Column Density Distribution Function Research Articles

An Exploration of AGN and Stellar Feedback Effects in the Intergalactic Medium via the Low-redshift Lyα Forest

We explore the role of galactic feedback on the low-redshift Lyα (Lyα) forest (z ≲ 2) statistics and its potential to alter the thermal state of the intergalactic medium. Using the Cosmology and Astrophysics with Machine Learning Simulations (CAMELS) suite, we explore variations of the AGN and stellar feedback models in the IllustrisTNG and Simba subgrid models. We find that both AGN and stellar feedback in Simba play a role in setting the Lyα forest column density distribution function (CDD) and the Doppler width (b-value) distribution. The Simba AGN jet feedback mode is able to efficiently transport energy out to the diffuse IGM, causing changes in the shape and normalization of the CDD and a broadening of the b-value distribution. We find that stellar feedback plays a prominent role in regulating supermassive black hole growth and feedback, highlighting the importance of constraining stellar and AGN feedback simultaneously. In IllustrisTNG, the AGN feedback variations explored in CAMELS do not affect the Lyα forest, but varying the stellar feedback model does produce subtle changes. Our results imply that the low-z Lyα forest can be sensitive to changes in the ultraviolet background, stellar and black hole feedback, and that AGN jet feedback in particular can have a strong effect on the thermal state of the IGM.

The atomic-to-molecular hydrogen transition in the TNG50 simulation: Using realistic UV fields to create spatially resolved H i maps

ABSTRACT Cold gas in galaxies provides a crucial test to evaluate the realism of cosmological hydrodynamical simulations. To extract the atomic and molecular hydrogen properties of the simulated galaxy population, post-processing methods taking the local UV field into account are required. We improve upon previous studies by calculating realistic UV fields with the dust radiative transfer code SKIRT to model the atomic-to-molecular transition in TNG50, the highest-resolution run of the IllustrisTNG suite. Comparing integrated quantities such as the H i mass function, we study to what detail the UV field needs to be modelled in order to calculate realistic cold gas properties. We then evaluate new, spatially resolved comparisons for cold gas in galaxies by exploring synthetic maps of atomic hydrogen at redshift zero and compare them to 21-cm observations of local galaxies from the WHISP survey. In terms of non-parametric morphologies, we find that TNG50 H i maps are less concentrated than their WHISP counterparts (median ΔC ≈ 0.3), due in part to central H i deficits related to the ejective character of supermassive black hole feedback in TNG. In terms of the H i column density distribution function, we find discrepancies between WHISP and IllustrisTNG that depend on the total H i abundance in these data sets as well as the post-processing method. To fully exploit the synergy between cosmological simulations and upcoming deep H i/H2 data, we advocate the use of accurate methods to estimate the UV radiation field and to generate mock maps.

Examining the decline in the C iv content of the Universe over 4.3 ≲ z ≲ 6.3 using the E-XQR-30 sample

ABSTRACT Intervening C iv absorbers are key tracers of metal-enriched gas in galaxy haloes over cosmic time. Previous studies suggest that the C iv cosmic mass density ($\Omega _{\rm C \, \small {IV}}$) decreases slowly over 1.5 $\lesssim \, z\lesssim$ 5 before declining rapidly at z ≳ 5, but the cause of this downturn is poorly understood. We characterize the $\Omega _{\rm C \, \small {IV}}$ evolution over 4.3 ≲ z ≲ 6.3 using 260 absorbers found in 42 XSHOOTER spectra of z ∼ 6 quasars, of which 30 come from the ESO Large Program XQR-30. The large sample enables us to robustly constrain the rate and timing of the downturn. We find that $\Omega _{\rm C \, \small {IV}}$ decreases by a factor of 4.8 ± 2.0 over the ∼300 Myr interval between z ∼ 4.7 and ∼5.8. The slope of the column density (log N) distribution function does not change, suggesting that C iv absorption is suppressed approximately uniformly across 13.2 ≤ log N/cm−2 < 15.0. Assuming that the carbon content of galaxy haloes evolves as the integral of the cosmic star formation rate density (with some delay due to stellar lifetimes and outflow travel times), we show that chemical evolution alone could plausibly explain the fast decline in $\Omega _{\rm C \, \small {IV}}$ over 4.3 ≲ z ≲ 6.3. However, the C iv/C ii ratio decreases at the highest redshifts, so the accelerated decline in $\Omega _{\rm C \, \small {IV}}$ at z ≳ 5 may be more naturally explained by rapid changes in the gas ionization state driven by evolution of the UV background towards the end of hydrogen reionization.

