The relationship between the intergalactic medium and galaxies

In this thesis we study the relationship between the intergalactic medium (IGM) and galaxies at z<1, in a statistical manner. Galaxies are mostly surveyed in emission using optical spectroscopy, while the IGM is mostly surveyed in absorption in the ultra-violet (UV) spectra of background quasi-stellar objects (QSOs). We present observational results investigating the connection between the IGM and galaxies using two complementary methods: • We use galaxy voids as tracers of both underdense and overdense regions. We use archival data to study the properties of H I absorption line systems within and around galaxy voids at z<0.1. Typical galaxy voids have sizes 10 Mpc and so our results constrain the very large-scale association. This sample contains 106 H I absorption systems and 1054 galaxy voids. • We use a sample of H I absorption line systems and galaxies from pencil beam surveys to measure the H I–galaxy cross-correlation at z<1. Our sample is composed of a combination of archival and new data taken by the author and collaborators. This survey covers transverse separations between H I and galaxies from ∼ 100 kpc (proper) up to ∼ 10 Mpc, filling the gap between the very large scales and those associated with the so-called circumgalactic medium (CGM). This sample contains 654 H I absorption systems and 17509 galaxies. Our results hint towards a picture in which there are at least three types of association between the diffuse gas in the Universe and galaxies at z 1: • One-to-one direct association because galaxies do contain diffuse gas. • Indirect association because both the IGM and galaxies trace the same over-dense underlying dark matter distribution. We provide quantitative evidence for this association. Moreover, we show that not all galaxies are related to the diffuse gas in the same way. In particular, a non negligible fraction of ‘non-star-forming’ galaxies might reside in environments devoid of diffuse H I. • No association because there are regions in the Universe that contain a significant amount of diffuse gas but that are devoid of galaxies. In these regions, only the IGM follows the underdense underlying dark matter distribution because galaxies are not present. We provide quantitative evidence for this scenario

On the Incidence of C IV Absorbers Along the Sightlines to Gamma-Ray Bursts

We report on the statistics of strong (W_r > 0.15 A) C IV absorbers at z=1.5-3.5 toward high-redshift gamma-ray bursts (GRBs). In contrast with a recent survey for strong Mg II absorption systems at z < 2, we find that the number of C IV absorbers per unit redshift dN/dz does not show a significant deviation from previous surveys using quasi-stellar objects (QSOs) as background sources. We find that the number density of C IV toward GRBs is dN/dz(z~1.5)= 2.2 +2.8/-1.4, dN/dz(z~2.5)= 2.3 +1.8/-1.1 and dN/dz(z~3.5)= 1.1 +2.6/-0.9. These numbers are consistent with previous C IV surveys using QSO spectra. Binning the entire dataset, we set a 95% c.l. upper limit to the excess of C IV absorbers along GRB sightlines at twice the incidence observed along QSO sightlines. Furthermore, the distribution of equivalent widths of the GRB and QSO samples are consistent with being drawn from the same parent population. Although the results for Mg II and C IV absorbers along GRB sightlines appear to contradict one another, we note that the surveys are nearly disjoint: the C IV survey corresponds to higher redshift and more highly ionized gas than the Mg II survey. Nevertheless, analysis on larger statistical samples may constrain properties of the galaxies hosting these metals (e.g. mass, dust content) and/or the coherence-length of the gas giving rise to the metal-line absorption.Comment: Accepted version (for publication in ApJ), results unchanged, 18 pages, 3 tables, 5 figure

A clumpy and anisotropic galaxy halo at z=1 from gravitational-arc tomography

Every star-forming galaxy has a halo of metal-enriched gas extending out to at least 100 kpc, as revealed by the absorption lines this gas imprints on the spectra of background quasars. However, quasars are sparse and typically probe only one narrow pencil beam 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 circum-galactic medium remains poorly constrained. As a result, our understanding of the physical processes that drive the re-cycling 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 II absorption -a standard tracer of enriched gas- in an intervening galaxy system at redshift 0.98 (i.e., ~8 Gyr ago). Our gravitational-arc tomography unveils a clumpy medium in which the absorption-strength decreases with increasing impact parameter, in good agreement with the statistics towards quasars; furthermore, we find strong evidence that the gas is not distributed isotropically. Interestingly, we detect little kinematic variation over a projected area of ~600 kpc squared, with all line-of-sight velocities confined to within a few tens of km/s of each other. These results suggest that the detected absorption originates from entrained recycled material, rather than in a galactic outflow.Comment: Published online in Nature on 31 January 201

