HI Column Density Distribution Research Articles

FIREbox: simulating galaxies at high dynamic range in a cosmological volume

ABSTRACT We introduce a suite of cosmological volume simulations to study the evolution of galaxies as part of the Feedback in Realistic Environments project. FIREbox, the principal simulation of the present suite, provides a representative sample of galaxies (∼1000 galaxies with $M_{\rm star}\gt 10^8\, M_\odot$ at z = 0) at a resolution ($\Delta {}x\sim {}20\, {\rm pc}$ , $m_{\rm b}\sim {}6\times {}10^4\, M_\odot$ ) comparable to state-of-the-art galaxy zoom-in simulations. FIREbox captures the multiphase nature of the interstellar medium in a fully cosmological setting (L = 22.1 Mpc) thanks to its exceptionally high dynamic range (≳106) and the inclusion of multichannel stellar feedback. Here, we focus on validating the simulation predictions by comparing to observational data. We find that star formation rates, gas masses, and metallicities of simulated galaxies with $M_{\rm star}\lt 10^{10.5-11}\, M_\odot$ broadly agree with observations. These galaxy scaling relations extend to low masses ($M_{\rm star}\sim {}10^7\, M_\odot$ ) and follow a (broken) power-law relationship. Also reproduced are the evolution of the cosmic HI density and the HI column density distribution at z ∼ 0–5. At low z , FIREbox predicts a peak in the stellar-mass–halo-mass relation but also a higher abundance of massive galaxies and a higher cosmic star formation rate density than observed, showing that stellar feedback alone is insufficient to reproduce the properties of massive galaxies at late times. Given its high resolution and sample size, FIREbox offers a baseline prediction of galaxy formation theory in a ΛCDM Universe while also highlighting modelling challenges to be addressed in next-generation galaxy simulations.

Efficient adiabatic hydrodynamical simulations of the high-redshift intergalactic medium

We present a post-processing tool for GADGET-2 adiabatic simulations to model various observed properties of the Ly$\alpha$ forest at $2.5 \leq z \leq 4$ that enables an efficient parameter estimation. In particular, we model the thermal and ionization histories that are not computed self-consistently by default in GADGET-2. We capture the effect of pressure smoothing by running GADGET-2 at an elevated temperature floor and using an appropriate smoothing kernel. We validate our procedure by comparing different statistics derived from our method with those derived using self-consistent simulations with GADGET-3. These statistics are: line of sight density field power spectrum, flux probability distribution function, flux power spectrum, wavelet statistics, curvature statistics, HI column density (${\rm N_{HI}}$) distribution function, linewidth ($b$) distribution and $b$ versus $\log {\rm N_{HI}}$ scatter. For the temperature floor of $10^4$ K and typical signal-to-noise of 25, the results agree well within 20 percent of the self-consistent GADGET-3 simulation. However, this difference is smaller than the expected $1\sigma$ sample variance for an absorption path length of $\sim 5.35$ at $z=3$. Moreover for a given cosmology, we gain a factor of $\sim N$ in computing time for modelling the intergalactic medium under $N \gg 1$ different thermal histories. In addition, our method allows us to simulate the non-equilibrium evolution of thermal and ionization state of the gas and include heating due to non-standard sources like cosmic rays and high energy $\gamma$-rays from Blazars.

