Cold, neutral interstellar gas, the reservoir for star formation, is traced through the absorption of the 21-cm continuum radiation by neutral hydrogen (H i). Although detected in one hundred cases in the host galaxies of distant radio sources, only recently have column densities approaching the maximum value observed in Lyman-$\alpha$ absorption systems ($N_{{\textrm{H}\,\scriptsize{\textrm{I}}}}\sim 10^{22}$ $\textrm{cm}^{-2}$) been found. Here, we explore the implications these have for the hypothesis that the detection rate of H i absorption is dominated by photo-ionisation from the active galactic nucleus (AGN). We find, with the addition all of the current searches for H i absorption at $z\geq0.1$, a strong correlation between the H i absorption strength and the ionising photon rate, with the maximum value at which H i is detected remaining close to the theoretical value in which all of the neutral gas would be ionised in a large spiral galaxy ($Q_{{\textrm{H}\,\scriptsize{\textrm{I}}}} = 2.9\times10^{56}$ ionising photons s$^{-1}$). We also rule out other effects (excitation by the radio continuum and changing gas properties) as the dominant cause for the decrease in the detection rate with redshift. Furthermore, from the maximum theoretical column density, we find that the five high column density systems have spin temperatures close to those of the Milky Way ($T_{\textrm{spin}} \lesssim 300$ K), whereas, from our model of a gaseous galactic disc, the H i detection at $Q_{{\textrm{H}\,\scriptsize{\textrm{I}}}} =2.9\times10^{56}$ s$^{-1}$ yields $T_{\textrm{spin}}\sim10\,000$ K, consistent with the gas being highly ionised.

A new determination of the temperature of the intergalactic medium (IGM) over $3.9 \leq z \leq 4.3$ is presented. We applied the curvature method on a sample of 10 high-resolution quasar spectra from the Ultraviolet and Visual Echelle Spectrograph on the VLT/ESO. We measured the temperature at mean density by determining the temperature at the characteristic overdensity, which is tight function of the absolute curvature irrespective of $\unicode{x03B3}$. Under the assumption of fiducial value of $\unicode{x03B3} = 1.4$, we determined the values of temperatures at mean density $T_{0} = 7893^{+1417}_{-1226}$ K and $T_{0} = 8153^{+1224}_{-993}$ K for redshift range of $3.9 \leq z \leq 4.1$ and $4.1 \leq z \leq 4.3$, respectively. Even though the results show no strong temperature evolution over the studied redshift range, our measurements are consistent with an IGM thermal history that includes a contribution from He ii reionisation.

We present the physical properties of Lyα emitters (LAEs) in a “DLA-concentrated regions” where there are 3 or more DLA within (50 Mpc)3 cubic box. We observed LAEs in a DLA-concentrated region at z = 2.3, the J1230+34 field, with Subaru/Suprime-Cam. In the 50 Mpc scale, we found no deferences in properties of LAEs such as Lyα luminosity function in the DLA-concentrated region compared to other fields at similar redshift. On the other hand, we found a ∼10 Mpc scale LAE overdensity around a strong DLA with NHI = 1021.08 cm−2.

Theories that attempt to unify the four fundamental interactions and alternative theories of gravity predict time and/or spatial variation of the fundamental constants of nature. Different versions of these theories predict different behaviours for these variations. As a consequence, experimental and observational bounds are an important tool to check the validity of such proposals. In this paper, we review constraints on the possible variation of the fundamental constants from astronomical observations and geophysical experiments designed to test the constancy of the fundamental constants of nature over different timescales. We also focus on the limits that can be obtained from white dwarfs, which can constrain the variation of the constants with the gravitational potential.

