Spectra Of Background Quasars Research Articles

Searching for the imprints of AGN feedback on the Lyman alpha forest around luminous red galaxies

ABSTRACT We explore the potential of using the low-redshift Lyman $\alpha$ (Ly $\alpha$) forest surrounding luminous red galaxies (LRGs) as a tool to constrain active galactic nuclei (AGNs) feedback models. Our analysis is based on snapshots from the Illustris and IllustrisTNG simulations at a redshift of $z=0.1$. These simulations offer an ideal platform for studying the influence of AGN feedback on the gas surrounding galaxies, as they share the same initial conditions and underlying code but incorporate different feedback prescriptions. Both simulations show significant impacts of feedback on the temperature and density of the gas around massive haloes. Following our previous work, we adjusted the UV background in both simulations to align with the observed number density of Ly $\alpha$ lines ($\rm dN/dz$) in the intergalactic medium and study the Ly $\alpha$ forest around massive haloes hosting LRGs, at impact parameters ($r_{\perp }$) ranging from 0.1 to 100 pMpc. Our findings reveal that $\rm dN/dz$, as a function of $r_{\perp }$, is approximately 1.5 to 2 times higher in IllustrisTNG compared to Illustris up to $r_{\perp }$ of $\sim 10$ pMpc. To further assess whether existing data can effectively discern these differences, we search for archival data containing spectra of background quasars probing foreground LRGs. Through a feasibility analysis based on these data, we demonstrate that ${\rm dN/dz} (r_{\perp })$ measurements can distinguish between feedback models of IllustrisTNG and Illustris with a precision exceeding 16$\sigma$. This underscores the potential of ${\rm dN/dz} (r_{\perp })$ measurements around LRGs as a valuable benchmark observation for discriminating between different feedback models.

Strong nebular emissions associated with Mg ii absorptions detected in the SDSS spectra of background quasars

ABSTRACT We present long-slit spectroscopic observations of 40 Galaxy On Top of Quasars (GOTOQs) at ${0.37 \leqslant z \leqslant 1.01}$ using the South African Large Telescope. Using this and available photometric data, we measure the impact parameters of the foreground galaxies to be in the range of 3–16 kpc with a median value of 8.6 kpc. This is the largest sample of galaxies producing Mg ii absorption at such low impact parameters. These quasar–galaxy pairs are ideal for probing the disc–halo interface. At such impact parameters, we do not find any anticorrelation between rest equivalent width (REW) of Ca ii, Mn ii, Fe ii, Mg ii, and Mg i absorption and impact parameter. These sight lines are typically redder than those of strong Mg ii absorbers, with the colour excess, E(B − V) for our sample ranging from −0.191 to 0.422, with a median value of 0.058. In the E(B − V) versus W3935 plane, GOTOQs occupy the same region as Ca ii absorbers. For a given E(B − V), we find larger W3935 than what has been found in the Milky Way, probably due to a smaller dust-to-gas ratio in GOTOQs. Galaxy parameters could be measured for twelve cases, and their properties seem to follow the trends found for strong Mg ii absorbers. Measuring the host galaxy properties for the full sample using HST photometry or AO-assisted ground-based imaging is important to gain insights into the relationship between the stellar mass of galaxies and the metal line REW distributions at low impact parameters.

MUSEQuBES: mapping the distribution of neutral hydrogen around low-redshift galaxies