The astrophysical consequences of intervening galaxy gas on fast radio bursts

We adopt and analyze results on the incidence and physical properties of damped Ly$\alpha$ systems (DLAs) to predict the astrophysical impact of gas in galaxies on observations of Fast Radio Bursts (FRBs). Three DLA measures form the basis of this analysis: (i) the HI column density distribution, parameterized as a double power-law; (ii) the incidence of DLAs with redshift (derived here), $\ell(z)=A+B \arctan(z-C)$ with $A=0.236_{-0.021}^{+0.016}, B=0.168_{-0.017}^{+0.010}, C=2.87_{-0.13}^{+0.17}$ and (iii) the electron density, parameterized as a log-normal deviate with mean $10^{-2.6} cm^{-3}$ and dispersion 0.3dex. Synthesizing these results, we estimate that the average rest-frame dispersion measure from the neutral medium of a single, intersecting galaxy is DM$^{NM}_{DLA}=0.25$ pc/cm^3. Analysis of AlIII and CII* absorption limits the putative warm ionized medium to contribute DM$^{WIM}_{DLA}<20$pc/cm^3. Given the low incidence of DLAs, we find that a population of FRBs at z=2 will incur DM(z=2)=0.01 pc/cm^3 on average, with a 99% c.l. upper bound of 0.22 pc/cm^3. Assuming that turbulence of the ISM in external galaxies is qualitatively similar to our Galaxy, we estimate that the angular broadening of an FRB by intersecting galaxies is negligible ($\theta<0.1$mas). The temporal broadening is also predicted to be small, $\tau \approx 0.3$ms for a z=1 galaxy intersecting a z=2 FRB for an observing frequency of $\nu=1$GHz. Even with $\nu=600$MHz, the fraction of sightlines broadened beyond 25ms is only approximately 0.1%. We conclude that gas within the ISM of intervening galaxies has a minor effect on the detection of FRBs and their resultant DM distributions. Download the repository at https://github.com/FRBs/FRB to repeat and extend the calculations presented here.

Simulated metal and H i absorption lines at the conclusion of reionization

We present a theoretical study of intergalactic metal absorption lines imprinted in the spectra of distant quasars during and after the Epoch of Reionization (EoR). We use high resolution hydrodynamical simulations at high redshift ($4 <z<8$), assuming a uniform UV background Haardt--Madau 12, post-processing with CLOUDY photoionization models and Voigt profile fitting to accurately calculate column densities of the ions CII, CIV, SiII, SiIV and OI in the intergalactic medium (IGM). In addition, we generate mock observations of neutral Hydrogen (HI) at $z<6$. Our simulations successfully reproduce the evolution of the cosmological mass density ($\Omega$) of CII and CIV, with $\Omega_{CII}$ exceeding $\Omega_{CIV}$ at $z >6$, consistent with the current picture of the tail of the EoR. The simulated CII exhibits a bimodal distribution with large absorptions in and around galaxies, and some traces in the lower density IGM. We find some discrepancies between the observed and simulated column density relationships among different ionic species at $z=6$, probably due to uncertainties in the assumed UV background. Finally, our simulations are in good agreement with observations of the HI column density distribution function at $z = 4$ and the HI cosmological mass density $\Omega_{HI}$ at $4 < z < 6$.

Low-redshift Lyman limit systems as diagnostics of cosmological inflows and outflows

We use cosmological hydrodynamic simulations with stellar feedback from the FIRE project to study the physical nature of Lyman limit systems (LLSs) at z<1. At these low redshifts, LLSs are closely associated with dense gas structures surrounding galaxies, such as galactic winds, dwarf satellites, and cool inflows from the intergalactic medium. Our analysis is based on 14 zoom-in simulations covering the halo mass range M_h~10^9-10^13 Msun at z=0, which we convolve with the dark matter halo mass function to produce cosmological statistics. We find that the majority of cosmologically-selected LLSs are associated with halos in the mass range 10^10 < M_h < 10^12 Msun. The incidence and HI column density distribution of simulated absorbers with columns 10^16.2 < N_HI < 2x10^20 cm^-2 are consistent with observations. High-velocity outflows (with radial velocity exceeding the halo circular velocity by a factor >~2) tend to have higher metallicities ([X/H] ~ -0.5) while very low metallicity ([X/H] < -2) LLSs are typically associated with gas infalling from the intergalactic medium. However, most LLSs occupy an intermediate region in metallicity-radial velocity space, for which there is no clear trend between metallicity and radial kinematics. Metal-enriched inflows arise in the FIRE simulations as a result of galactic winds that fall back onto galaxies at low redshift. The overall simulated LLS metallicity distribution has a mean (standard deviation) [X/H] = -0.9 (0.4) and does not show significant evidence for bimodality, in contrast to recent observational studies but consistent with LLSs arising from halos with a broad range of masses and metallicities.

