Damped Lyman-α Research Articles

Isotopic abundance of carbon in the DLA towards QSO B1331+170

ABSTRACT Chemical evolution models predict a gradual build-up of 13C in the Universe, based on empirical nuclear reaction rates and assumptions on the properties of stellar populations. However, old metal-poor stars within the Galaxy contain more 13C than is predicted, suggesting that further refinements to the models are necessary. Gas at high-redshift provides important supplementary information at metallicities $-2\lesssim \left[{\rm Fe/H}\right]\lesssim -1$, for which there are only a few measurements in the Galaxy. We obtained new, high-quality, VLT/ESPRESSO observations of the QSO B1331$+$170 and used them to measure 12C/13C in the damped Lyman-$\alpha$ system (DLA) at $z_{\rm abs}=1.776$, with $\left[{\rm Fe/H}\right]$ = −1.27. ai-vpfit, an artificial intelligence tool based on genetic algorithms and guided by a spectroscopic information criterion, was used to explore different possible kinematic structures of the carbon gas. Three hundred independent ai-vpfit models of the absorption system were produced using pre-set 12C/13C values, ranging from 4 to 500. Our results show that ${\rm ^{12}C / ^{13}C}=28.5^{+51.5}_{-10.4}$, suggesting a possibility of 13C production at low metallicity.

Evidence of Pop III stars’ chemical signature in neutral gas at z ∼ 6

Aims. This study explores the metal enrichment signatures attributed to the first generation of stars (Pop III) in the Universe, focusing on the E-XQR-30 sample – a collection of 42 high signal-to-noise ratio spectra of quasi-stellar objects (QSOs) with emission redshifts ranging from 5.8 to 6.6. We aim to identify traces of Pop III metal enrichment by analyzing neutral gas in the interstellar medium of primordial galaxies and their satellite clumps, detected in absorption. Methods. To chase the chemical signature of Pop III stars, we studied metal absorption systems in the E-XQR-30 sample, selected through the detection of the neutral oxygen absorption line at 1302 Å. The O I line is a reliable tracer of neutral hydrogen and allowed us to overcome the challenges posed by the Lyman-α forest’s increasing saturation at redshifts above ∼5 to identify damped Lyman-α systems (DLAs). We detected and analyzed 29 O I systems at z ≥ 5.4, differentiating between proximate DLAs (PDLAs) and intervening DLAs. Voigt function fits were applied to obtain ionic column densities, and relative chemical abundances were determined for 28 systems. These were then compared with the predictions of theoretical models. Results. Our findings expand the study of O I systems at z ≥ 5.4 fourfold. No systematic differences were observed in the average chemical abundances between PDLAs and intervening DLAs. The chemical abundances in our sample align with literature systems at z > 4.5, suggesting a similar enrichment pattern for this class of absorption systems. A comparison between these DLA-analogs at 4.5 < z < 6.5 with a sample of very metal-poor DLAs at 2 < z < 4.5 shows in general similar average values for the relative abundances, with the exception of [C/O], [Si/Fe] and [Si/O] which are significantly larger for the high-z sample. Furthermore, the dispersion of the measurements significantly increases in the high-redshift bin. This increase is predicted by the theoretical models and indicates a potential retention of Pop III signatures in the probed gas. Conclusions. This work represents a significant advancement in the study of the chemical properties of highly neutral gas at z ≥ 5.4, shedding light on its potential association with the metal enrichment from Pop III stars. Future advancements in observational capabilities, specifically high-resolution spectrographs, are crucial for refining measurements and addressing current limitations in the study of these distant absorption systems.

Strong damped Lyman-α absorption in young star-forming galaxies at redshifts 9 to 11.

Primordial neutral atomic gas, mostly composed of hydrogen, is the raw material for star formation in galaxies. However, there are few direct constraints on the amount of neutral atomic hydrogen (Hi) in galaxies at early cosmic times. We analyzed James Webb Space Telescope (JWST) near-infrared spectroscopy of distant galaxies, at redshifts ≳8. From a sample of 12 galaxies, we identified three that show strong damped Lyman-α absorption due to Hi in their local surroundings. The galaxies are located at spectroscopic redshifts of 8.8, 10.2, and 11.4, corresponding to 400 to 600 million years after the Big Bang. They have Hi column densities ≳1022 cm-2, which is an order of magnitude higher than expected for a fully neutral intergalactic medium, and constitute a gas-rich population of young star-forming galaxies.