ABSTRACT We present a detailed study of cool, neutral gas traced by Lyα around 4595 z < 0.5 galaxies using stacks of background quasar spectra. The galaxies are selected from our MUSEQuBES low-z survey along with data from the literature. These galaxies, with a median stellar mass of ${\rm log}_{10}(M_*/\rm {\rm M}_{\odot })$ =10.0, are probed by 184 background quasars giving rise to 5054 quasar-galaxy pairs. The median impact parameter is b = 1.5 pMpc (median b/Rvir = 10.4) with 204 (419) quasar-galaxy pairs probing b/Rvir < 1(2). We find excess absorption out to at least ≈15Rvir transverse distance and ≈±600 $\rm km~s^{-1}$ along the line of sight. We show that the median stacked profile for the full sample, dominated by the pairs with b > Rvir, can be explained by a galaxy-absorber two-point correlation function with r0 = 7.6 pMpc and γ = −1.57. There are strong indications that the inner regions (≲Rvir) of the rest equivalent width profile are better explained by a log-linear (or a Gaussian) relation, whereas the outer regions are well described by a power-law, consistent with galaxy-absorber large-scale clustering. Using a sub-sample of 339 galaxies (442 quasar-galaxy pairs, median b/Rvir = 1.6) with star formation rate measurements, we find that the Lyα absorption is significantly stronger for star-forming galaxies compared to passive galaxies, but only within the virial radius. The Lyα absorption at b ≈ Rvir for a redshift-controlled sample peaks at $M_* \sim 10^9~ {\rm {\rm M}_{\odot }}~ (M_{\rm halo} \sim 10^{11}~ \rm {\rm M}_{\odot })$.

Canonical Hubble-Tension-Resolving Early Dark Energy Cosmologies Are Inconsistent with the Lyman-α Forest.

Current cosmological data exhibit discordance between indirect and some direct inferences of the present-day expansion rate H_{0}. Early dark energy (EDE), which briefly increases the cosmic expansion rate prior to recombination, is a leading scenario for resolving this "Hubble tension" while preserving a good fit to cosmic microwave background (CMB) data. However, this comes at the cost of changes in parameters that affect structure formation in the late-time universe, including the spectral index of scalar perturbations n_{s}. Here, we present the first constraints on axionlike EDE using data from the Lyman-α forest, i.e., absorption lines imprinted in background quasar spectra by neutral hydrogen gas along the line of sight. We consider two independent measurements of the one-dimensional Lyα forest flux power spectrum from the Sloan Digital Sky Survey (SDSS eBOSS) and from the MIKE/HIRES and X-Shooter spectrographs. We combine these with a baseline dataset comprised of Planck CMB data and baryon acoustic oscillation (BAO) measurements. Combining the eBOSS Lyα data with the CMB and BAO dataset reduces the 95% confidence level (C.L.) upper bound on the maximum fractional contribution of EDE to the cosmic energy budget f_{EDE} from 0.07 to 0.03 and constrains H_{0}=67.9_{-0.4}^{+0.4} km/s/Mpc (68% C.L.), with maximum aposteriori value H_{0}=67.9 km/s/Mpc. Similar results are obtained for the MIKE/HIRES and X-Shooter Lyα data. Our Lyα-based EDE constraints yield H_{0} values that are in >4σ tension with the SH0ES distance-ladder measurement and are driven by the preference of the Lyα forest data for n_{s} values lower than those required by EDE cosmologies that fit Planck CMB data. Taken at face value, the Lyα forest severely constrains canonical EDE models that could resolve the Hubble tension.

MusE GAs FLOw and Wind (MEGAFLOW) IX. The impact of gas flows on the relations between the mass, star formation rate, and metallicity of galaxies

ABSTRACTWe study the link between gas flow events and key galaxy scaling relations: the relations between star formation rate (SFR) and stellar mass (the main sequence, MS); gas metallicity and stellar mass (the mass–metallicity relation, MZR); and gas metallicity, stellar mass, and SFR (the fundamental metallicity relation, FMR). Using all star-forming galaxies (SFGs) in the 22 MUSE fields of the MusE GAs FLOw and Wind (MEGAFLOW) survey, we derive the MS, MZR, and FMR scaling relations for 385 SFGs with $M_\star = 10^{8} - 10^{11.5}\, {\rm M}_\odot$ at redshifts 0.35 < z < 0.85. Using the MUSE data and complementary X-Shooter spectra at 0.85 < z < 1.4, we determine the locations of 21 SFGs associated with inflowing or outflowing circumgalactic gas (i.e. with strong Mg ii absorption in background quasar spectra) relative to these scaling relations. Compared to a control sample of galaxies without gas flows (i.e. without Mg ii absorption within 70 kpc of the quasar), SFGs with inflow events (i.e. Mg ii absorption along the major axis) are preferentially located above the MS, while SFGs with outflow events (i.e. Mg ii absorption along the minor axis) are preferentially more metal rich. Our observations support the scenario in which gas accretion increases the SFR while diluting the metal content and where circumgalactic outflows are found in more metal-rich galaxies.