Constraining a halo model for cosmological neutral hydrogen

We describe a combined halo model to constrain the distribution of neutral hydrogen (HI) in the post-reionization universe. We combine constraints from the various probes of HI at different redshifts: the low-redshift 21-cm emission line surveys, intensity mapping experiments at intermediate redshifts, and the Damped Lyman-Alpha (DLA) observations at higher redshifts. We use a Markov Chain Monte Carlo (MCMC) approach to combine the observations and place constraints on the free parameters in the model. Our best-fit model involves a relation between neutral hydrogen mass $M_{\rm HI}$ and halo mass $M$ with a non-unit slope, and an upper and a lower cutoff. We find that the model fits all the observables but leads to an underprediction of the bias parameter of DLAs at $z \sim 2.3$. We also find indications of a possible tension between the HI column density distribution and the mass function of HI-selected galaxies at $z\sim 0$. We provide the central values of the parameters of the best-fit model so derived. We also provide a fitting form for the derived evolution of the concentration parameter of HI in dark matter haloes, and discuss the implications for the redshift evolution of the HI-halo mass relation.

The Effelsberg-Bonn H i Survey: Milky Way gas

The Effelsberg-Bonn HI Survey (EBHIS) is a new 21-cm survey performed with the 100-m telescope at Effelsberg. It covers the whole northern sky out to a redshift of z~0.07 and comprises HI line emission from the Milky Way and the Local Volume. We aim to substitute the northern-hemisphere part of the Leiden/Argentine/Bonn Milky Way HI survey (LAB) with this first EBHIS data release, which presents the HI gas in the Milky Way regime. The use of a seven-beam L-band array made it feasible to perform this all-sky survey with a 100-m class telescope in a reasonable amount of observing time. State-of-the-art fast-Fourier-transform spectrometers provide the necessary data read-out speed, dynamic range, and spectral resolution to apply software radio-frequency interference mitigation. EBHIS is corrected for stray radiation and employs frequency-dependent flux-density calibration and sophisticated baseline-removal techniques to ensure the highest possible data quality. Detailed analyses of the resulting data products show that EBHIS is not only outperforming LAB in terms of sensitivity and angular resolution, but also matches the intensity-scale of LAB extremely well, allowing EBHIS to be used as a drop-in replacement for LAB. Data products are made available to the public in a variety of forms. Most important, we provide a properly gridded Milky Way HI column density map in HEALPix representation. To maximize the usefulness of EBHIS data, we estimate uncertainties in the HI column density and brightness temperature distributions, accounting for systematic effects.

COLD AND WARM ATOMIC GAS AROUND THE PERSEUS MOLECULAR CLOUD. II. THE IMPACT OF HIGH OPTICAL DEPTH ON THE HI COLUMN DENSITY DISTRIBUTION AND ITS IMPLICATION FOR THE HI-TO-H2TRANSITION

We investigate the impact of high optical depth on the HI saturation observed in the Perseus molecular cloud by using Arecibo HI emission and absorption measurements toward 26 radio continuum sources. The spin temperature and optical depth of individual HI components are derived along each line-of-sight, enabling us to estimate the correction for high optical depth. We examine two different methods for the correction, Gaussian decomposition and isothermal methods, and find that they are consistent (maximum correction factor ~ 1.2) likely due to the relatively low optical depth and insignificant contribution from the diffuse radio continuum emission for Perseus. We apply the correction to the optically thin HI column density on a pixel-by-pixel basis, and find that the total HI mass increases by ~10%. Using the corrected HI column density image and far-infrared data from the IRIS Survey, we then derive the H2 column density on ~0.4 pc scales. For five dark and star-forming sub-regions, the HI surface density is uniform with Sigma_HI ~ 7-9 solar mass/pc2, in agreement with the minimum HI surface density required for shielding H2 against photodissociation. As a result, Sigma_H2/Sigma_HI and Sigma_HI+Sigma_H2 show a tight relation. Our results are consistent with predictions for H2 formation in steady state and chemical equilibrium, and suggest that H2 formation is mainly responsible for the Sigma_HI saturation in Perseus. We also compare the optically thick HI with the observed "CO-dark" gas, and find that the optically thick HI only accounts for ~20% of the "CO-dark" gas in Perseus.