Cold molecules in H I 21 cm absorbers across redshifts ∼0.1–4

Absorption lines at high-redshift in front of quasars are quite rare in the millimeter (mm) domain. Only five associated and five intervening systems have been reported in the literature. Nevertheless, these discoveries provide very useful information that is complementary to emission lines, allowing, for instance, to distinguish between inflows and outflows. These lines are also good candidates for studying the variations of the fundamental constants of physics. Here we report the findings of our search for CO and other molecules in emission and absorption in front of a sample of 30 targets, comprising 16 associated and 14 intervening H I 21 cm absorbers. The observations were made with the IRAM-30 m telescope simultaneously at 3 mm and 2 mm, exploring several lines of the CO ladder and HCO+, depending on the redshift. We detected eight targets in emission, of which five are new. The derived molecular gas masses range from 109 to 7 × 1011 M⊙ and the highest redshift detection (z = 3.387) corresponds to a relatively average-metallicity damped Lyman-α absorber for this redshift. We also report four new detections in absorption. Two of the associated CO absorption line detections at high-redshift (z = 1.211 and 1.275) result from high-spatial-resolution follow-up observations with NOEMA. The disparity between the mm molecular and H I 21 cm absorption lines for these and another intervening system detected in HNC at z = 1.275 is attributable to radio and mm sight lines tracing different media. We compare the atomic and molecular column densities of 14 known high-redshift (z > 0.1) molecular absorption line systems. The associated H I absorption lines are broad and exhibit multiple components, and the molecular absorption generally corresponds to the broader and weaker 21 cm absorption component. This indicates two distinct phases: one near galaxy centers with a larger CO-to-H I abundance ratio, and another with lower molecular abundance in the outer regions of the galaxy. In comparison, intervening absorption profiles correspond primarily to H I-dominated gas structure in galaxy outskirts, except for gas at low impact parameters in gravitationally lensed systems. The comparison of interferometric and single-dish observations presented here shows that the detection of absorption requires sufficient spatial resolution to overcome the dilution by emission and will be an important criterion for mm follow-up of 21 cm absorbers from ongoing large-scale surveys.

Modelling post-reionization H i distributions in fuzzy dark matter cosmologies using conditional normalizing flows

ABSTRACT Upcoming 21-cm intensity mapping experiments like the Square Kilometer Array (SKA) hold significant potential to constrain the properties of dark matter. In this work, we model neutral hydrogen (H i) distributions using high-resolution hydrodynamical N-body simulations of both cold dark matter (CDM) and fuzzy dark matter (FDM) cosmologies in the post-reionization redshift range of z = 3.42−4.94. We show that the H i abundance decreases in FDM-like cosmologies. Extreme FDM models with m ∼ 10−22 eV are at odds with a range of measurements. Due to the increased halo bias, the H i bias increases, paralleled by the damped Lyman-α (DLA) bias which we infer from the cross-section of DLAs. The distribution of the latter in extreme FDM models has a high median at the low-mass end, which can be traced to the high column density of cosmic filaments. FDM models exhibit a very similar abundance of DLAs compared to CDM while sub-DLAs are already less abundant. We study the prospects of detecting the brightest H i peaks with SKA1-Low at z = 4.94, indicating moderate signal-to-noise ratios (SNRs) at angular resolution θA = 2 arcmin with a rapidly declining SNR for lower values of θA. After training the conditional normalizing flow network HIGlow on 2D H i maps, we interpolate its latent space of axion masses to predict the peak flux for a new, synthetic FDM cosmology, finding good agreement with expectations. This work thus underscores the potential of normalizing flows in capturing complex, non-linear structures within H i maps, offering a versatile tool for conditional sample generation and prediction tasks.