A Giant Arc on the Sky

ABSTRACT We present the serendipitous discovery of a ‘Giant Arc on the Sky’ at $z$ ∼ 0.8. The Giant Arc (GA) spans ∼1 Gpc (proper size, present epoch) and appears to be almost symmetrical on the sky. It was discovered via intervening Mg ii absorbers in the spectra of background quasars, using the catalogues of Zhu & Ménard. The use of Mg ii absorbers represents a new approach to the investigation of large-scale structures (LSSs) at redshifts $0.45 \la z \la 2.25$. We present the observational properties of the GA, and we assess it statistically using methods based on (i) single-linkage hierarchical clustering (∼4.5σ); (ii) the Cuzick-Edwards test (∼3.0σ); and (iii) power-spectrum analysis (∼4.8σ). Each of these methods has distinctive attributes and powers, and we advise considering the evidence from the ensemble. We discuss our approaches to mitigating any post hoc aspects of analysing significance after discovery. The overdensity of the GA is δρ/ρ ∼ 1.3 ± 0.3. The GA is the newest and one of the largest of a steadily accumulating set of very large LSSs that may (cautiously) challenge the Cosmological Principle, upon which the ‘standard model’ of cosmology is founded. Conceivably, the GA is the precursor of a structure like the Sloan Great Wall (but the GA is about twice the size), seen when the Universe was about half its present age.

Exploratory Study of the Transverse Proximity Effect around BAL Quasars

We aim to determine why there exists anisotropic H i absorption around quasars; i.e., the environments around quasars are highly biased toward producing strong H i absorption in the transverse direction while there exists a significant deficit of H i absorption within a few megaparsecs of quasars along the line of sight. The most plausible explanation for this opposite trend is that the transverse direction is shielded from quasar UV radiation by dust torus. However, a critical weakness of this explanation is that we do not have any information on the inclination angle of our sightline relative to the torus. In this study, we examine environments of quasars with broad-absorption troughs in their spectra (i.e., BAL quasars) because it is widely believed that BAL troughs are observed if the central continuum is viewed from the side through their powerful outflows near the dust torus. With closely separated 12 projected quasar pairs at different redshifts with a separation angle of θ < 120″, we examine H i absorption at foreground BAL quasars in the spectra of background quasars. We confirm that there exists optically thick gas around two of 12 BAL quasars, and that the mean H i absorption strength is EWrest ∼ 1 Å. This is consistent with past results of studies of non-BAL quasars, although not statistically significant. The origins of optically thick H i absorbers around BAL and non-BAL quasars could be different since their column densities are different by ∼3 orders of magnitude. A larger sample is required to narrow down possible scenarios explaining the anisotropic H i absorption around quasars.

Resolving the H I in damped Lyman α systems that power star formation.

Reservoirs of dense atomic gas (primarily hydrogen) contain approximately 90 per cent of the neutral gas at a redshift of 3, and contribute to between 2 and 3 per cent of the total baryons in the Universe1-4. These 'damped Lyman α systems'-so called because they absorb Lyman α photons within and from background sources-have been studied for decades, but only through absorption lines present in the spectra of background quasars and γ-ray bursts5-10. Such pencil beams do not constrain the physical extent of the systems. Here we report integral-field spectroscopy of a bright, gravitationally lensed galaxy at a redshift of 2.7 with two foreground damped Lyman α systems. These systems are greater than 238 kiloparsecs squared in extent, with column densities of neutral hydrogen varying by more than an order of magnitude on scales of less than 3 kiloparsecs. The mean column densities are between 1020.46 and 1020.84 centimetres squared and the total masses are greater than 5.5 × 108-1.4 × 109 times the mass of the Sun, showing that they contain the necessary fuel for the next generation of star formation, consistent with relatively massive, low-luminosity primeval galaxies at redshifts greater than 2.