The Low-redshift Lyα Forest as a Constraint for Models of AGN Feedback

We study the sensitivity of the z = 0.1 Lyα forest observables, such as the column density distribution function (CDD), flux PDF, flux power spectrum, and line-width distribution, to subgrid models of active galactic nucleus (AGN) feedback using the Illustris and IllustrisTNG (TNG) cosmological simulations. The two simulations share an identical ultraviolet background (UVB) prescription and similar cosmological parameters, but TNG features an entirely reworked AGN feedback model. Due to changes in the AGN radio-mode model, the original Illustris simulations have a factor of 2–3 fewer Lyα absorbers than TNG at column densities N H i < 1015.5 cm−2. We compare the simulated forest statistics to UV data from the Cosmic Origins Spectrograph (COS) and find that neither simulation can reproduce the slope of the absorber distribution. Both Illustris and TNG also produce significantly smaller line-width distributions than observed in the COS data. We show that TNG is in much better agreement with the observed z = 0.1 flux power spectrum than Illustris. We explore which statistics can disentangle the effects of AGN feedback from alternative UVB models by rescaling the UVB of Illustris to produce a CDD match to TNG. While this UVB rescaling is degenerate with the effect of AGN feedback on the CDD, the amplitude and shape of the flux PDF and 1D flux power spectrum change in a way distinct from the scaling of the UVB. Our study suggests that the z = 0.1 Lyα forest observables can be used as a diagnostic of AGN feedback models.

Limits on non-canonical heating and turbulence in the intergalactic medium from the low redshift Lyman α forest

ABSTRACT We examine the column density distribution function (CDDF) and Doppler parameter distribution from hydrodynamical simulations and Cosmic Origins Spectrograph (COS) observations of the Ly α forest at redshift 0 ≤ z ≤ 0.2. Allowing for a factor of two uncertainty in the metagalactic H $\, \rm \scriptstyle I \, $ photoionization rate, our hydrodynamical simulations are in good agreement (1–1.5σ) with the shape and amplitude of the observed CDDF at H $\, \rm \scriptstyle I \, $ column densities $10^{13.3}\rm \, cm^{-2}\le N_{\rm H\,{\small I}}\le 10^{14.5}\rm \, cm^{-2}$. However, the Doppler widths of the simulated lines remain too narrow with respect to the COS data. We argue that invoking AGN feedback does not resolve this discrepancy. We also disfavour enhanced photoheating rates as a potential solution, as this requires an unphysically hard UV background spectrum. If instead appealing to a non-canonical source of heating, an additional specific heat injection of $u \lesssim 6.9\rm \, eV\, m_{\rm p}^{-1}$ is required at z ≲ 2.5 for gas that has $N_{\rm H\,{\small I}}\simeq 10^{13.5}\rm \, cm^{-2}$ by z = 0.1. Alternatively, there may be an unresolved line of sight turbulent velocity component of $v_{\rm turb}\lesssim 8.5\rm \, km\, s^{-1}(N_{\rm H\,{\small I}}/10^{13.5}\rm \, cm^{-2})^{0.21}$ for the coldest gas in the diffuse IGM.

The evolution of the Si ivcontent in the Universe from the epoch of reionization to cosmic noon

ABSTRACTWe investigate the abundance and distribution of metals in the high-redshift intergalactic medium and circum-galactic medium through the analysis of a sample of almost 600 Si iv absorption lines detected in high- and intermediate-resolution spectra of 147 quasars. The evolution of the number density of Si iv lines, the column density distribution function, and the cosmic mass density are studied in the redshift interval 1.7 ≲ z ≲ 6.2 and for log N(Si iv) ≥ 12.5. All quantities show a rapid increase between z ∼ 6 and z ≲ 5 and then an almost constant behaviour to z ∼ 2 in very good agreement with what is already observed for C iv absorption lines. The present results are challenging for numerical simulations: When simulations reproduce our Si iv results, they tend to underpredict the properties of C iv, and when the properties of C iv are reproduced, the number of strong Si iv lines [log N(Si iv) &amp;gt; 14] is overpredicted.