Proximate molecular quasar absorbers

Proximate molecular quasar absorbers (PH2) are an intriguing population of absorption systems that was recently uncovered through strong H2absorption at a small velocity separation from the background quasars. We performed a multi-wavelength spectroscopic follow-up of 13 such systems with VLT/X-shooter. Here, we present the observations and study the overall chemical enrichment measured from the H I, H2, and metal lines. We combined this with an investigation of the neutral gas kinematics with respect to the quasar host. We find gas-phase metallicities in the range 2% to 40% of the solar value, that is, in the upper-half range of H I-selected proximate damped Lyman-αsystems, but similar to what is seen in intervening H2-bearing systems. This is likely driven by similar selection effects that play against the detection of most metal- and molecule-rich systems in absorption. Differences are seen in the abundance of dust (from [Zn/Fe]) and its depletion pattern when compared to intervening systems, however, possibly indicating different dust production or destruction close to the active galactic nucleus. We also note the almost ubiquitous presence of a high-ionisation phase traced by N Vin proximate systems. In spite of the hard UV field from the quasars, we found no strong overall deficit of neutral argon, at least when compared to intervening damped Lyman-αsystems. The reason likely is that argon is mostly neutral in the H2phase, which accounts for a large fraction of the total amount of metals. We measured the quasar systemic redshifts through emission lines from both ionised gas and CO(3–2) emission, the latter being detected in all six cases for which we obtained 3 mm data from complementary NOEMA observations. For the first time, we observe a trend between the line-of-sight velocity with respect to systemic redshift and metallicity of the absorbing gas. This suggests that high-metallicity neutral and molecular gas is more likely to be located in outflows, while low-metallicity gas could be more clustered in velocity space around the quasar host, possibly with an infalling component.

Observations of cold extragalactic gas clouds at z = 0.45 towards PKS 1610-771

ABSTRACT We present results from MUSE observations of a 21-cm ${\rm H\, {\small I}}$ absorption system detected with the Australian Square Kilometre Array Pathfinder radio telescope at redshift z = 0.4503 towards the z = 1.71 quasar PKS 1610-771. We identify four galaxies (A, B, X, and Y) at the same redshift as the 21-cm ${\rm H\, {\small I}}$ Damped Lyman-α (DLA) absorption system, with impact parameters ranging from less than 10 kpc to almost 200 kpc from the quasar sightline. ${\rm Ca\, {\small II}}$ and ${\rm Na\, {\small I}}$ absorption is seen in the MUSE spectrum of the background QSO, with velocities coinciding with the initial ${\rm H\, {\small I}}$ 21-cm detection, but tracing less dense and warmer gas. This metal-line component aligns with the rotating ionized disc of galaxy B (impact parameter 18 kpc from the QSO) and appears to be corotating with the galaxy disc. In contrast, the 21-cm ${\rm H\, {\small I}}$ absorber is blueshifted relative to the galaxies nearest the absorber and has the opposite sign to the velocity field of galaxy B. Since galaxies A and B are separated by only 17 kpc on the sky and 70 km s−1 in velocity, it appears likely that the 21-cm detection traces extragalactic clouds of gas formed from their interaction. This system reveals that the cold 100 K neutral gas critical for star formation can be associated with complex structures beyond the galaxy disc, and is a first case study made in preparation for future large 21-cm absorption surveys like the ASKAP First Large Absorption Survey in ${\rm H\, {\small I}}$.

Extremely strong DLAs at high redshift: gas cooling and H2 formation

ABSTRACT We present a spectroscopic investigation with the Very Large Telescope/X-shooter of seven candidate extremely strong damped Lyman-α absorption systems [ESDLAs, N(H i) ≥ 5 × 1021 cm−2] observed along quasar sightlines. We confirm the extremely high column densities, albeit slightly (0.1 dex) lower than the original ESDLA definition for four systems. We measured low-ionization metal abundances and dust extinction for all systems. For two systems, we also found strong associated H$\rm _2$ absorption $\log N(\rm{H\rm _2})\,\text{(cm$^{-2}$)}=18.16\pm 0.03$ and 19.28 ± 0.06 at z = 3.26 and 2.25 towards J2205+1021 and J2359+1354, respectively, while for the remaining five we measured conservative upper limits on the H$\rm _2$ column densities of typically $\log N(\rm{H\rm _2})\,\text{(cm$^{-2}$)}\,\lt\, 17.3$. The increased H2 detection rate (10–55 per cent at 68 per cent confidence level) at high H i column density compared with the overall damped Lyman-α population (∼5–10 per cent) confirms previous works. We find that these seven ESDLAs have similar observed properties as those previously studied towards quasars and γ-ray burst afterglows, suggesting they probe inner regions of galaxies. We use the abundance of ionized carbon at the excited fine-structure level to calculate the cooling rates through the C ii λ158 μm emission, and compare them with the cooling rates from damped Lyman-α systems in the literature. We find that the cooling rate distribution of ESDLAs also presents the same bimodality as previously observed for the general (mostly lower H i column density) damped Lyman-α population.

