstellar medium (ISM) are strong tracers of star formation and are some of the main targets of line intensity mapping (LIM) surveys. In this work we present an empirical multiline emission model that simultaneously covers the mean, scatter, and correlations of [C ii], CO J = 1–0 to J = 5–4, and [C i] lines in the redshift range 1 ≤ z ≤ 9. We assume that the galaxy ISM line emission luminosity versus halo mass relations can be described by double power laws with redshift-dependent lognormal scatter. The model parameters are then derived by fitting to the state-of-the-art semianalytic simulation results that have successfully reproduced multiple submillimeter line observations at 0 ≤ z ≲ 6. We cross-check the line emission statistics predicted by the semianalytic simulation and our empirical model, finding that at z ≥ 1 our model reproduces the simulated line intensities with fractional error less than about 10%. The fractional difference is less than 25% for the power spectra. Grounded on physically motivated and self-consistent galaxy simulations, this computationally efficient model will be helpful in forecasting ISM emission-line statistics for upcoming LIM surveys.

A high-resolution investigation of the multiphase ISM in a galaxy during the first two billion years

ABSTRACT We have carried out the first spatially resolved investigation of the multiphase interstellar medium (ISM) at high redshift, using the z = 4.24 strongly lensed submillimetre galaxy H-ATLASJ142413.9+022303 (ID141). We present high-resolution (down to ∼350 pc) ALMA observations in dust continuum emission and in the CO(7–6), $\rm H_2O (2_{1,1} - 2_{0,2})$, [C i] (1–0), and [C i] (2–1) lines, the latter two allowing us to spatially resolve the cool phase of the ISM for the first time. Our modelling of the kinematics reveals that the system appears to be dominated by a rotationally-supported gas disc with evidence of a nearby perturber. We find that the [C i] (1–0) line has a very different distribution to the other lines, showing the existence of a reservoir of cool gas that might have been missed in studies of other galaxies. We have estimated the mass of the ISM using four different tracers, always obtaining an estimate in the range of $\rm 3.2{\!-\!}3.8 \times 10^{11}\ M_{\odot }$, significantly higher than our dynamical mass estimate of $\rm 0.8{\!-\!}1.3 \times 10^{11}\ M_{\odot }$. We suggest that this conflict and other similar conflicts reported in the literature is because the gas-to-tracer ratios are ≃4 times lower than the Galactic values used to calibrate the ISM in high-redshift galaxies. We demonstrate that this could result from a top-heavy initial mass function and strong chemical evolution. Using a variety of quantitative indicators, we show that, extreme though it is at z = 4.24, ID141 will likely join the population of quiescent galaxies that appears in the Universe at z ∼ 3.