The column densities of molecular gas across cosmic time: bridging observations and simulations

ABSTRACT Observations of the cosmic evolution of different gas phases across time indicate a marked increase in the molecular gas mass density towards z ∼ 2–3. Such a transformation implies an accompanied change in the global distribution of molecular hydrogen column densities ($N_{\rm {H_2}}$). Using observations by PHANGS-ALMA/SDSS and simulations by GRIFFIN/IllustrisTNG we explore the evolution of this H2 column density distribution function [$f(N_{\rm {H}_2})$]. The H2 (and H i) column density maps for TNG50 and TNG100 are derived in post-processing and are made available through the IllustrisTNG online API. The shape and normalization of $f(N_{\rm {H}_2})$ of individual main-sequence star-forming galaxies are correlated with the star formation rate (SFR), stellar mass (M*), and H2 mass ($M_{\rm {H}_2}$) in both observations and simulations. TNG100, combined with H2 post-processing models, broadly reproduces observations, albeit with differences in slope and normalization. Also, an analytically modelled f(N), based on exponential gas discs, matches well with the simulations. The GRIFFIN simulation gives first indications that the slope of $f(N_{\rm {H}_2})$ might not majorly differ when including non-equilibrium chemistry in simulations. The $f(N_{\rm {H}_2})$ by TNG100 implies that higher molecular gas column densities are reached at z = 3 than at z = 0. Further, denser regions contribute more to the molecular mass density at z = 3. Finally, H2 starts dominating compared to H i only at column densities above log($N_{\rm {H}_2} / \rm {cm}^{-2}) \sim 21.8\!-\!22$ at both redshifts. These results imply that neutral atomic gas is an important contributor to the overall cold gas mass found in the ISM of galaxies including at densities typical for molecular clouds at z = 0 and 3.

The H i Column Density Distribution of the Galactic Disk and Halo

Abstract We present a census of neutral gas in the Milky Way disk and halo down to limiting column densities of N(H i) ∼ 1014 cm−2 using measurements of H i Lyman series absorption from the Far Ultraviolet Spectroscopic Explorer. Our results are drawn from an analysis of 25 AGN sight lines spread evenly across the sky with Galactic latitude ∣b∣ ≳ 20°. By simultaneously fitting multi-component Voigt profiles to 11 Lyman series absorption transitions covered by FUSE (Lyβ–Lyμ) plus HST measurements of Lyα, we derive the kinematics and column densities of a sample of 152 H i absorption components. While saturation prevents accurate measurements of many components with column densities 17 ≲ log N(H i) ≲ 19, we derive robust measurements at log N(H i) ≲ 17 and log N(H i) ≳ 19. We derive the first ultraviolet H i column density distribution function (CDDF) of the Milky Way, both globally and for low-velocity (ISM), intermediate-velocity clouds (IVCs), and high-velocity clouds (HVCs). We find that IVCs and HVCs show statistically indistinguishable CDDF slopes, with β IVC = − 1.01 − 0.14 + 0.15 and β HVC = − 1.05 − 0.06 + 0.07 . Overall, the CDDF of the Galactic disk and halo appears shallower than that found by comparable extragalactic surveys, suggesting a relative abundance of high column density gas in the Galactic halo. We derive the sky-covering fractions as a function of H i column density, finding an enhancement of IVC gas in the northern hemisphere compared to the south. We also find evidence for an excess of inflowing H i over outflowing H i, with −0.88 ± 0.40 M ⊙ yr−1 of HVC inflow versus ≈0.20 ± 0.10 M ⊙ yr−1 of HVC outflow, confirming an excess of inflowing HVCs seen in UV metal lines.

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 &amp; 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 &amp;lt; z &amp;lt; 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 &amp;gt;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 &amp;lt; 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.