Spatially resolved gas flows around the Milky Way

ABSTRACT We present spatially resolved measurements of cool gas flowing into and out of the Milky Way (MW), using archival ultraviolet spectra of background quasars from the Hubble Space Telescope/Cosmic Origins Spectrograph. We co-add spectra of different background sources at close projected angular separation on the sky. This novel stacking technique dramatically increases the signal-to-noise ratio of the spectra, allowing the detection of low-column-density gas (down to EW &amp;gt; 2 mÅ). We identify absorption as inflowing or outflowing, by using blue/redshifted high-velocity cloud absorption components in the Galactocentric rest frame, respectively. The mass surface densities of both inflowing and outflowing gases vary by more than an order of magnitude across the sky, with mean values of 〈Σin〉 ≳ 104.6 ± 0.1$\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}$ for inflowing gas and 〈Σout〉 ≳ 103.5 ± 0.1$\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}$ for outflowing gas. The mass flow rate surface densities (mass flow rates per unit area) also show large variation across the sky, with $\langle \dot{\Sigma }(d)_{\rm in}\rangle \gtrsim (10^{-3.6\pm 0.1})(d/12\, \mathrm{kpc})^{-1}$ $\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}\, \mathrm{yr}^{-1}$ for inflowing gas and $\langle \dot{\Sigma }(d)_{\rm out}\rangle \gtrsim (10^{-4.8\pm 0.1})(d/12\, \mathrm{kpc})^{-1}$ $\mathrm{ M}_{\odot }\, \mathrm{kpc}^{-2}\, \mathrm{yr}^{-1}$ for outflowing gas. The regions with highest surface mass density of inflowing gas are clustered at smaller angular scales (θ &amp;lt; 40°). This indicates that most of the mass in inflowing gas is confined to small, well-defined structures, whereas the distribution of outflowing gas is spread more uniformly throughout the sky. Our study confirms that the MW is predominantly accreting gas, but it is also losing a non-negligible mass of gas via outflow.

The Magellanic Corona as the key to the formation of the Magellanic Stream.

The dominant gaseous structure in the Galactic halo is the Magellanic Stream. This extended network of neutral and ionized filaments surrounds the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC), the two most massive satellite galaxies of the Milky Way1-4. Recent observations indicate that the LMC and SMC are on their first passage around the Galaxy5, that the Magellanic Stream is made up of gas stripped from both clouds2,6,7 and that the majority of this gas is ionized8,9. Although it has long been suspected that tidal forces10,11 and ram-pressure stripping12,13 contributed to the formation of the Magellanic Stream, models have not been able to provide a full understanding of its origins3. Several recent developments-including the discovery of dwarf galaxies associated with the Magellanic group14-16, determination of the high mass of the LMC17, detection of highly ionized gas near stars in the LMC18,19 and predictions of cosmological simulations20,21-support the existence of a halo of warm (roughly 500,000 kelvin) ionized gas around the LMC (the 'Magellanic Corona'). Here we report that, by including this Magellanic Corona in hydrodynamic simulations of the Magellanic Clouds falling onto the Milky Way, we can reproduce the Magellanic Stream and its leading arm. Our simulations explain the filamentary structure, spatial extent, radial-velocity gradient and total ionized-gas mass of the Magellanic Stream. We predict that the Magellanic Corona will be unambiguously observable via high-ionization absorption lines in the ultraviolet spectra of background quasars lying near the LMC.

