Interstellar Medium Of High-redshift Galaxies Research Articles

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 interstellar medium of high-redshift galaxies: Gathering clues from C III] and [C II] lines

Context.A tight relation between [C II] 158 μm line luminosity and the star formation rate (SFR) has been observed for local galaxies. At high redshift (z > 5), galaxies instead deviate downwards from the local Σ[C II]− ΣSFRrelation. This deviation might be caused by different interstellar medium (ISM) properties in galaxies at early epochs.Aims.To test this hypothesis, we combined the [C II] and SFR data with C III] 1909 Å line observations and our physical models. We additionally investigated how ISM properties, such as burstiness,κs, total gas density,n, and metallicity,Z, affect the deviation from the Σ[C II]− ΣSFRrelation in these sources.Methods.We present the VLT/X-shooter observations targeting the C III]λ1909 line emission in three galaxies at 5.5 < z < 7.0. We include archival X-shooter data of two other sources at 5.5 < z < 7.0 and the VLT/MUSE archival data of six galaxies atz ∼ 2. We extend our sample of galaxies with eleven star-forming systems at 6 < z < 7.5, with either C III] or [C II] detection reported in the literature.Results.We detected C III]λλ1907, 1909 line emission in HZ10 and we derived the intrinsic, integrated flux of the C III]λ1909 line. We constrained the ISM properties for our sample of galaxies,κs,n, andZ, by applying our physically motivated model based on the MCMC algorithm. For the most part, high-zstar-forming galaxies show subsolar metallicities. The majority of the sources have log(κs) ≳ 1, that is, they overshoot the Kennicutt–Schmidt (KS) relation by about one order of magnitude.Conclusions.Our findings suggest that the whole KS relation might be shifted upwards at early times. Furthermore, all the high-zgalaxies of our sample lie below the Σ[C II]− ΣSFRlocal relation. The total gas density,n, shows the strongest correlation with the deviation from the local Σ[C II]− ΣSFRrelation, namely, low-density high-zsystems have lower [C II] surface brightness, in agreement with theoretical models.

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.

Measuring dynamical masses from gas kinematics in simulated high-redshift galaxies

ABSTRACT Advances in instrumentation have recently extended detailed measurements of gas kinematics to large samples of high-redshift galaxies. Relative to most nearby, thin disc galaxies, in which gas rotation accurately traces the gravitational potential, the interstellar medium (ISM) of $z$ ≳ 1 galaxies is typically more dynamic and exhibits elevated turbulence. If not properly modelled, these effects can strongly bias dynamical mass measurements. We use high-resolution FIRE-2 cosmological zoom-in simulations to analyse the physical effects that must be considered to correctly infer dynamical masses from gas kinematics. Our analysis covers a range of galaxy properties from low-redshift Milky-Way-mass galaxies to massive high-redshift galaxies (M⋆ > 1011 M⊙ at $z$ = 1). Selecting only snapshots where a disc is present, we calculate the rotational profile $\bar{v}_\phi (r)$ of the cool ($10^{3.5}\,\lt {\it T}\lt 10^{4.5}~\rm {K}$) gas and compare it to the circular velocity $v_{\rm c}=\sqrt{GM_{\rm enc}/r}$. In the simulated galaxies, the gas rotation traces the circular velocity at intermediate radii, but the two quantities diverge significantly in the centre and in the outer disc. Our simulations appear to over-predict observed rotational velocities in the centres of massive galaxies (likely from a lack of black hole feedback), so we focus on larger radii. Gradients in the turbulent pressure at these radii can provide additional radial support and bias dynamical mass measurements low by up to 40 per cent. In both the interior and exterior, the gas’ motion can be significantly non-circular due to e.g. bars, satellites, and inflows/outflows. We discuss the accuracy of commonly used analytic models for pressure gradients (or ‘asymmetric drift’) in the ISM of high-redshift galaxies.

Dust and gas content of high-redshift galaxies hosting obscured AGN in the Chandra Deep Field-South