OH in the diffuse interstellar medium: physical modelling and prospects with upcoming SKA precursor/pathfinder surveys

ABSTRACT Hydroxyl ($\rm OH$) is known to form efficiently in cold gas (T ∼ 100 K) along with the molecule $\rm H_2$ and can be used as an efficient tracer of the diffuse molecular gas in the interstellar medium (ISM). Using a simple formalism describing the $\rm H\, I/H_2$ transition and a reduced network of major chemical reactions, we present a semi-analytical prescription to estimate the abundances of O-bearing molecules in the diffuse ISM. We show that predictions based on our prescription are in good agreement with the estimates obtained using the meudon pdr code which utilizes the full reaction network. We investigate the dependence of the relative abundances of $\rm OH/H\, I$ and $\rm OH/H_2$ on the variations of physical conditions i.e. the metallicity, number density (n), cosmic ray ionization rate (ζ), and strength of UV field (χ) in the medium. We find that the $\rm OH/H\, I$ abundances observed in the Galactic ISM can be reproduced by models with n ∼ 50 cm−3, χ ∼ 1 (Mathis field), and ζ ∼ 3 × 10−17 s−1, with a variation of about 1 dex allowed around these values. Using the constrained $\rm H_2$ column density distribution function at z ∼ 3, we estimate the $\rm OH$ column density distribution function and discuss future prospects with the upcoming large radio absorption line surveys.

The relation between Lyαabsorbers and local galaxy filaments

Context.The intergalactic medium (IGM) is believed to contain the majority of baryons in the universe and to trace the same dark matter structure as galaxies, forming filaments and sheets. Lyαabsorbers, which sample the neutral component of the IGM, have been extensively studied at low and high redshift, but the exact relation between Lyαabsorption, galaxies, and the large-scale structure is observationally not well constrained.Aims.In this study, we aim at characterising the relation between Lyαabsorbers and nearby over-dense cosmological structures (galaxy filaments) at recession velocities Δv ≤ 6700 km s−1by using archival observational data from various instruments.Methods.We analyse 587 intervening Lyαabsorbers in the spectra of 302 extragalactic background sources obtained with the Cosmic Origins Spectrograph (COS) installed on theHubbleSpace Telescope (HST). We combine the absorption line information with galaxy data of five local galaxy filaments from the V8k catalogue.Results.Along the 91 sightlines that pass close to a filament, we identify 215 (227) Lyαabsorption systems (components). Among these, 74 Lyαsystems are aligned in position and velocity with the galaxy filaments, indicating that these absorbers and the galaxies trace the same large-scale structure. The filament-aligned Lyαabsorbers have a ∼90% higher rate of incidence (d𝒩/dz = 189 for logN(H I) ≥ 13.2) and a slightly shallower column density distribution function slope (−β = −1.47) relative to the general Lyαpopulation atz = 0, reflecting the filaments’ matter over-density. The strongest Lyαabsorbers are preferentially found near galaxies or close to the axis of a filament, although there is substantial scatter in this relation. Our sample of absorbers clusters more strongly around filament axes than a randomly distributed sample would do (as confirmed by a Kolmogorov–Smirnov test), but the clustering signal is less pronounced than for the galaxies in the filaments.

The evolution of the low-density H i intergalactic medium from z = 3.6 to 0: data, transmitted flux, and H i column density,,

ABSTRACT We present a new, uniform analysis of the H i transmitted flux (F) and H i column density ($N_{\mathrm{H\,{\small I}}}$) distribution in the low-density IGM as a function of redshift z for 0 &amp;lt; z &amp;lt; 3.6 using 55 HST/COS FUV (Δz = 7.2 at z &amp;lt; 0.5), five HST/STIS + COS NUV (Δz = 1.3 at z ∼ 1) and 24 VLT/UVES, and Keck/HIRES (Δz = 11.6 at 1.7 &amp;lt; z &amp;lt; 3.6) AGN spectra. We performed a consistent, uniform Voigt profile analysis to combine spectra taken with different instruments, to reduce systematics and to remove metal-line contamination. We confirm previously known conclusions on firmer quantitative grounds in particular by improving the measurements at z ∼ 1. Two flux statistics at 0 &amp;lt; F &amp;lt; 1, the mean H i flux and the flux probability distribution function (PDF), show that considerable evolution occurs from z = 3.6 to z = 1.5, after which it slows down to become effectively stable for z &amp;lt; 0.5. However, there are large sightline variations. For the H i column density distribution function (CDDF, f ∝ $N_{\rm H\,{\small I}}^{-\beta }$) at $\log (N_{\mathrm{H\,{\small I}}}/1\, {\mathrm{cm}^{-2}})$ ∈ [13.5, 16.0], β increases as z decreases from β = 1.60 at z ∼ 3.4 to β = 1.82 at z ∼ 0.1. The CDDF shape at lower redshifts can be reproduced by a small amount of clockwise rotation of a higher-z CDDF with a slightly larger CDDF normalization. The absorption line number per z (dn/dz) shows a similar evolutionary break at z ∼ 1.5 as seen in the flux statistics. High-$N_{\mathrm{H\,{\small I}}}$ absorbers evolve more rapidly than low-$N_{\mathrm{H\,{\small I}}}$ absorbers to decrease in number or cross-section with time. The individual dn/dz shows a large scatter at a given z. The scatter increases towards lower z, possibly caused by a stronger clustering at lower z.

