Distant (high-redshift) galaxies are faint and small (on the sky), so studying them through direct imaging is challenging, particularly from the ground. An alternative way to detect such galaxies is to analyze the spectra of luminous background objects (quasars and gamma-ray bursts), and look for the tell-tale absorption lines that arise due to foreground galaxies lying along the line-of-sight. These lines contain a wealth of information on the chemical properties (elemental abundances) and physical properties (temperature, density, ionization state) of the absorbing gas. The strongest absorbers, known as damped Lyman-alpha (DLA) systems, are thought to trace gas in and around galaxy disks and protogalaxies. I have led a program to survey the "high-ion" absorption in a large sample of DLAs at z~2-3 observed with the UVES spectrograph on the VLT, to investigate and characterize the warm-hot gas in these absorbers. The lines studies are those of five-times-ionized oxygen (O VI), four-times-ionized nitrogen (N V), and three-times-ionized carbon (C IV). The strength and velocity spread of these high-ion lines and their correlation with other DLA properties give important observational constraints on the kinematics and structure of the DLA galaxies. These surveys reveal that a significant fraction of metals and baryons in DLAs exist in ionized gas (plasma). The figure on the right shows that the high-ion (C IV) velocity widths in DLAs are almost always broader than the low-ion (Si II) velocity widths (from Fox et al. 2007, A&A, 473, 791). Galactic winds are one possible energy source for the high ion kinematics.

The intergalactic medium (IGM), the filamentary web of plasma existing between galactic structures (see figure at right), occupies the vast majority of the volume of the Universe. The IGM is detected as a series of hydrogen absorption lines (known as the Ly-alpha forest) detected blueward of Ly-alpha emission in quasar spectra. It has been known for several decades that the IGM is metal-enriched, because metal lines are detected at the same redshift as Ly-alpha forest lines, down to low H I column densities, corresponding to low-density regions. What is less well known is how homogeneous the metal enrichment is: do the metals exist in relatively small, enriched "pockets", or are they widespread with a large volume filling factor? And how were the metals transported from their sites of origin (in the cores of massive stars) to the IGM where they are observed? Were most IGM metals produced at very high redshift in an early generation of Population III stars, or are they dispersed later? To address these questions, I'm involved in the study of high signal-to-noise, high spectral resolution optical quasar spectra (taken with VLT/UVES), focusing on the detailed absorption-line profiles in individual high-redshift IGM systems. In particular, studying pairs of quasars lying close together on the sky (separations on the order of arcminutes) allows us to probe the transverse structure of the IGM.

Gamma-ray bursts (GRBs) are the most energetic events in the Universe, typically releasing as much energy in a few seconds as the Sun will emit in its entire 10 billion year lifetime. Aside from their interest as the endpoints of massive star evolution, GRBs are ideal background sources for absorption-line spectroscopy, thanks to their enormous luminosities and smooth power-law continua. Optical spectroscopy of GRB afterglows (particularly the long-duration bursts, lasting for more than 2 seconds) can be used to study both the intervening IGM along the line-of-sight and the interstellar medium (ISM) of the host galaxy, Using the UVES spectrograph on the VLT in Chile in rapid-response mode (in which ongoing exposures are automatically interrupted to slew the telescope to the GRB, which is then observed within minutes), I am part of a team to determine physical conditions and chemical abundances in the host galaxy's ISM. Certain exotic absorption lines (from excited levels of singly-ionized iron and nickel) show variation over short timescales (minutes to hours), allowing us to track the evolving ionization and excitation level of the gas in real time. In combination with photo-excitation modeling, this allows one to derive the distance from the burst to the absorbing cloud. The figure on the right shows such time-variation in the Fe II* and Ni II* lines in the spectrum of GRB 060418 (from Vreeswijk et al. 2007, A&A, 468, 83).

The Milky Way is surrounded by a network of high-velocity clouds (HVCs) that do not rotate with the Galactic disk, but rather trace inflowing and outflowing gas streams passing through the hot Galactic corona. HVCs are studied in 21 cm emission from neutral hydrogen (see map at right, courtesy Bart Wakker), and in UV and optical absorption in the spectra of background sources. The infalling HVCs play an important role in galaxy evolution, as carriers of low-metallicity fuel to power future star formation in the disk. Observational constraints on HVCs are important to understand these physical processes. My work on HVCs has focused on absorption-line studies of high-ionization species (O VI, N V, C IV), whose detection indicates the presence of hot plasma at temperatures of a few hundred thousand Kelvin. These "high ions" are thought to arise at the turbulent boundaries between the clouds and the even hotter (million-degree) surrounding corona. The long-term goal of this work is to determine the overall ionization fraction in HVCs, allowing us to investigate whether most accreting gas clouds make it safely into the Galactic disk, or whether they become fully ionized and "evaporate" into the hot corona (see Fox et al. 2010, ApJ, 718, 1046).

Most of the baryonic (observable) matter in the Universe is outside galaxies, so to detect and characterize the baryons, one has to study the IGM. I am involved in absorption-line studies of the low-redshift IGM using the Hubble Space Telescope, with the goal of completing the inventory of baryons, and studying the IGM absorbers' physical conditions, chemical enrichment, proximity to galaxies, and relation to galaxy evolution processes such as inflow and outflow. The Cosmic Origins Spectrograph (COS) installed in May 2009 on HST is addressing these questions in detail (see cartoon on right), providing much higher sensitivity than previous space-based UV spectrographs.