J1721+8842: a gravitationally lensed binary quasar with a proximate damped Lyman-α absorber

High-redshift binary quasars provide key insights into mergers and quasar activity, and are useful tools for probing the spatial kinematics and chemistry of galaxies along the line of sight. However, only three sub-10-kpc binaries have been confirmed above z = 1. Gravitational lensing would provide a way to easily resolve such binaries, study them in higher resolution, and provide more sightlines, though the required alignment with a massive foreground galaxy is rare. Through image deconvolution of StanCam Nordic Optical Telescope (NOT) monitoring data, we reveal two further point sources in the known, z ≈ 2.38, quadruply lensed quasar (quad) J1721+8842. An ALFOSC/NOT long-slit spectrum shows that the brighter of these two sources is a quasar with z = 2.369 ± 0.007 based on the C III] line, while the C III] redshift of the quad is z = 2.364 ± 0.003. Lens modelling using point-source positions rules out a single source model, favouring an isothermal lens mass profile with two quasar sources separated by ∼6.0 kpc (0.73″) in projection. Given the resolving ability from lensing and current lensed quasar statistics, this discovery suggests a large population of undiscovered, unlensed sub-10-kpc binaries. We also analyse spectra of two images of the quad, showing narrow Lyα emission within the trough of a proximate damped Lyman-α absorber (PDLA). An apparent mismatch between the continuum and narrow line flux ratios provides a new potential tool for simultaneously studying microlensing and the quasar host galaxy. Signs of the PDLA are also seen in the second source, but a deeper spectrum is still required to confirm this. Thanks to the multiple lines of sight from lensing and two quasar sources, this system offers simultaneous subparsec- and kiloparsec-scale probes of a PDLA.

C ii*/C ii ratio in high-redshift DLAs: ISM phase separation drives the observed bimodality of [C ii] cooling rates

ABSTRACT We discuss observations of C ii*/C ii ratios and cooling rates due to [C ii] 158μm emission in high-redshift intervening damped Lyman-α (DLA) systems towards quasars. We show that the observed bimodality in the C ii cooling rates actually reflects a bimodality in the C ii*/C ii−metallicity plane that can be naturally explained by phase segregation of the neutral medium, without invoking differences in star-formation scenarios. Assuming realistic distributions of the physical parameters to calculate the phase diagrams, we also reproduce qualitatively the metallicity dependence of this bimodality. We emphasize that high-z DLAs mostly probe low-metallicity gas ($Z\lesssim 0.1 \, \mathrm{Z}_{\odot }$), where heating is dominated by cosmic rays (and/or turbulence), and not by photoelectric heating. Therefore, even if the gas of DLA is predominantly cold (where the cooling is dominated by [C ii]), the excitation of C ii can be used to derive the cosmic ray ionization rate (and/or turbulent heating), but not the UV field, as was previously considered. Alternatively, if the gas in DLA is predominantly warm, C ii*/C ii can be used to constrain its number density. Finally, we also discuss the importance of the ionized medium, which, if also present along the line of sight, can significantly increase the average C ii*/C ii ratio.