MusE GAs FLOw and Wind (MEGAFLOW) IV. A two sightline tomography of a galactic wind

ABSTRACTGalactic outflows are thought to eject baryons back out to the circumgalactic medium. Studies based on metal absorption lines (Mg ii in particular) in the spectra of background quasars indicate that the gas is ejected anisotropically, with galactic winds likely leaving the host in a bi-conical flow perpendicular to the galaxy disc. In this paper, we present a detailed analysis of an outflow from a z = 0.7 ‘green-valley’ galaxy [log (M*/M⊙) = 9.8; $\mbox{SFR}=0.5\, \mathrm{M}_{\odot }\, \mathrm{yr}^{-1}$] probed by two background sources from the MusE GAs FLOw and Wind (MEGAFLOW) survey. Thanks to a fortuitous configuration with a background quasar (SDSSJ1358 + 1145) and a bright background galaxy at z = 1.4, both at impact parameters of $\approx\! 15\, \hbox{kpc}$, we can – for the first time – probe both the receding and approaching components of a putative galactic outflow around a distant galaxy. We measure a significant velocity shift between the Mg ii absorption from the two sightlines ($84\pm 17\, \hbox{km~s$^{-1}$}$), which is consistent with the expectation from our simple fiducial wind model, possibly combined with an extended disc contribution.

Resolving 3D Disk Orientation Using High-resolution Images: New Constraints on Circumgalactic Gas Inflows

Abstract We constrain gas inflow speeds in star-forming galaxies with color gradients consistent with inside-out disk growth. Our method combines new measurements of disk orientation with previously described circumgalactic absorption in background quasar spectra. Two quantities, a position angle and an axis ratio, describe the projected shape of each galactic disk on the sky, leaving an ambiguity about which side of the minor axis is tipped toward the observer. This degeneracy regarding the 3D orientation of disks has compromised previous efforts to measure gas inflow speeds. We present Hubble Space Telescope and Keck/LGSAO imaging that resolves the spiral structure in five galaxies at redshift z ≈ 0.2. We determine the sign of the disk inclination for four galaxies, under the assumption that spiral arms trail the rotation. We project models for both radial infall in the disk plane and circular orbits onto each quasar sightline. We compare the resulting line-of-sight velocities to the observed velocity range of Mg ii absorption in spectra of background quasars, which intersect the disk plane at radii between 69 and 115 kpc. For two sightlines, we constrain the maximum radial inflow speeds as 30–40 km s−1. We also rule out a velocity component from radial inflow in one sightline, suggesting that the structures feeding gas to these growing disks do not have unity covering factor. We recommend appropriate selection criteria for building larger samples of galaxy–quasar pairs that produce orientations sensitive to constraining inflow properties.

Circumgalactic Pressure Profiles Indicate Precipitation-limited Atmospheres for M * ∼ 109–1011.5 M ⊙

Cosmic gas cycles in and out of galaxies, but outside of galaxies it is difficult to observe except for the absorption lines that circumgalactic clouds leave in the spectra of background quasars. Using photoionization modeling of those lines to determine cloud pressures, we find that galaxies are surrounded by extended atmospheres that confine the clouds and have a radial pressure profile that depends on galaxy mass. Motivated by observations of the universe's most massive galaxies, we compare those pressure measurements with models predicting the critical pressure at which cooler clouds start to precipitate out of the hot atmosphere and rain toward the center. We find excellent agreement, implying that the precipitation limit applies to galaxies over a wide mass range.

The nature of strong H i absorbers probed by cosmological simulations: satellite accretion and outflows

ABSTRACT We use cosmological hydrodynamic zoom simulations to study the neutral gas distribution in and around galaxies that gives rise to high column density H i Ly α absorption (damped Ly α systems (DLAs) and sub-DLAs) in background quasar spectra. Such simulations often sacrifice numerical resolution for volume that affects the lower density galaxy halo gas, and simulations have difficulties reproducing the span of projected separations (b) between absorbing clouds and their hosts. Our simulations produce (sub-)DLAs over the entire probed parameter space ($b\lesssim 50\, \mathrm{kpc}$ and metallicity −4 ≲[M/H]≲ 0.5) at all redshifts (z ∼ 0.4 − 3.0), enclosing spectroscopically confirmed absorber-galaxy pairs. Recovering (sub-)DLAs at b ≳ 20 − 30 kpc from a massive host galaxy requires high numerical resolution and efficient feedback, and we show that these lines-of-sight are associated with dwarf satellites in the main halo, stripped metal-rich gas, and outflows. H i disc- and halo gas significantly contributes to (sub-)DLAs around galaxies. At large redshifts the halo plays an increasingly important role, while at 0.4 &lt; z &lt; 1 the disc and halo contribute with ∼60(80) and ∼40(20) per cent to column densities above the sub-DLA(DLA) lower limits. The distribution of b for sub-DLAs and DLAs overlap at z ∼ 2 − 3, but evolves so that sub-DLAs on average are located at twice larger b by z ∼ 0.5. A weak correlation suggests that sub-DLA covering fractions increase with stellar mass more rapidly than those of DLA. This can explain why sub-DLAs are preferentially selected in more massive galaxies in the low-z Universe.