Context. Obscured active galactic nuclei (AGN) represent a significant fraction of the entire AGN population, especially at high redshift (∼70% at z = 3 − 5). They are often characterized by the presence of large gas and dust reservoirs that are thought to sustain and possibly obscure vigorous star formation processes that make these objects shine at FIR and submillimeter wavelengths. Studying the physical properties of obscured AGN and their host galaxies is crucial to shedding light on the early stages of a massive system lifetime. Aims. We aim to investigate the contribution of the interstellar medium (ISM) to the obscuration of quasars in a sample of distant highly star forming galaxies and to unveil their morphological and kinematics properties. Methods. We exploit Atacama Large Millimeter/submillimeter Array Cycle 4 observations of the continuum (∼2.1 mm) and high-J CO emission of a sample of six X-ray selected, FIR detected galaxies hosting an obscured AGN at zspec > 2.5 in the 7 Ms Chandra Deep Field-South. We measured the masses and sizes of the dust and molecular gas by fitting the images, visibilities, and spectra, and we derived the gas density and column density on the basis of a uniform sphere geometry. Finally, we compared the measured column densities with those derived from the Chandra X-ray spectra. Results. We detected both the continuum and line emission for three sources for which we measured both the flux density and size. For the undetected sources, we derived an upper limit on the flux density from the root mean square of the images. We found that the detected galaxies are rich in gas and dust (molecular gas mass in the range < 0.5–2.7 × 1010 M⊙ for αCO = 0.8 and up to ∼2 × 1011 M⊙ for αCO = 6.5, and dust mass < 0.9–4.9 × 108 M⊙) and generally compact (gas major axis 2.1–3.0 kpc, dust major axis 1.4–2.7 kpc). The column densities associated with the ISM are on the order of 1023 − 24 cm−2, which is comparable with those derived from the X-ray spectra. For the detected sources we also derived dynamical masses in the range 0.8–3.7 × 1010 M⊙. Conclusions. We conclude that the ISM of high redshift galaxies can substantially contribute to nuclear obscuration up to the Compton-thick (> 1024 cm−2) regime. In addition, we found that all the detected sources show a velocity gradient reminding one rotating system, even though two of them show peculiar features in their morphology that can be associated with a chaotic, possibly merging, structure.

Near-infrared spectroscopic observations of high redshift C I absorbers

We study a sample of 17 z > 1.5 absorbers selected based on the presence of strong C I absorption lines in Sloan Digital Sky Survey (SDSS) spectra and observed with the European Southern Observatory Very Large Telescope (ESO-VLT) spectrograph X-shooter. We derive metallicities, depletion onto dust, and extinction by dust, and analyse the absorption from Mg II, Mg I, Ca II, and Na I that are redshifted into the near infrared wavelength range. We show that most of these C I absorbers have high metallicity and dust content. We detect nine Ca II absorptions with W(Ca IIλ3934) > 0.23 Å out of 14 systems where we have appropriate wavelength coverage. The observed equivalent widths are similar to what has been measured in other lower redshift surveys of Ca II systems. We detect ten Na I absorptions in the 11 systems where we could observe this absorption. The median equivalent width (W(Na Iλ5891) = 0.68 Å) is larger than what is observed in local clouds with similar H I column densities but also in z < 0.7 Ca II systems detected in the SDSS. The systematic presence of Na I absorption in these C I systems strongly suggests that the gas is neutral and cold, maybe part of the diffuse molecular gas in the interstellar medium of high-redshift galaxies. Most of the systems (12 out of 17) have W(Mg IIλ2796) > 2.5 Å and six of them have log N(H I) < 20.3, with the extreme case of J1341+1852 that has log N(H I) = 18.18. The Mg II absorptions are spread over more than Δυ ~ 400 km s−1 for half of the systems; three absorbers have Δυ > 500 km s−1. The kinematics are strongly perturbed for most of these systems, which indicates that these systems probably do not arise in quiet disks and must be close to regions with intense star-formation activity and/or are part of interacting objects. All this suggests that a large fraction of the cold gas at high redshift arises in disturbed environments.

If it does not kill them, it makes them stronger: collisional evolution of star clusters with tidal shocks

Abstract The radii of young (≲100 Myr) star clusters correlate only weakly with their masses. This shallow relation has been used to argue that impulsive tidal perturbations, or ‘shocks’, by passing giant molecular clouds (GMCs) preferentially disrupt low-mass clusters. We show that this mass–radius relation is in fact the result of the combined effect of two-body relaxation and repeated tidal shocks. Clusters in a broad range of environments including those like the solar neighbourhood evolve towards a typical radius of a few parsecs, as observed, independent of the initial radius. This equilibrium mass–radius relation is the result of a competition between expansion by relaxation and shrinking due to shocks. Interactions with GMCs are more disruptive for low-mass clusters, which helps to evolve the globular cluster mass function (GCMF). However, the properties of the interstellar medium in high-redshift galaxies required to establish a universal GCMF shape are more extreme than previously derived, challenging the idea that all GCs formed with the same power-law mass function.