It has recently been shown that the abundance of cold neutral gas may follow a similar evolution as the star formation history. This is physically motivated, since stars form out of this component of the neutral gas and if the case, would resolve the long-standing issue that there is a clear disparity between the total abundance of neutral gas and star-forming activity over the history of the Universe. Radio-band 21-cm absorption traces the cold gas and comparison with the Lyman-α absorption, which traces all of the gas, provides a measure of the cold gas fraction, or the spin temperature, Tspin. The recent study has shown that the spin temperature (degenerate with the ratio of the absorber/emitter extent) appears to be anti-correlated with the star formation density, ψ*, with 1/Tspin undergoing a similar steep evolution as ψ* over redshifts of 0 ≲ z ≲ 3, whereas the total neutral hydrogen exhibits little evolution. Above z ∼ 3, where ψ* shows a steep decline with redshift, there are insufficient 21-cm data to determine whether 1/Tspin continues to follow ψ*. Knowing this is paramount in ascertaining whether the cold neutral gas does trace the star formation over the Universe’s history. We explore the feasibility of resolving this with 21-cm observations of the largest contemporary sample of reliable damped Lyman-α absorption systems and conclude that, while today’s largest radio interferometers can reach the required sensitivity at z ≲ 3.5, the Square Kilometre Array is required to probe higher redshifts.

We present the first measurement of differences in MgII absorption strength in multiple intervening absorbers, which are also identified as (sub-)Damped Lyman alpha absorption systems, in the four spectra of the quadruply lensed quasar H1413+1143, often referred to the “Cloverleaf”, from highly spatial resolution and high signal-to-noise spectroscopy with an optical multi-mode spectrograph, the Kyoto tridimentional spectrograph II on board the Subaru telescope. The detection of significant MgII absorptions in multiple components in the spatially-resolved spectra suggests that chemical enrichment differs at least on scale of about 10 kpc within the separation of sightlines. For, a DLA system at redshift zabs = 1.66, the rest equivalent widths of MgII absorption lines change by factors up to 6, which is similar to those of HI absorption lines. This suggests that (inhomogeneous) cold absorbers which give rise to strong HI/MgII absorptions dwell on a scale within 10 kpc in the circumgalactic medium (CGM).

Absorption 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.

The large scatter in Lyman-α opacity at z > 5.3 has been an ongoing mystery, prompting a flurry of numerical models. A uniform ultra-violet background has been ruled out at those redshifts, but it is unclear whether any proposed models produce sufficient inhomogeneities. In this paper we provide an update on the measurement which first highlighted the issue: Lyman-α effective optical depth along high-z quasar lines of sight. We nearly triple on the previous sample size in such a study thanks to the cooperation of the DES-VHS, SHELLQs, and SDSS collaborations as well as new reductions and spectra. We find that a uniform UVB model is ruled out at 5.1 < z < 5.3, as well as higher redshifts, which is perplexing. We provide the first such measurements at z ∼ 6. None of the numerical models we confronted to this data could reproduce the observed scatter.

We have discovered that warm gas flows along galaxy major and minor axes detected out to 200 kpc. Our results are derived from a sample of HST-imaged isolated galaxies with nearby background quasars used to probe their 105K CGM detected in HST/COS UV spectra (traced by Ovi absorption). We constrain the geometry of the gas to reside between 20-40 degrees of the projected major axis and within 60 degrees of the projected minor axis, with little-to-no gas found in between. Furthermore, strong absorption systems tend to be found along the minor axes of star-forming galaxies. All of our results are consistent with the current view of the CGM originating from major axis-fed inflows/recycled gas and from minor axis-driven outflows.

Our view of galaxies has been transformed in recent years with diffuse halo gas surrounding galaxies that contains at least as many metals and baryons as their disks. While single sight lines through galaxy halos seen in absorption have provided key new constraints, they provide only average properties. Our massive neighbor, the Andromeda (M31) galaxy, provides an unique way to study its circumgalactic medium whereby we can study it using not one or two, but ~36 sightlines thanks to its proximity. With our Large HST program — Project AMIGA (Absorption Maps In the Gas of Andromeda), our goals are to determine the spatial distribution of the halo properties of a L* galaxy using 36 background targets at different radii and azimuths. In this brief paper, I discuss briefly the scientific rationale of Project AMIGA and some early science results. In particular, for the first time we have demonstrated that M31 has a gaseous halo that extends to Rvir with as much as metal and baryonic masses than in its disk and has substantial change in its ionization properties with more highly ionized gas found at R ~ Rvir than cooler gas found near the disk.