Investigating the physics and environment of Lyman limit systems in cosmological simulations

In this work, I investigate the properties of Lyman limit systems (LLSs) using state-of-the-art zoom-in cosmological galaxy formation simulations with on the fly radiative transfer, which includes both the cosmic UV background (UVB) and local stellar sources. I compare the simulation results to observations of the incidence frequency of LLSs and the HI column density distribution function over the redshift range $z=2-5$ and find good agreement. I explore the connection between LLSs and their host halos and find that LLSs reside in halos with a wide range of halo masses with a nearly constant covering fraction within a virial radius. Over the range $z=2-5$, I find that more than half of the LLSs reside in halos with $M < 10^{10}h^{-1}M_\odot$, indicating that absorption line studies of LLSs can probe these low-mass galaxies which H$_2$-based star formation models predict to have very little star formation. I study the physical state of individual LLSs and test a simple model (Schaye 2001) which encapsulates many of their properties. I confirm that LLSs have a characteristic absorption length given by the Jeans length and that they are in photoionization equilibrium at low column densities. Finally, I investigate the self-shielding of LLSs to the UVB and explore how the non-sphericity of LLSs affects the photoionization rate at a given $N_{\rm HI}$. I find that at $z\approx 3$, LLSs have an optical depth of unity at a column density of $\sim 10^{18} {\rm cm}^{-2}$ and that this is the column density which characterizes the onset of self-shielding.

A consistent determination of the temperature of the intergalactic medium at redshift 〈z〉 = 2.4

We present new measurements of the thermal state of the intergalactic medium (IGM) at $z\sim2.4$ derived from absorption line profiles in the Ly$\alpha$ forest. We use a large set of high-resolution hydrodynamical simulations to calibrate the relationship between the temperature-density ($T$--$\Delta$) relation in the IGM and the distribution of HI column densities, $N_{\rm HI}$, and velocity widths, $b_{\rm HI}$, of discrete Ly$\alpha$ forest absorbers. This calibration is then applied to the measurement of the lower cut-off of the $b_{\rm HI}$--$N_{\rm HI}$ distribution recently presented by Rudie et al. (2012). We infer a power-law $T$--$\Delta$ relation, $T=T_{0}\Delta^{\gamma-1}$, with a temperature at mean density, $T_{0}=[1.00^{+0.32}_{-0.21}]\times10^{4}\rm\,K$ and slope $(\gamma-1)=0.54\pm0.11$. The slope is fully consistent with that advocated by the analysis of Rudie et al (2012); however, the temperature at mean density is lower by almost a factor of two, primarily due to an adjustment in the relationship between column density and physical density assumed by these authors. These new results are in excellent agreement with the recent temperature measurements of Becker et al. (2011), based on the curvature of the transmitted flux in the Ly$\alpha$ forest. This suggests that the thermal state of the IGM at this redshift is reasonably well characterised over the range of densities probed by these methods.

The evolution of H i and C iv quasar absorption line systems at 1.9 &lt;z&lt; 3.2

We have investigated the distribution and evolution of ~3100 intergalactic HI absorbers with HI column densities log N(HI) = [12.75, 17.0] at 1.9 < z < 3.2, using 18 high resolution, high S/N quasar spectra obtained from the ESO VLT/UVES archive. We used two sets of Voigt profile fitting analysis, one including all the available high-order Lyman lines to obtain reliable HI column densities of saturated lines, and another using only the Ly-alpha lines. There is no significant difference between the results from the two fits. Combining our results with literature data, the mean number density at 0 < z < 4 is not well described by a single power law and strongly suggests that its evolution slows down at z < 1.5 at the high and low column density ranges. We also divided our entire HI absorbers at 1.9 < z < 3.2 into two samples, the unenriched forest and the CIV-enriched forest, depending on whether HI lines are associated with CIV at log N(CIV) > 12.2 within a given velocity range. The entire HI column density distribution function (CDDF) can be described as the combination of these two well-characterised populations which overlap at log N(HI) ~ 15. At log N(HI) < 15, the unenriched forest dominates, showing a similar power-law distribution to the entire forest. The CIV-enriched forest dominates at log N(HI) > 15, with its distribution function proportional to N(HI)^(-1.45). However, it starts to flatten out at lower N(HI), since the enriched forest fraction decreases with decreasing N(HI). The deviation from the power law at log N(HI) = [14, 17] shown in the CDDF for the entire HI sample is a result of combining two different HI populations with a different CDDF shape. The total HI mass density relative to the critical density is Omega(HI) ~ 1.6 x 10^(-6) h^(-1), where the enriched forest accounts for ~40% of Omega(HI).