Cosmic metal density evolution in neutral gas: insights from observations and cosmological simulations

ABSTRACT We contrast the latest observations of the cosmic metal density in neutral gas ($\rho _{ {met,neu}}$) with three cosmological galaxy evolution simulations: L-Galaxies 2020, TNG100, and EAGLE. We find that the fraction of total metals that are in neutral gas is <40 per cent at 3 ≲ $z$ ≲ 5 in these simulations, whereas observations of damped Lyman-α (DLA) systems suggest ≳ 85 per cent. In all three simulations, hot, low-density gas is also a major contributor to the cosmic metal budget, even at high redshift. By considering the evolution in cosmic SFR density ($\rho _{ {\rm {\small {sfr}}}}$), neutral gas density ($\rho _{ {HI}}$), and mean gas-phase metallicity ($[\langle {} {M/H}\rangle ]_{ {neu}}$), we determine two possible ways in which the absolute $\rho _{ {met,neu}}$ observed in DLAs at high redshift can be matched by simulations: (i) the $\rho _{ {\rm {\small {sfr}}}}$ at $z$ ≳ 3 is greater than inferred from current FUV observations, or (ii) current high-redshift DLA metallicity samples have a higher mean host mass than the overall galaxy population. If the first is correct, TNG100 would match the ensemble data best, however there would be an outstanding tension between the currently observed $\rho _{ {\rm {\small {sfr}}}}$ and $\rho _{ {met,neu}}$. If the second is correct, L-Galaxies 2020 would match the ensemble data best, but would require an increase in neutral gas mass inside subhaloes above $z$ ∼ 2.5. If neither is correct, EAGLE would match the ensemble data best, although at the expense of overestimating $[\langle {} {M/H}\rangle ]_{ {neu}}$. Modulo details related to numerical resolution and H i mass modelling in simulations, these incompatibilities highlight current tensions between key observed cosmic properties at high redshift.

Damped Lyman-α absorbers from Sloan digital sky survey DR16Q with Gaussian processes

ABSTRACT We present a new catalogue of Damped Lyman-α absorbers from SDSS DR16Q, as well as new estimates of their statistical properties. Our estimates are computed with the Gaussian process models presented in Garnett et al., Ho, Bird & Garnett with an improved model for marginalizing uncertainty in the mean optical depth of each quasar. We compute the column density distribution function (CDDF) at 2 < z < 5, the line density (dN/dX), and the neutral hydrogen density (ΩDLA). Our Gaussian process model provides a posterior probability distribution of the number of DLAs per spectrum, thus allowing unbiased probabilistic predictions of the statistics of DLA populations even with the noisiest data. We measure a non-zero column density distribution function for $N_{\rm {HI}}\lt 3 \times 10^{22} \, \rm {cm}^{-2}$ with $95{{\ \rm per\ cent}}$ confidence limits, and $N_{\rm {HI}}\lesssim 10^{22} \, \rm {cm}^{-2}$ for spectra with signal-to-noise ratios >4. Our results for DLA line density and total hydrogen density are consistent with previous measurements. Despite a small bias due to the poorly measured blue edges of the spectra, we demonstrate that our new model can measure the DLA population statistics when the DLA is in the Lyman-β forest region. We verify our results are not sensitive to the signal-to-noise ratios and redshifts of the background quasars although a residual correlation remains for detections from zQSO < 2.5, indicating some residual systematics when applying our models on very short spectra, where the SDSS spectral observing window only covers part of the Lyman-α forest.

HD molecules at high redshift: cosmic ray ionization rate in the diffuse interstellar medium

ABSTRACT We present a systematic study of deuterated molecular hydrogen (HD) at high redshift, detected in absorption in the spectra of quasars. We present four new identifications of HD lines associated with known $\rm H_2$-bearing Damped Lyman-α systems. In addition, we measure upper limits on the $\rm HD$ column density in 12 recently identified $\rm H_2$-bearing DLAs. We find that the new $\rm HD$ detections have similar $N({\rm HD})/N(\rm H_2)$ ratios as previously found, further strengthening a marked difference with measurements through the Galaxy. This is likely due to differences in physical conditions and metallicity between the local and the high-redshift interstellar media. Using the measured N(HD)/N(H2) ratios together with priors on the UV flux (χ) and number densities (n), obtained from analysis of $\rm H_2$ and associated C i lines, we are able to constrain the cosmic ray ionization rate (CRIR, ζ) for the new $\rm HD$ detections and for eight known HD-bearing systems where priors on n and χ are available. We find significant dispersion in ζ, from a few × 10−18 s−1 to a few × 10−15 s−1. We also find that ζ strongly correlates with χ – showing almost quadratic dependence, slightly correlates with Z, and does not correlate with n, which probably reflects a physical connection between cosmic rays and star-forming regions.