Connecting cosmological accretion to strong Ly α absorbers

ABSTRACT We present an analytical model for the cosmological accretion of gas on to dark matter haloes, based on a similarity solution applicable to spherical systems. Performing simplified radiative transfer, we compute how the accreting gas turns increasingly neutral as it self-shields from the ionizing background, and obtain the column density, $N_{\rm H\,{\small I}}$, as a function of impact parameter. The resulting column-density distribution function (CDDF) is in excellent agreement with observations. The analytical expression elucidates (1) why haloes over a large range in mass contribute about equally to the CDDF as well as (2) why the CDDF evolves so little with redshift in the range z = 2–5. We show that the model also predicts reasonable damped Lyman-αabsorber(DLA) line widths (v90), bias, and molecular fractions. Integrating over the CDDF yields the mass density in neutral gas, $\Omega _{\rm H\,{\small I}}$, which agrees well with observations. $\Omega _{\rm H\,{\small I}}(z)$ is nearly constant even though the accretion rate on to haloes evolves. We show that this occurs because the fraction of time that the inflowing gas is neutral depends on the dynamical time of the halo, which is inversely proportional to the accretion rate. Encapsulating results from cosmological simulations, the simple model shows that most Lyman-limit systems and DLAs are associated with the cosmological accretion of gas on to haloes.

Damped Ly α absorbers and atomic hydrogen in galaxies: the view of the GAEA model

ABSTRACT Using the GAEA semi-analytic model, we analyse the connection between Damped Ly α systems (DLAs) and H i in galaxies. Our state-of-the-art semi-analytic model is tuned to reproduce the local galaxy H i mass function, and that also reproduces other important galaxy properties, including the galaxy mass–gas metallicity relation. To produce catalogues of simulated DLAs we throw 105 random lines of sight in a composite simulated volume: dark matter haloes with log$(\frac{M_{200}}{ {\rm M}_{\odot }}) \ge 11.5$ are extracted from the Millennium Simulation, while for $9.2 \le \log (\frac{M_{200}}{ \mathrm{M}_{\odot }})\lt 11.5$ we use the Millennium II, and for $8 \le \log (\frac{M_{200}}{\mathrm{M}_{\odot }}) \lt 9.2$ a halo occupation distribution model. At 2 &amp;lt; z &amp;lt; 3, where observational data are more accurate, our fiducial model predicts the correct shape of the column density distribution function, but its normalization falls short of the observations, with the discrepancy increasing at higher redshift. The agreement with observations is significantly improved increasing both the H i masses and the disc radii of model galaxies by a factor of 2, as implemented ‘a posteriori’ in our 2M−2R model. In the redshift range of interest, haloes with $M_{200} \ge {10}^{11} \, \mathrm{M}_{\odot }$ give the major contribution to ΩDLA, and the typical DLA host halo mass is $\sim \!{10}^{11} \, \mathrm{M}_{\odot }$. The simulated DLA metallicity distribution is in relatively good agreement with observations, but our model predicts an excess of DLAs at low metallicities. Our results suggest possible improvements for the adopted modelling of the filtering mass and metal ejection in low-mass haloes.