Strong absorption lines in quasar spectra primarily probe low-mass galaxies and detecting these in emission has previously been difficult. Dedicated surveys for the host galaxies of damped Lyman-α (DLA) systems have often resulted in non-detections and upper limits. Targeting the most metal-rich absorbers has proven to be a viable method, because these galaxies are brighter. By combining DLA metallicities and deriving host galaxy stellar masses, we find that metal-rich DLAs (with >10% solar metallicity) and their host galaxies follow the same redshift-dependent scaling relation between stellar mass and metallicity as luminosity-selected galaxies. We derive a prediction for an absorber galaxy mass that depends on the DLA metallicity.

Observations of metal absorption lines in the spectra of QSOs out to z > 6 are providing an important probe into the enrichment and ionization state of the intergalactic medium (IGM) at the tail end of reionization. Using simulations with four different feedback models, including the Illustris and Sherwood simulations, we investigate how the overall incidence rate and equivalent width distribution of metal-line absorbers varies with the galactic wind scheme. The low-ionization absorbers are reasonably insensitive to the feedback implementation, with all models reasonably close to the observed incidence rate of O i absorbers. However, all of our models struggle to reproduce the observations of C iv, which is probing overdensities close to the mean at z ~ 6, suggesting that the metals are not being transported out into the IGM efficiently enough in these simulations.

We use high-resolution Keck, VLT, or Hubble Space Telescope spectra of background quasars to examine the kinematic properties of the multiphase, metal-enriched circumgalactic medium in the outskirts of galaxies at 0.08 < zgal < 1.0, focusing on the low-ionization Mgii and high-ionization Ovi doublets. The absorption kinematics of low-ionization gas in the circumgalactic medium depend strongly on the star formation activity and the location about the host galaxy, where the largest velocity dispersions are associated with blue, face-on galaxies probed along the minor axis. Conversely, high-ionization gas kinematics are independent of galaxy star formation activity and orientation.

Understanding the distribution of gas in galaxies and its interaction with the IGM is crucial to complete the picture of galaxy evolution. At all redshifts, absorption features seen in QSO spectra serve as a unique probe of the gaseous content of foreground galaxies and the IGM, extending out to 200 kpc. Studies show that star formation history is intimately related to the co-evolution of galaxies and the IGM. In order to study the environments traced by absorption systems and the role of inflows and outflows, it is critical to measure the emission properties of host galaxies and their halos. We overcome the challenge of detecting absorption host galaxies with the MUSE integral field spectrograph on VLT. MUSE’s large field of view and sensitivity to emission lines has allowed a never-before seen match between the number density of absorbers along QSO sightlines and the number density of emission line galaxies within 200 kpc of the QSO. These galaxies represent a sample for which previously elusive connections can be made between mass, metallicity, SFR, and absorption.

We present new EAGLE (“Evolution and Assembly of GaLaxies and their Environments”, Schaye et al. 2015) zoom simulations of a range of Milky Way-like galaxies that can uniquely reproduce the metallicities of galactic stars, the ISM, and now the circumgalactic medium (CGM) as probed by quasar absorption line surveys. The surprising result is that the average L* galaxy loses more oxygen to the CGM out to hundreds of kpcs from a galaxy than it retains in its stars. These zooms not only follow the nucleosynthetic yields of 11 elements, they follow the non-equilibrium ionization and cooling of 133 ions, which allow direct comparison to observations of the COS-Halos survey of O VI in galactic halos out to 150 kpc (Tumlinson et al. 2011). The result is a new understanding of the galactic nucleosynthetic yield budget in simulations where galaxies are dramatically shaped by supernovae and black hole feedback. We are now closer to reconciling observed stellar, ISM, and now CGM metallicities with the nucleosynthetic production of the stellar component.