Nature of the DLA towards Q 0528−250: High pressure and strong UV field revealed by excitation of C i, H2, and Si ii

ABSTRACT We present the detection of excited fine-structure energy levels of singly ionized silicon and neutral carbon associated with the proximate damped Lyman-α system at zabs = 2.811 towards Q 0528−250. This absorber has an apparent relative velocity that is inconsistent with the Hubble flow indicating motion along the line-of-sight towards the quasar, i.e. zabs > zem. We measure the metallicity of the system to be [Zn/H] = −0.68 ± 0.02. Using the relative populations of the fine-structure levels of Si ii and C i, as well as the populations of H2 rotational levels, we constrain the physical conditions of the gas. We derive hydrogen number densities of $n_{\rm H}=190^{+70}_{-50}$ cm−3 and $260^{+30}_{-20}$ cm−3 in two velocity components where both C i and H2 are detected. Taking into account the kinetic temperature in each component, ∼150 K, we infer high values of thermal pressure in the cold neutral medium probed by the observations. The strengths of the UV field in Draine’s unit are $I_{\rm UV} = 10^{+5}_{-3}$ and $14^{+3}_{-3}$ in each of these two components, respectively. Such enhanced UV fluxes and thermal pressure compared to intervening DLAs are likely due to the proximity of the quasar. The typical size of the absorber is ∼104 au. Assuming the UV flux is dominated by the quasar, we constrain the distance between the quasar and the absorber to be ∼150−200 kpc. This favours a scenario where the absorption occurs in a companion galaxy located in the group where the quasar-host galaxy resides. This is in line with studies in emission that revealed the presence of several galaxies around the quasar.

Physical conditions in two high-redshift H2-bearing GRB-DLAs, 120815A and 121024A

ABSTRACT The gamma-ray burst (GRB) afterglows provide a unique opportunity to study the interstellar medium (ISM) of star-forming galaxies at high-redshift. The GRB-DLAs (damped Lyman-α absorbers) contain a large neutral hydrogen column density, N(H i), and are observed against the GRB afterglow. A large fraction of GRB-DLAs show presence of molecular hydrogen (H2) which is an indicator of star-formation. Hence it is important to study those GRB-DLAs which have H2 lines to decipher and understand their physical conditions. The GRB-DLAs 121024A and 120815A, situated at redshift 2.30 and 2.36, respectively, are two such important H2-bearing GRB-DLAs. Besides H2, these two GRB-DLAs also show many metal lines. In this work we have carried out a detail numerical study on the H2 lines, as well as on those metal lines, in GRB-DLAs 121024A and 120815A self-consistently. We use the spectral synthesis code cloudy for this study. This modelling helps us to determine the underlying physical conditions which give rise to such lines and hence to understand these two GRB-DLAs in much more detail than any other previous investigation. We find that the hydrogen densities for these two H2-bearing DLAs are ≥60 cm−3. Moreover our study infers that the linear sizes are ≤17.7 pc for these two GRB-DLAs, and the mean gas temperatures averaged over the cloud thickness, are ≤140 K. Overall, we find that these two H2-bearing GRB-DLAs are denser, cooler, and smaller compared to those without H2.

Observational estimate of the partial covering probability of quasar emission regions by distant H2 absorption clouds