A JOINT MODEL OF THE X-RAY AND INFRARED EXTRAGALACTIC BACKGROUNDS. I. MODEL CONSTRUCTION AND FIRST RESULTS

We present an extragalactic population model of the cosmic background light to interpret the rich high-quality survey data in the X-ray and IR bands. The model incorporates star-formation and supermassive black hole (SMBH) accretion in a co-evolution scenario to fit simultaneously 617 data points of number counts, redshift distributions and local luminosity functions (LFs) with 19 free parameters. The model has four main components, the total IR LF, the SMBH accretion energy fraction in the IR band, the star-formation SED and the unobscured SMBH SED extinguished with a HI column density distribution. As a result of the observational uncertainties about the star-formation and SMBH SEDs, we present several variants of the model. The best-fit reduced chi^2 reaches as small as 2.7-2.9 of which a significant amount (>0.8) is contributed by cosmic variances or caveats associated with data. Compared to previous models, the unique result of this model is to constrain the SMBH energy fraction in the IR band that is found to increase with the IR luminosity but decrease with redshift up to z ~ 1.5; this result is separately verified using aromatic feature equivalent width data. The joint modelling of X-ray and mid-IR data allows for improved constraints on the obscured AGN, especially the Compton-thick AGN population. All variants of the model require that Compton-thick AGN fractions decrease with the SMBH luminosity but increase with redshift while the type-1 AGN fraction has the reverse trend.

The H i column density distribution function in faint dwarf galaxies

We present the HI column density distribution function,f(NHI), as measured from dwarf galaxies observed as part of the Faint Irregular Galaxy GMRT (FIGGS) survey. We find that the shape of the dwarf galaxy f(NHI) is significantly different from the f(NHI) for high redshift Damped Lyman − α absorbers (DLAs) or the f(NHI) for a representative sample of z = 0 gas rich galaxies. The dwarf f(NHI) falls much more steeply at high HI column densities as compared to the other determinations. While ∼ 10% of the cross section above NHI = 10 20.3 atoms cm at z = 0 is provided by dwarf galaxies, the fraction falls to . 1% by NHI ∼ 10 atoms cm. In the local universe, the contribution to the high NHI end of the f(NHI) distribution comes predominantly from the inclined disks of large galaxies. Dwarf galaxies, both because of their smaller scale lengths, and their larger intrinsic axial ratios do not produce large HI column densities even when viewed edge-on. If high column density DLAs/GRB hosts correspond to galaxies like the local dwarfs, this would require either that (i) the absorption arises from merging and not isolated systems or (ii) the observed lines of sight are strongly biased towards high column density regions.

ON THE ORIGIN OF THE HIGH COLUMN DENSITY TURNOVER IN THE H I COLUMN DENSITY DISTRIBUTION

We study the high column density regime of the HI column density distribution function and argue that there are two distinct features: a turnover at NHI ~ 10^21 cm^-2 which is present at both z=0 and z ~ 3, and a lack of systems above NHI ~ 10^22 cm^-2 at z=0. Using observations of the column density distribution, we argue that the HI-H2 transition does not cause the turnover at NHI ~ 10^21 cm^-2, but can plausibly explain the turnover at NHI > 10^22 cm^-2. We compute the HI column density distribution of individual galaxies in the THINGS sample and show that the turnover column density depends only weakly on metallicity. Furthermore, we show that the column density distribution of galaxies, corrected for inclination, is insensitive to the resolution of the HI map or to averaging in radial shells. Our results indicate that the similarity of HI column density distributions at z=3 and z=0 is due to the similarity of the maximum HI surface densities of high-z and low-z disks, set presumably by universal processes that shape properties of the gaseous disks of galaxies. Using fully cosmological simulations, we explore other candidate physical mechanisms that could produce a turnover in the column density distribution. We show that while turbulence within GMCs cannot affect the DLA column density distribution, stellar feedback can affect it significantly if the feedback is sufficiently effective in removing gas from the central 2-3 kpc of high-redshift galaxies. Finally, we argue that it is meaningful to compare column densities averaged over ~ kpc scales with those estimated from quasar spectra which probe sub-pc scales due to the steep power spectrum of HI column density fluctuations observed in nearby galaxies.