FeLoBAL quasars may be an early evolutionary stage of merger-triggered quasar activity. We test this hypothesis using HST imaging of four FeLoBAL quasars. The host galaxy colors are consistent with early-type quiescent galaxies or heavily-obscured starbursts.

The accurate description of the properties of the Lyman-α forest is a spectacular success of the Cold Dark Matter theory of cosmological structure formation. After a brief review of early models, it is shown how numerical simulations have demonstrated the Lyman-α forest emerges from the cosmic web in the quasi-linear regime of overdensity. The quasi-linear nature of the structures allows accurate modeling, providing constraints on cosmological models over a unique range of scales and enabling the Lyman-α forest to serve as a bridge to the more complex problem of galaxy formation.

The intergalactic medium (IGM) accounts for ≳ 90% of baryons at all epochs and yet its three dimensional distribution in the cosmic web remains mostly unknown. This is so because the only feasible way to observe the bulk of the IGM is through intervening absorption line systems in the spectra of bright background sources, which limits its characterization to being one-dimensional. Still, an averaged three dimensional picture can be obtained by combining and cross-matching multiple one-dimensional IGM information with three-dimensional galaxy surveys. Here, we present our recent and current efforts to map and characterize the IGM in the cosmic web using galaxies as tracers of the underlying mass distribution. In particular, we summarize our results on: (i) IGM around star-forming and non-star-forming galaxies; (ii) IGM within and around galaxy voids; and (iii) IGM in intercluster filaments. With these datasets, we can directly test the modern paradigm of structure formation and evolution of baryonic matter in the Universe.

The physical properties and dynamical behavior of Broad Absorption Line (BAL) outflows are crucial themes in understanding the connections between galactic centers and their hosts. FeLoBALs (identified with the presence of low-ionization Fe II BALs) are a peculiar class of quasar outflows that constitute ~ 1% of the BAL population. With their large column densities and apparent outflow kinetic luminosities, FeLoBALs appear to be exceptionally powerful and are strong candidates for feedback in galaxy evolution. We conducted variability studies of 12 FeLoBAL quasars with emission redshifts 0.69 ≤ z ≤ 1.93, spanning both weekly and multi-year timescales in the quasar's rest frame. We detected absorption-line variability from low-ionization species (Fe II, Mg II) in four of our objects, with which we established a representative upper limit for the distance of the absorber from the supermassive black hole (SMBH) to be ~20 parsecs. Our goals are to understand the mechanisms producing the variability (e.g. ionization changes or gas traversing our line of sight) and place new constraints on the locations, structure, and kinetic energies of the outflows.

The first extragalactic diffuse interstellar band (DIB) detections were of λ4430 in the Large and Small Magellanic Clouds (LMC and SMC) in the 1960s and 70s. Driven mainly by the increased sensitivity afforded by 8-10 m-class telescopes, the last 13 years have witnessed an explosion of DIB discoveries throughout the nearby and distant universe. This review focuses on the history of extragalactic DIB studies, including some of the important results that have come out of this field, and looks to the future for what can be learned about DIBs in external galaxies with the next generation of large telescopes. So far, DIBs have been observed in the Magellanic Clouds, starburst galaxies, DLAs, and nearby (≤30 Mpc distant) spiral galaxies, and are found to be ubiquitous in the diffuse interstellar medium (ISM) of extragalactic environments wherever dust is present. Important results include the finding that DIB carriers are significantly more closely related to dust than to neutral hydrogen, and that the λ6283 DIB tends to be anomalously weak in low-metallicity sightlines.