Detecting multiple DLAs per spectrum in SDSS DR12 with Gaussian processes

ABSTRACT We present a revised version of our automated technique using Gaussian processes (GPs) to detect damped Lyman α absorbers (DLAs) along quasar (QSO) sightlines. The main improvement is to allow our GP pipeline to detect multiple DLAs along a single sightline. Our DLA detections are regularized by an improved model for the absorption from the Lyman α forest that improves performance at high redshift. We also introduce a model for unresolved sub-DLAs that reduces misclassifications of absorbers without detectable damping wings. We compare our results to those of two different large-scale DLA catalogues and provide a catalogue of the processed results of our GP pipeline using 158 825 Lyman α spectra from SDSS data release 12. We present updated estimates for the statistical properties of DLAs, including the column density distribution function, line density (dN/dX), and neutral hydrogen density (ΩDLA).

ALMACAL – VI. Molecular gas mass density across cosmic time via a blind search for intervening molecular absorbers

ABSTRACT We are just starting to understand the physical processes driving the dramatic change in cosmic star formation rate between z ∼ 2 and the present day. A quantity directly linked to star formation is the molecular gas density, which should be measured through independent methods to explore variations due to cosmic variance and systematic uncertainties. We use intervening CO absorption lines in the spectra of mm-bright background sources to provide a census of the molecular gas mass density of the Universe. The data used in this work are taken from ALMACAL, a wide and deep survey utilizing the ALMA calibrator archive. While we report multiple Galactic absorption lines and one intrinsic absorber, no extragalactic intervening molecular absorbers are detected. However, due to the large redshift path surveyed (Δz = 182), we provide constraints on the molecular column density distribution function beyond z ∼ 0. In addition, we probe column densities of N(H2) &gt; 1016 atoms cm−2, 5 orders of magnitude lower than in previous studies. We use the cosmological hydrodynamical simulation IllustrisTNG to show that our upper limits of $\rho ({\rm H}_2)\lesssim 10^{8.3}\, \text{M}_{\odot }\, \text{Mpc}^{-3}$ at 0 &lt; z ≤ 1.7 already provide new constraints on current theoretical predictions of the cold molecular phase of the gas. These results are in agreement with recent CO emission-line surveys and are complementary to those studies. The combined constraints indicate that the present decrease of the cosmic star formation rate history is consistent with an increasing depletion of molecular gas in galaxies compared to z ∼ 2.

Sub-damped Lyman α systems in the XQ-100 survey – I. Identification and contribution to the cosmological H i budget

Abstract Sub-damped Lyman α systems (subDLAs; H i column densities of 19.0 ≤ logN(H i) &lt; 20.3) are rarely included in the cosmic H i census performed at redshifts zabs ≳ 1.5, yet are expected to contribute significantly to the overall H i mass budget of the Universe. In this paper, we present a blindly selected sample of 155 subDLAs found along 100 quasar sightlines (with a redshift path-length ΔX = 475) in the XQ-100 legacy survey to investigate the contribution of subDLAs to the H i mass density of the Universe. The impact of X-Shooter’s spectral resolution on Ly α absorber identification is evaluated, and found to be sufficient for reliably finding absorbers down to a column density of logN(H i) ≥ 18.9. We compared the implications of searching for subDLAs solely using H i absorption versus the use of metal lines to confirm the identification, and found that metal-selection techniques would have missed 75 subDLAs. Using a bootstrap Monte Carlo simulation, we computed the column density distribution function (f(N, X)) and the cosmological H i mass density ($\Omega _{\rm H\,{\small I}}$) of subDLAs and compared with our previous work based on the XQ-100 damped Lyman α systems. We do not find any significant redshift evolution in f(N, X) or $\Omega _{\rm H\,{\small I}}$ for subDLAs. However, subDLAs contribute 10–20 per cent of the total $\Omega _{\rm H\,{\small I}}$ measured at redshifts 2 &lt; z &lt; 5, and thus have a small but significant contribution to the H i budget of the Universe.