Modelling the supernova-driven ISM in different environments

We use hydrodynamical simulations in a $(256\;{\rm pc})^3$ periodic box to model the impact of supernova (SN) explosions on the multi-phase interstellar medium (ISM) for initial densities $n = 0.5-30$ cm$^{-3}$ and SN rates $1-720$ Myr$^{-1}$. We include radiative cooling, diffuse heating, and the formation of molecular gas using a chemical network. The SNe explode either at random positions, at density peaks, or both. We further present a model combining thermal energy for resolved and momentum input for unresolved SNe. Random driving at high SN rates results in hot gas ($T\gtrsim 10^6$ K) filling $> 90$% of the volume. This gas reaches high pressures ($10^4 < P/k_\mathrm{B} < 10^7$ K cm$^{-3}$) due to the combination of SN explosions in the hot, low density medium and confinement in the periodic box. These pressures move the gas from a two-phase equilibrium to the single-phase, cold branch of the cooling curve. The molecular hydrogen dominates the mass ($>50$%), residing in small, dense clumps. Such a model might resemble the dense ISM in high-redshift galaxies. Peak driving results in huge radiative losses, producing a filamentary ISM with virtually no hot gas, and a small molecular hydrogen mass fraction ($\ll 1$%). Varying the ratio of peak to random SNe yields ISM properties in between the two extremes, with a sharp transition for equal contributions. The velocity dispersion in HI remains $\lesssim 10$ km s$^{-1}$ in all cases. For peak driving the velocity dispersion in H$_\alpha$ can be as high as $70$ km s$^{-1}$ due to the contribution from young, embedded SN remnants.

The ESO UVES Advanced Data Products Quasar Sample – IV. On the deficiency of argon in DLA systems

In this work, we study argon abundances in the interstellar medium of high-redshift galaxies (2<z(abs)<4.2) detected as Damped Lya absorbers (DLA) in the spectra of background quasars. We use high-resolution quasar spectra obtained from the ESO-UVES advanced data products (EUADP) database. We present 3 new measurements and 5 upper limits of ArI. We further compiled DLAs/sub-DLA data from the literature with measurements available of argon and alpha-capture elements (S or Si), making up a total of 37 systems, i.e. the largest DLA argon sample investigated so far. We confirm that argon is generally deficient in DLAs, with a mean value [Ar/alpha]= -0.4+/-0.06dex (standard error of the mean). The [Ar/alpha] ratios show a weak, positive trend with increasing NHI and increasing absorption redshift, and a weak, negative trend with dust-free metallicity, [S/H]. Detailed analysis of the abundance ratios indicates that ArI ionisation, rather than dust depletion or nucleosynthetic evolution, is responsible for the argon deficiency. Altogether, the observational evidence is consistent with a scenario of argon ionisation dominated by quasar metagalactic radiation modulated by local HI self-shielding inside the DLA host galaxies. Our measurements and limits of argon abundances suggest that the cosmic reionisation of HeII is completed above z=3, but more measurements at z(abs)>3.5 are required to probe the final stages of this process of cosmic reionisation.

Are globular clusters the natural outcome of regular high-redshift star formation?

AbstractWe summarise the recent progress in understanding the formation and evolution of globular clusters (GCs) in the context of galaxy formation and evolution. It is discussed that an end-to-end model for GC formation and evolution should capture four different phases: (1) star and cluster formation in the high-pressure interstellar medium of high-redshift galaxies, (2) cluster disruption by tidal shocks in the gas-rich host galaxy disc, (3) cluster migration into the galaxy halo, and (4) the final evaporation-dominated evolution of GCs until the present day. Previous models have mainly focussed on phase 4. We present and discuss a simple model that includes each of these four steps – its key difference with respect to previous work is the simultaneous addition of the high-redshift formation and early evolution of young GCs, as well as their migration into galaxy haloes. The new model provides an excellent match to the observed GC mass spectrum and specific frequency, as well as the relations of GCs to the host dark matter halo mass and supermassive black hole mass. These results show (1) that the properties of present-day GCs are reproduced by assuming that they are the natural outcome of regular high-redshift star formation (i.e. they form according to same physical processes that govern massive cluster formation in the local Universe), and (2) that models only including GC evaporation strongly underestimate their integrated mass loss over a Hubble time.

Faint AGN inz≳ 6 Lyman-break galaxies powered by cold accretion and rapid angular momentum transport

We develop a radiation pressure-balanced model for the interstellar medium of high-redshift galaxies that describes many facets of galaxy formation at z>~6, including star formation rates and distributions and gas accretion onto central black holes. We first show that the vertical gravitational force in the disk of such a model is dominated by the disk self-gravity supported by the radiation pressure of ionizing starlight on gas. Constraining our model to reproduce the UV luminosity function of Lyman-break galaxies (LBGs), we limit the available parameter-space to wind mass-loading factors 1--4 times the canonical value for momentum-driven winds. We then focus our study by exploring the effects of different angular momentum transport mechanisms in the galactic disk and find that accretion driven by gravitational torques, such as from linear spiral waves or non-linear orbit crossings, can build up black hole masses by z=6 consistent with the canonical M-sigma relation with a duty cycle of unity, while accretion mediated by a local viscosity such as in an alpha-disk results in negligible BH accretion. Both gravitational torque models produce X-ray emission from active galactic nuclei (AGN) in high-redshift LBGs in excess of the estimated contribution from high-mass X-ray binaries. Using a recent analysis of deep Chandra observations by Cowie et al., we can already begin to rule out the most extreme regions of our parameter-space: the inflow velocity of gas through the disk must either be less than one percent of the disk circular velocity or the X-ray luminosity of the AGN must be substantially obscured. Moderately deeper future observations or larger sample sizes will be able to probe the more reasonable range of angular momentum transport models and obscuring geometries.