ABSTRACT We have searched high spectral resolution spectra of quasars known to exhibit high redshift (z > 1.7) intervening H2-bearing damped Lyman-α (DLA) systems for partial coverage of the quasar emission by intervening H2 clouds. Partial coverage manifests itself by the presence of non-zero residual flux in the core of saturated H2 absorption lines. The residual flux can be observed either only at the bottom of absorption lines redshifted on top of quasar emission lines, in which case part of the broad line region (BLR) is not covered, or in all absorption lines, in case some continuum source is not covered. Among 35 H2 absorption clouds in 14 quasar spectra obtained with the VLT-UVES or Keck-HIRES spectrographs, we detect partial coverage of the BLR for 13 clouds. This result suggests that the probability of partial coverage of the QSO BLR by a distant H2 absorption cloud is about 40 per cent. For four systems towards Q 0013−0029, Q 0405−4418, Q 0812+3208, and J 2100−0641, partial coverage is detected for the first time. We determine the theoretical probability of partial coverage of the BLR by a distant H2 cloud as a function of the ratio between the cloud and the BLR sizes. Using this model, we obtain an estimate of the characteristic BLR radius of $50^{+19}_{-23}\,\rm{light \,days}$. This is similar to the estimate of the BLR size obtained by reverberation-mapping analysis $({\sim}100\,\rm{light\, days})$.

Testing galaxy formation simulations with damped Lyman-α abundance and metallicity evolution

ABSTRACT We examine the properties of damped Lyman-α absorbers (DLAs) emerging from a single set of cosmological initial conditions in two state-of-the-art cosmological hydrodynamic simulations: simba and technicolor dawn. The former includes star formation and black hole feedback treatments that yield a good match with low-redshift galaxy properties, while the latter uses multifrequency radiative transfer to model an inhomogeneous ultraviolet background (UVB) self-consistently and is calibrated to match the Thomson scattering optical depth, UVB amplitude, and Ly α forest mean transmission at z > 5. Both simulations are in reasonable agreement with the measured stellar mass and star formation rate functions at z ≥ 3, and both reproduce the observed neutral hydrogen cosmological mass density, $\Omega _{\rm H\, \small{I}}(z)$. However, the DLA abundance and metallicity distribution are sensitive to the galactic outflows’ feedback and the UVB amplitude. Adopting a strong UVB and/or slow outflows underproduces the observed DLA abundance, but yields broad agreement with the observed DLA metallicity distribution. By contrast, faster outflows eject metals to larger distances, yielding more metal-rich DLAs whose observational selection may be more sensitive to dust bias. The DLA metallicity distribution in models adopting an H2-regulated star formation recipe includes a tail extending to [M/H] ≪ −3, lower than any DLA observed to date, owing to curtailed star formation in low-metallicity galaxies. Our results show that DLA observations play an important role in constraining key physical ingredients in galaxy formation models, complementing traditional ensemble statistics such as the stellar mass and star formation rate functions.

Chemical enrichment and host galaxies of extremely strong intervening DLAs towards quasars

We present the results from VLT/X-shooter spectroscopic observations of 11 extremely strong intervening damped Lyman-α absorbers (ESDLAs) that were initially selected as high N(H I) (i.e. ≥5 × 1021 cm−2) candidates from the Sloan Digital Sky Survey (SDSS). We confirm the high H I column densities, which we measure to be in the range log N(H I) = 21.6 − 22.4. Molecular hydrogen is detected with high column densities (N(H2)≥1018 cm−2) in 5 out of 11 systems, 3 of which are reported here for the first time, and we obtain conservative upper limits on N(H2) for the remaining 6 systems. We also measure the column density of various metal species (Zn II, Fe II, Si II, Cr II, and C I), quantify the absorption-line kinematics (Δv90), and estimate the extinction of the background quasar light (AV) by dust in the absorbing gas. We compare the chemical properties of this sample of ESDLAs, supplemented with literature measurements, to that of DLAs located at the redshift of long-duration γ-ray bursts (GRB-DLAs). We confirm that the two populations are almost indistinguishable in terms of chemical enrichment and gas kinematics. In addition, we find no marked differences in the incidence of H2. All this suggests that ESDLAs and GRB-DLAs probe similar galactic environments. We search for the galaxy counterparts of ESDLAs and find associated emission lines in 3 out of 11 systems, 2 of which are reported here for the first time (at zabs = 2.304 and 2.323 towards the quasars SDSS J002503.03+114547.80 and SDSS J114347.21+142021.60, respectively). The measured separations between the quasar sightlines and the emission associated with the ESDLA galaxy (for a total of five sightlines) are all very small (ρ < 3 kpc). Because our observations are complete up to ρ ∼ 7 kpc, we argue that the emission counterparts of the remaining systems are more likely below the detection limit than outside the search area. While the small impact parameters are similar to what is observed for GRB-DLAs, the associated star formation rates are on average lower than for GRB host galaxies. This is explained by long-duration GRBs being associated with the death of massive stars and therefore pinpointing regions of active star formation in the GRB host galaxies. Our observations support the suggestion from the literature that ESDLAs could act as blind analogues of GRB-DLAs, probing neutral gas with high column density in the heart of high-redshift galaxies, without any prior on the instantaneous star formation rate.