Multiphase circumgalactic medium probed with MUSE and ALMA

Galaxy halos appear to be missing a large fraction of their baryons, most probably hiding in the circumgalactic medium (CGM), a diffuse component within the dark matter halo that extends far from the inner regions of the galaxies. A powerful tool to study the CGM gas is offered by absorption lines in the spectra of background quasars. Here, we present optical (MUSE) and mm (ALMA) observations of the field of the quasar Q1130-1449 which includes a log [N(H I)/cm^-2]=21.71+/-0.07 absorber at z=0.313. Ground-based VLT/MUSE 3D spectroscopy shows 11 galaxies at the redshift of the absorber down to a limiting SFR>0.01 M_sun yr^-1 (covering emission lines of [OII], Hbeta, [OIII], [NII] and Halpha), 7 of which are new discoveries. In particular, we report a new emitter with a smaller impact parameter to the quasar line-of-sight (b=10.6 kpc) than the galaxies detected so far. Three of the objects are also detected in CO(1-0) in our ALMA observations indicating long depletion timescales for the molecular gas and kinematics consistent with the ionised gas. We infer from dedicated numerical cosmological RAMSES zoom-in simulations that the physical properties of these objects qualitatively resemble a small group environment, possibly part of a filamentary structure. Based on metallicity and velocity arguments, we conclude that the neutral gas traced in absorption is only partly related to these emitting galaxies while a larger fraction is likely the signature of gas with surface brightness almost four orders of magnitude fainter that current detection limits. Together, these findings challenge a picture where strong-HI quasar absorbers are associated with a single bright galaxy and favour a scenario where the HI gas probed in absorption is related to far more complex galaxy structures.

Neutral Gas Phase Metallicities Associated with Dwarf Galaxies

AbstractAbsorption line spectroscopy of foreground gas in the spectra of background quasars has revealed some clear cases where neutral gas is present and associated with dwarf galaxies. Spectroscopy of Lyα and low-ionization metal lines can be combined to derive neutral gas phase metallicities. The damped Lyα absorbers (DLAs) in quasar spectra are the clearest cases of absorption by predominantly neutral gas regions. Here we present some results on neutral gas phase metallicities for cases where the DLA is clearly associated with a dwarf galaxy. We find that the neutral gas phase metallicities in these systems are similar to those in other DLAs. We argue that there may be many unrecognized cases where a DLA is actually associated with a dwarf galaxy even though there is a luminous galaxy within 100 kpc of the quasar sightline.

Quasars Probing Quasars. IX. The Kinematics of the Circumgalactic Medium Surrounding z ∼ 2 Quasars

Abstract We examine the kinematics of the gas in the environments of galaxies hosting quasars at z ∼ 2. We employ 148 projected quasar pairs to study the circumgalactic gas of the foreground quasars in absorption. The sample selects foreground quasars with precise redshift measurements, using emission lines with precision ≲300 km s−1 and average offsets from the systemic redshift ≲ ∣ 100 km s − 1 ∣ . We stack the background quasar spectra at the foreground quasar’s systemic redshift to study the mean absorption in C ii, C iv, and Mg ii. We find that the mean absorptions exhibit large velocity widths σ v ≈ 300 km s−1. Further, the mean absorptions appear to be asymmetric about the systemic redshifts. The mean absorption centroids exhibit small redshift relative to the systemic δv ≈ +200 km s−1, with large intrinsic scatter in the centroid velocities of the individual absorption systems. We find the observed widths are consistent with gas in gravitational motion and Hubble flow. However, while the observation of large widths alone does not require galactic-scale outflows, the observed offsets suggest that the gas is on average outflowing from the galaxy. The observed offsets also suggest that the ionizing radiation from the foreground quasars is anisotropic and/or intermittent.