Absorption signatures of warm-hot gas at low redshift: broad H i Lyα absorbers

We investigate the physical state of HI absorbing gas at low redshift (z = 0.25) using a subset of cosmological, hydrodynamic simulations from the OWLS project, focusing in particular on broad (b_HI > 40 km/s) HI Lyman-Alpha absorbers (BLAs), which are believed to originate in shock-heated gas in the warm-hot intergalactic medium (WHIM). Our fiducial model, which includes radiative cooling by heavy elements and feedback by supernovae and active galactic nuclei, predicts that by z = 0.25 nearly 60 per cent of the gas mass ends up at densities and temperatures characteristic of the WHIM and we find that half of this fraction is due to outflows. The standard HI observables (distribution of HI column densities N_HI, distribution of Doppler parameters b_HI, b_HI - N_HI correlation) and the BLA line number density predicted by our simulations are in remarkably good agreement with observations. BLAs arise in gas that is hotter, more highly ionised and more enriched than the gas giving rise to typical Lyman-Alpha forest absorbers. The majority of the BLAs arise in warm-hot (log (T/K) ~ 5) gas at low (log Delta < 1.5) overdensities. On average, thermal broadening accounts for at least 60 per cent of the BLA line width, which in turn can be used as a rough indicator of the thermal state of the gas. Detectable BLAs account for only a small fraction of the true baryon content of the WHIM at low redshift. In order to detect the bulk of the mass in this gas phase, a sensitivity at least one order of magnitude better than achieved by current ultraviolet spectrographs is required. We argue that BLAs mostly trace gas that has been shock-heated and enriched by outflows and that they therefore provide an important window on a poorly understood feedback process.

The nature of H i absorbers in gamma-ray burst afterglows: clues from hydrodynamic simulations

In recent work, we have shown that it is possible to link quantitatively many aspects of damped Lyman alpha (DLA) absorbers in the spectra of quasars to high resolution simulations of galaxy formation. Using runs from the same series of hydrodynamic numerical studies, we consider the expected properties of Lyman alpha absorbers seen in the spectra of high redshift (z>2) gamma ray burst afterglows (GRB-DLAs). If GRBs are associated with the death of massive stars, their afterglows provide insights into otherwise unprobed regions of protogalactic objects, but detailed physical interpretations are currently embryonic. We find that median impact parameters (measured from the potential minimum) are approximately 1 kpc for GRBs compared with 4 kpc for QSO-DLAs. However, an equally important difference is that GRB-DLAs are predominantly associated with halos of mass 10^10<M_vir/M_sol<10^12, an order of magnitude larger than the hosts of QSO-DLAs. Accordingly, there are differences in the stellar properties of hosts. Our simulations accurately predict the form of the GRB-DLA HI column density distribution, producing quantitative agreement for N_HI>10^19 cm^-2, but they somewhat underpredict the incidence of low column densities N_HI<10^19 cm^-2. Line-of-sight neutral gas metallicities predicted by our simulations (10^-2 < Z/Z_sol < 1) are consistent with the modest observational constraints. Because of large internal dispersions in gas metallicities, this agreement is not significantly compromised by imposing a cut-off on the metallicity of stars able to launch GRBs (Z_star<Z_sol/3), confounding claims that the observed metallicity of GRB-DLAs poses a challenge to current GRB models. (Abridged.)

On the Origin of Anomalous Velocity Clouds in the Milky Way

We report that neutral hydrogen (HI) gas clouds, resembling High Velocity Clouds (HVCs) observed in the Milky Way (MW), appear in MW-sized disk galaxies formed in high-resolution Lambda Cold Dark Matter (LCDM) cosmological simulations which include gas-dynamics, radiative cooling, star formation, supernova feedback, and metal enrichment. Two such disk galaxies are analyzed, and HI column density and velocity distributions in all-sky Aitoff projections are constructed. The simulations demonstrate that LCDM is able to create galaxies with sufficient numbers of anomalous velocity gas clouds consistent with the HVCs observed within the MW, and that they are found within a galactocentric radius of 150 kpc. We also find that one of the galaxies has a polar gas ring, with radius 30 kpc, which appears as a large structure of HVCs in the Aitoff projection. Such large structures may share an origin similar to extended HVCs observed in the MW, such as Complex C.