A first quantification of the effects of absorption for H I intensity mapping experiments

Context. HI intensity mapping (IM) will be used to do precision cosmology, using many existing and upcoming radio observatories. It will measure the integrated HI 21 cm emission signal from “voxels” of the sky at different redshifts. The signal will be contaminated due to absorption, the largest component of which will be the flux absorbed by the HI emitting sources themselves from the potentially bright flux incident on them from background radio continuum sources. Aims. We, for the first time, provide a quantitative estimate of the magnitude of the absorbed flux compared to the emitted HI flux. The ratio of the two fluxes was calculated for various voxels placed at redshifts between 0.1 and 2.5. Methods. We used a cosmological sky simulation of the atomic HI emission line, and summed over the emitted and absorbed fluxes for all sources within voxels at different redshifts. In order to determine the absorbed flux, for each HI source the flux incident from background radio continuum sources was estimated by determining the numbers, sizes, and redshift distribution of radio continuum sources that lie behind it, based on existing observations and simulations. The amount of this incident flux that is absorbed by each HI source was calculated using a relation between integrated optical depth with HI column density determined using observations of damped Lyman-α systems (DLAs) and sub-DLAs. Results. We find that for the same co-moving volume of sky, the HI emission decreases quickly with increasing redshift, while the absorption varies much less with redshift and follows the redshift distribution of faint sources that dominate the number counts of radio continuum sources. This results in the fraction of absorption compared to emission to be negligible in the nearby Universe (up to a redshift of ∼0.5), increases to about 10% at a redshift of one, and continues to increase to about 30% up to a redshift of 2.5. These numbers can vary significantly due to the uncertainty on the exact form of the following relations: firstly, the number counts of radio continuum sources at sub-mJy flux densities; secondly, the relation between integrated optical depth and HI column density of HI sources; and thirdly, the redshift distribution of radio continuum sources up to the highest redshifts. Conclusions. Absorption of the flux incident from background radio continuum sources might become an important contaminant to HI IM signals beyond redshifts of 0.5. The impact of absorption needs to be quantified more accurately using inputs from upcoming deep surveys of radio continuum sources, H I absorption, and HI emission with the Square Kilometre Array and its precursors.

X-shooter observations of strong H2-bearing DLAs at high redshift

ABSTRACT We present results from spectroscopic observations with X-shooter at the Very Large Telescope of seven H2-bearing damped Lyman-α systems (DLAs) at high redshifts (zabs ∼ 2.5–3). These DLAs were originally selected from the presence of strong H2 lines directly seen at the DLA redshift in low-resolution low signal-to-noise ratio SDSS spectra. We confirm the detection of molecular hydrogen in all of them. We measure the column densities of H i, H2 in various rotational levels, and metal species, and associated dust extinction. The metallicities, obtained from undepleted species, are in the range log Z = −0.8 to −0.2. We discuss the chemical enrichment in these clouds and compare their properties with that of other molecular-rich systems selected by other means. In particular, we show that three different methods of pre-selection of H2-bearing DLAs in the SDSS have their own biases but complement each other mostly in terms of chemical enrichment. We use the rotational excitation of H2 molecules together with the fine-structure energy levels of neutral carbon to constrain the physical conditions in the gas with the help of numerical modelling as well as analytical expressions for the surface density at which atomic to molecular conversion happens. We find that the H2-bearing medium revealed by the studied DLAs has typical values for the kinetic temperature, hydrogen density, and UV radiation field of T ∼ 100 K, nH ∼ 100 cm−3, and IUV , respectively, about twice the intensity of the Draine field. Detailed studies combining different selections should therefore bring important clues to understand the H i-H2 transition at high redshift.