Damped Lyα systems as probes of chemical evolution over cosmological timescales

AbstractWe review the current state of knowledge of damped Lyα systems (DLAs) selected in absorption on quasar sightlines. These objects are extremely useful to study the interstellar medium of high-redshift galaxies and the nucleosynthesis in the early Universe. The characteristics of this galaxy population has been investigated for years and slowly we are getting information on their puzzling nature. Imaging atz&lt;1 shows that DLAs are associated with a mixing bag of galaxies with no especially large contribution from dwarf galaxies. Evidence for a mild evolution of the cosmic mean metallicity with time is observed. The star formation histories of these high-redshift galaxies begin to be accessible and indicate that DLAs tend to be young, gas-dominated galaxies with low star formation rates per unit area. Finally, indirect estimation of the DLA stellar masses from the mass-metallicity relations observed for emission-selected star-forming galaxies atz= 2−3 points to intermediate-mass galaxies withM*&lt; 109M⊙.

The nitrogen and oxygen abundances in the neutral gas at high redshift

We study the Oxygen and Nitrogen abundances in the interstellar medium of high-redshift galaxies. We use high resolution and high signal-to-noise ratio spectra of Damped Lyman-alpha (DLA) systems detected along the line-of-sight to quasars to derive robust abundance measurements from unsaturated metal absorption lines. We present results for a sample of 16 high-redshift DLAs and strong sub-DLAs (log N(HI)>19.5, 2.4 -1 and nine systems have [N/O]<-1.15. In the diagram [N/O] versus [O/H], a loose plateau is possibly present at [N/O]=-0.9 that is below the so-called primary plateau as seen in local metal-poor dwarf galaxies ([N/O] in the range -0.57 to -0.74). No system is seen above this primary plateau whereas the majority of the systems lie well below with a large scatter. All this suggests a picture in which DLAs undergo successive star-bursts. During such an episode, the [N/O] ratio decreases sharply because of the rapid release of Oxygen by massive stars whereas inbetween two bursts, Nitrogen is released by low and intermediate-mass stars with a delay and the [N/O] ratio increases.

Dust Extinction of Gamma-Ray Burst Host Galaxies: Identification of Two Classes?

Dust in the host galaxies of gamma-ray bursts (GRBs) dims and reddens their afterglow spectra. Knowledge of the nature of this dust is crucial for correcting for extinction, providing clues to the nature of GRB progenitors, and probing the interstellar medium of high-redshift galaxies as well as the nature of cosmic dust when the universe was much younger and galaxies were much less evolved. The dust and extinction properties of GRB host galaxies are still poorly known. Unlike previous work, we derive in this Letter the extinction curves for 10 GRB host galaxies without a priori assumption of any specific extinction types (such as that of the Milky Way, or the Small/Large Magellanic Clouds). It is found that there appears to exist two different types of extinction curves: one is relatively flat and gray, the other displays a steeper dependence on inverse wavelength, closely resembling that of the Milky Way but with the 2175$\Angstrom$ feature removed.

Heavy‐Element Abundances and Dust Depletions in the Host Galaxies of Three Gamma‐Ray Bursts

We have derived the column densities of heavy elements in three gamma-ray burst (GRB) optical transients, associated with the circumburst or interstellar medium (ISM) of the host galaxy. In comparison with the same elements observed in damped Lyα (DLA) systems along QSO sight lines, we find evidence for much higher column densities of Zn II. The gap between the QSO-DLA and GRB-DLA distributions is smoothly bridged by observations of the interstellar absorption in the Milky Way and the Magellanic Clouds. Very small [Fe/Zn], [Si/Zn], and [Cr/Zn] values in GRB-DLAs indicate large dust depletions. Once the dust-to-metals ratios are determined, we find an optical extinction AV ≈ 1 mag, to be compared with typical AV 0.1 in most QSO-DLAs. Our inference of high dust content appears to be in contradiction with the typical low reddening previously found in GRBs. One possible way to reconcile is a dust grain size distribution biased toward big grains, which would give a gray extinction. Possibly, the small dust grains have been destroyed by the GRBs themselves. Our findings support the idea that primarily optically selected QSOs probe mainly low-gas/dust regions of high-redshift galaxies, while the more powerful GRBs can be detected through denser regions of their ISM (molecular clouds and star-forming regions). Therefore, GRB-DLAs and QSO-DLAs together provide a more complete picture of the global properties of the interstellar medium in high-redshift galaxies.