QSO absorption spectroscopy provides a sensitive probe of both neutral medium and diffuse ionized gas in the distant Universe. It extends 21 cm maps of gaseous structures around low-redshift galaxies both to lower gas column densities and to higher redshifts. Specifically, DLAs of N(H I) ≳ 2 × 1020 cm−2 probe neutral gas in the ISM of distant star-forming galaxies, LLS of N(H I) > 1017 cm−2 probe optically thick HVCs and gaseous streams in and around galaxies, and strong Lyα absorbers of N(H I) ≈ 1014−17 cm−2 and associated metal-line absorption transitions, such as Mg II, C IV, and O VI, trace chemically enriched, ionized gas and starburst outflows. Over the last decade, an unprecedentedly large number of ∼ 10000 DLAs have been identified along random QSO sightlines to provide robust statistical characterizations of the incidence and mass density of neutral atomic gas at z ≲ 5. Extensive follow-up studies have yielded accurate measurements of chemical compositions and molecular gas content for this neutral gas cross-section selected sample from z ≈ 5 to z ≈ 0 (Sect. 2). Combining galaxy surveys with absorption-line observations of gas around galaxies has enabled comprehensive studies of baryon cycles between star-forming regions and low-density gas over cosmic time. DLAs, while being rare as a result of a small cross-section of neutral medium in the Universe, have offered a unique window into gas dynamics and chemical enrichment in the outskirts of star-forming disks (Sect. 3), as well as star formation physics at high redshifts (Sect. 4). Observations of strong Lyα absorbers and associated ionic transitions around galaxies have also demonstrated that galaxy mass is a dominant factor in driving the extent of chemically enriched halo gas and that chemical enrichment is well confined within galactic haloes for both low-mass dwarfs and massive galaxies (Sect. 5).
With new observations carried out using new, multiplex instruments, continuing progress is expected in further advancing our understanding of baryonic cycles in the outskirts of galaxies over the next few years. These include, but are not limited to: (1) direct constraints for the star formation relation in different environments (e.g., Gnedin and Kravtsov 2010), particularly for star-forming galaxies at z ≳ 2 in low surface density regimes of ΣSFR < 0.1 M⊙ yr−1 kpc−2 and Σgas ≈ 10−100 M⊙ pc−2; (2) an empirical understanding of galaxy environmental effects in distributing heavy elements to large distances based on deep galaxy surveys carried out in a large number of QSO fields (e.g., Johnson et al 2015); and (3) a three-dimensional map of gas flows in the circumgalactic space that combines absorption-line kinematics along multiple sightlines with optical morphologies of the absorbing galaxies and emission morphologies of extended gas around the galaxies (e.g., Rubin et al 2011, Chen et al 2014, Zahedy et al 2016). Wide-field IFUs on existing large ground-based telescopes substantially increase the efficiency in faint galaxy surveys (e.g., Bacon et al 2015) and in revealing extended low surface brightness emission features around high-redshift galaxies (e.g., Cantalupo et al 2014, Borisova et al 2016). The James Webb Space Telescope (JWST), which is scheduled to be launched in October 2018, will expand the sensitivity of detecting faint star-forming galaxies in the early Universe. Combining deep infrared images from JWST and CO (or dust continuum) maps from ALMA will lead to critical constraints for the star formation relation in low surface density regimes.
Acknowledgements The author wishes to dedicate this review to the memory of Arthur M. Wolfe for his pioneering and seminal work on the subject of damped Lyα absorbers and for inspiring generations of scientists to pursue original and fundamental research. The author thanks Nick Gnedin, Sean Johnson, Rebecca Pierce, Marc Rafelski, and Fakhri Zahedy for providing helpful input and comments. In preparing this review, the author has made use of NASA's Astrophysics Data System Bibliographic Services.