Imprints of the first billion years: Lyman limit systems atz∼ 5

Lyman Limit systems (LLSs) trace the low-density circumgalactic medium and the most dense regions of the intergalactic medium, so their number density and evolution at high redshift, just after reionisation, are important to constrain. We present a survey for LLSs at high redshifts, $z_{\rm LLS} =3.5$--5.4, in the homogeneous dataset of 153 optical quasar spectra at $z \sim 5$ from the Giant Gemini GMOS survey. Our analysis includes detailed investigation of survey biases using mock spectra which provide important corrections to the raw measurements. We estimate the incidence of LLSs per unit redshift at $z \approx 4.4$ to be $\ell(z) = 2.6 \pm 0.4$. Combining our results with previous surveys at $z_{\rm LLS} <4$, the best-fit power-law evolution is $\ell(z) = \ell_* [(1+z)/4]^\alpha$ with $\ell_* = 1.46 \pm 0.11$ and $\alpha = 1.70 \pm 0.22$ (68\% confidence intervals). Despite hints in previous $z_{\rm LLS} <4$ results, there is no indication for a deviation from this single power-law soon after reionization. Finally, we integrate our new results with previous surveys of the intergalactic and circumgalactic media to constrain the hydrogen column density distribution function, $f(N_{\rm HI},X)$, over 10 orders of magnitude. The data at $z \sim 5$ are not well described by the $f(N_{\rm HI},X)$ model previously reported for $z \sim 2$--3 (after re-scaling) and a 7-pivot model fitting the full $z \sim 2$--5 dataset is statistically unacceptable. We conclude that there is significant evolution in the shape of $f(N_{\rm HI},X)$ over this $\sim$2 billion year period.

Constraining the H2 column density distribution at z ∼ 3 from composite DLA spectra

Abstract We present the detection of the average H2 absorption signal in the overall population of neutral gas absorption systems at z∼ 3 using composite absorption spectra built from the Sloan Digital Sky Survey-III damped Lyman α catalogue. We present a new technique to directly measure the H2 column density distribution function $f_{\rm H_2}(N)$ from the average H2 absorption signal. Assuming a power-law column density distribution, we obtain a slope $\beta = -1.29 \pm 0.06(\rm stat) \pm 0.10 (\rm sys)$ and an incidence rate of strong H2 absorptions [with N(H2) ≳ 1018 cm−2] to be $4.0 \pm 0.5(\rm stat) \pm 1.0 (\rm sys)\, \hbox{ per cent}$ in H i absorption systems with N(H i) ≥1020 cm−2. Assuming the same inflexion point where $f_{\rm H_2}(N)$ steepens as at z = 0, we estimate that the cosmological density of H2 in the column density range $\log N(\rm H_2) ({\rm cm}^{-2})= 18{\text{--}}22$ is ${\sim } 15\hbox{ per cent}$ of the total. We find one order of magnitude higher H2 incident rate in a sub-sample of extremely strong damped Lyman α absorption systems (DLAs) [$\log N(\rm{H\,\small {I}}) ({\rm cm}^{-2}) \ge 21.7$], which, together with the derived shape of $f_{\rm H_2}(N)$, suggests that the typical H i–H2 transition column density in DLAs is log N(H)(cm−2) ≳ 22.3 in agreement with theoretical expectations for the average (low) metallicity of DLAs at high-z.

The abundance, distribution, and physical nature of highly ionized oxygen O vi, O vii, and O viii in IllustrisTNG

We explore the abundance, spatial distribution, and physical properties of the OVI, OVII, and OVIII ions of oxygen in circumgalactic and intergalactic media (the CGM, IGM, and WHIM). We use the TNG100 and TNG300 large volume cosmological magneto-hydrodynamical simulations. Modeling the ionization states of simulated oxygen, we find good agreement with observations of the low-redshift OVI column density distribution function (CDDF), and present its evolution for all three ions from z=0 to z=4. Producing mock quasar absorption line spectral surveys, we show that the IllustrisTNG simulations are fully consistent with constraints on the OVI content of the CGM from COS-Halos and other low redshift observations, producing columns as high as observed. We measure the total amount of mass and average column densities of each ion using hundreds of thousands of simulated galaxies spanning 10^11 < Mhalo/Msun < 10^15 corresponding to 10^9 < M*/Msun < 10^12 in stellar mass. Stacked radial profiles of OVI are computed in 3D number density and 2D projected column, decomposing into the 1-halo and 2-halo terms. Relating halo OVI to properties of the central galaxy, we find a correlation between the (g-r) color of a galaxy and the total amount of OVI in its CGM. In comparison to the COS-Halos finding, this leads to a dichotomy of columns around star-forming versus passive galaxies at fixed stellar (or halo) mass. We demonstrate that this correlation is a direct result of blackhole feedback associated with quenching and represents a causal consequence of galactic-scale baryonic feedback impacting the physical state of the circumgalactic medium.