A clumpy and anisotropic galaxy halo at redshift 1 from gravitational-arc tomography.

Every star-forming galaxy has a halo of metal-enriched gas that extends out to at least 100 kiloparsecs, as revealed by the absorption lines that this gas imprints on the spectra of background quasars. However, quasars are sparse and typically probe only one narrow beam of emission through the intervening galaxy. Close quasar pairs and gravitationally lensed quasars have been used to circumvent this inherently one-dimensional technique, but these objects are rare and the structure of the circumgalactic medium remains poorly constrained. As a result, our understanding of the physical processes that drive the recycling of baryons across the lifetime of a galaxy is limited. Here we report integral-field (tomographic) spectroscopy of an extended background source-a bright, giant gravitational arc. We can thus coherently map the spatial and kinematic distribution of Mg ɪɪ absorption-a standard tracer of enriched gas-in an intervening galaxy system at redshift 0.98 (around 8 billion years ago). Our gravitational-arc tomography unveils a clumpy medium in which the absorption strength decreases with increasing distance from the galaxy system, in good agreement with results for quasars. Furthermore, we find strong evidence that the gas is not distributed isotropically. Interestingly, we detect little kinematic variation over a projected area of approximately 600 square kiloparsecs, with all line-of-sight velocities confined to within a few tens of kilometres per second of each other. These results suggest that the detected absorption originates from entrained recycled material, rather than in a galactic outflow.

Early metal enrichment of gas-rich galaxies at z ∼ 5

We present abundance measurements of elements O, C, Si and Fe for three gas-rich galaxies at z~5 using observations from the Very Large Telescope (VLT) and the Keck telescope in order to better constrain the early chemical enrichment of gas-rich galaxies. These galaxies show strong Lyman-{\alpha} absorption in the spectra of background quasars, with neutral hydrogen column densities log N${_H{}_I}$(cm$^{-2}$)=20.10$\pm$0.15, 20.10$\pm$0.15, and 20.80$\pm$0.15. Using the undepleted element O, we find the metallicities [O/H] to be in the range of -2.51 to -2.05 dex. Our study has doubled the existing sample of measurements of undepleted elements at z>4.5. Combining our measurements with those from the literature, we find that the N${_H{}_I}$-weighted mean metallicity of z~5 absorbers is consistent with the prediction based on z<4.5 DLAs. Thus, we find no significant evidence of a sudden drop in metallicity at z>4.7 as reported by some prior studies. We also determine the extent of dust depletion using a combination of both the volatile element O and the refractory elements Si and/or Fe. Some of the absorbers show evidence of depletion of elements on dust grains, e.g. low [Si/O] or [Fe/O]. The relative abundances of these absorbers along with other z~5 absorbers from the literature show some peculiarities, e.g. low [C/O] in several absorbers and high [Si/O] in one absorber. Finally, we find that the metallicity vs. velocity dispersion relation of z~5 absorbers may be different from that of lower-redshift absorbers.

Measurement of the small-scale structure of the intergalactic medium using close quasar pairs.

The distribution of diffuse gas in the intergalactic medium (IGM) imprints a series of hydrogen absorption lines on the spectra of distant background quasars known as the Lyman-α forest. Cosmological hydrodynamical simulations predict that IGM density fluctuations are suppressed below a characteristic scale where thermal pressure balances gravity. We measured this pressure-smoothing scale by quantifying absorption correlations in a sample of close quasar pairs. We compared our measurements to hydrodynamical simulations, where pressure smoothing is determined by the integrated thermal history of the IGM. Our findings are consistent with standard models for photoionization heating by the ultraviolet radiation backgrounds that reionized the universe.