Signatures of extended discs and outflows in the circumgalactic medium using the Q0107 quasar triplet

We use H I absorption along the lines-of-sight to the Q0107 quasar triplet in order to model potential disc and outflow structures in the circumgalactic medium of intervening galaxies at z ≲ 1, as well as the intergalactic medium on scales of up to a few virial radii. We consider a sample of twelve isolated galaxies in the Q0107 field with position angles and inclinations measured from HST imaging as well as redshifts from our spectroscopic surveys, alongside 27 detected Ly α absorbers within 500 km s−1 of these galaxies. Building on previous work showing increased incidence of absorption close to the projected major and minor axes, we use model rotating discs and bi-conical outflows in attempting to reproduce the observed absorption. Requiring these models to match absorption in multiple lines-of-sight provides additional constraints over single-sightline observations. We identify four possible outflows with velocities ∼100 km s−1, two of which extend to or beyond the virial radius, with a variety of opening angles. Two galaxies have nearby co-rotating absorbers with rotation velocities ≲ vvir, that may probe disc-like structures, and we can rule out a disc/outflow origin for a further ten absorbers. These indicate that outflowing and co-rotating structures can extend to large scales but are either not ubiquitous, or do not always produce detectable Ly α. In some cases, disc models are successful even close to the minor axis of the galaxy, and some of our model outflows exhibit wide opening angles. These results imply that purely geometrical cuts are not sufficient to distinguish between discs and outflows in single line-of-sight studies

Modelling gas around galaxy pairs and groups using the Q0107 quasar triplet

We examine to what extent disc and outflow models can reproduce observations of H I gas within a few virial radii of galaxies in pairs and groups. Using highly sensitive HST/COS and FOS spectra of the Q0107 quasar triplet covering Ly α for z≲1, as well as a deep galaxy redshift survey including VIMOS, DEIMOS, GMOS, and MUSE data, we test simple disc and outflow models against the H I absorption along three lines-of-sight (separated by 200–500 kpc) through nine galaxy groups in this field. These can be compared with our previous results in which these models can often be fit to the absorption around isolated galaxies. Our models can reproduce ≈ 75 per cent of the 28 identified absorption components within 500 km s−1 of a group galaxy, so most of the H I around groups is consistent with a superposition of the CGM of the individual galaxies. Gas stripped in interactions between galaxies may be a plausible explanation for some of the remaining absorption, but neither the galaxy images nor the galaxy and absorber kinematics provide clear evidence of such stripped material, and these unexplained absorbers do not preferentially occur around close pairs of galaxies. We find H I column densities typically higher than at similar impact parameters around isolated galaxies (≈ 2.5σ), as well as more frequent detections of O VI than around isolated galaxies (30 per cent of sightlines to 7 per cent)

MApping the Most Massive Overdensities Through Hydrogen (MAMMOTH) I: Methodology

Modern cosmology predicts that a galaxy overdensity is associated to a large reservoir of the intergalactic gas, which can be traced by the Ly$\alpha$ forest absorption. We have undertaken a systematic study of the relation between Coherently Strong intergalactic Ly$\alpha$ Absorption systems (CoSLAs), which have highest optical depth ($\tau$) in $\tau$ distribution, and mass overdensities on the scales of $\sim$ 10 - 20 $h^{-1}$ comoving Mpc. On such large scales, our cosmological simulations show a strong correlation between the effective optical depth ($\tau_{\rm{eff}}$) of the CoSLAs and the 3-D mass overdensities. In moderate signal-to-noise spectra, however, the profiles of CoSLAs can be confused with high column density absorbers. For $z>2.6$, where the corresponding Ly$\beta$ is redshifted to the optical, we have developed the technique to differentiate between these two alternatives. We have applied this technique to SDSS-III quasar survey at $z = 2.6$ - 3.3, and we present a sample of five CoSLA candidates with $\tau_{\rm{eff}}$ on 15 $h^{-1}$ Mpc greater than $4.5\times$ the mean optical depth. At lower redshifts of $z < 2.6$, where the background quasar density is higher, the overdensity can be traced by intergalactic absorption groups using multiple sight lines. Our overdensity searches fully utilize the current and next generation of Ly$\alpha$ forest surveys which cover a survey volume of $> (1\ h^{-1}$ Gpc)$^3$. In addition, systems traced by CoSLAs will build a uniform sample of the most massive overdensities at $z > 2$ to constrain the models of structure formation, and offer a unique laboratory to study the interactions between galaxy overdensities and the intergalactic medium.Comment: 24 pages, 30 figures, 8 tables, submitted to the Astrophysical Journa