Reconciling the local galaxy population with damped Lyman α cross-sections and metal abundances

A comprehensive analysis of 355 high-quality Westerbork Synthesis Radio Telescope (WSRT) HI 21-cm line maps of nearby galaxies shows that the properties and incident rate of damped Lyman alpha absorption systems (DLAs) observed in the spectra of high-redshift QSOs are in good agreement with DLAs originating in gas discs of galaxies like those in the z approximate to 0 population. Comparison of low-z DLA statistics with the HI incidence rate and column density distribution f(N (HI)) for the local galaxy sample shows no evidence for evolution in the integral 'cross-section density' = l(-1) (l = mean free path between absorbers) below z approximate to 1.5, implying that there is no need for a hidden population of galaxies or H I clouds to contribute significantly to the DLA cross-section. Compared with z approximate to 4, our data indicate evolution of a factor of 2 in the comoving density along a line of sight. We find that dN/dz(z = 0) = 0.045 +/- 0.006. The idea that the local galaxy population can explain the DLAs is further strengthened by comparing the properties of DLAs and DLA galaxies with the expectations based on our analysis of local galaxies. The distribution of luminosities of DLA host galaxies, and of impact parameters between QSOs and the centres of DLA galaxies, is in good agreement with what is expected from local galaxies. Approximately 87 per cent of low-z DLA galaxies are expected to be fainter than L-*, and 37 per cent have impact parameters less than 1 arcsec at z = 0.5. The analysis shows that some host galaxies with very low impact parameters and low luminosities are expected to be missed in optical follow-up surveys. The well-known metallicity-luminosity relation in galaxies, in combination with metallicity gradients in galaxy discs, causes the expected median metallicity of low-z DLAs to be low (similar to 1/7 solar), which is also in good agreement with observations of low-z DLAs. We find that f(N H I) can be fitted satisfactorily with a gamma distribution, a single power law is not a good fit at the highest column densities N H I > 10(21) cm(-2). The vast majority (approximate to 81 per cent) of the H I gas in the local Universe resides in column densities above the classical DLA limit (N (H I) > 2 x 10(20) cm(-2)), with N (H I) similar to 10(21) cm(-2) dominating the cosmic H I mass density.

HI Column Density Distribution Function at z = 0: Connection to Damped Lyα Statistics

AbstractWe present a measurement of the HI column density distribution function f(Nhi) at the present epoch for column densities &gt; 1020 cm−2. These high column densities compare to those measured in damped Lyα lines seen in absorption against background quasars. Although observationally rare, it appears that the bulk of the neutral gas in the Universe is associated with these damped Lyα systems. In order to obtain a good anchor point at z = 0 we determine f(Nhi) in the local Universe by using 21 cm synthesis observations of a complete sample of spiral galaxies. We show that f(Nhi) for damped Lyα systems has changed significantly from high z to the present and that change is greatest for the highest column densities. The measurements indicate that low surface brightness galaxies make a minor contribution to the cross section for HI, especially for NHI &gt; 1021 cm−2.

APM z ≳ 4 survey: distribution and evolution of high column density H i absorbers

Eleven candidate damped Lya absorption systems were identified in 27 spectra of the quasars from the APM z>4 survey covering the redshift range 2.8<z(abs)<4.4 (8 with z(abs)>3.5). High resolution echelle spectra (0.8A FWHM) have been obtained for three quasars, including 2 of the highest redshift objects in the survey. Two damped systems have confirmed HI column densities of N(HI) >= 10^20.3 atoms cm^-2, with a third falling just below this threshold. We have discovered the highest redshift damped Lya absorber known at z=4.383 in QSO BR1202-0725. The APM QSOs provide a substantial increase in the redshift path available for damped surveys for z>3. We combine this high redshift sample with other quasar samples covering the redshift range 0.008 < z < 4.7 to study the redshift evolution and the column density distribution function for absorbers with log N(HI)>=17.2. In the HI column density distribution f(N)=kN^-beta we find evidence for breaks in the power law, flattening for 17.2< log N(HI)<21 and steepening for log N(HI)>21.2. The column density distribution function for the data with log N(HI)>=20.3 is better fit with the form f(N)=(f*/N*)(N/N*)^-beta exp(-N/N*). Significant redshift evolution in the number density per unit redshift is evident in the higher column density systems with an apparent decline in N(z) for z>3.5.