Significant progress has been made over the last decade in characterizing the cool circumgalactic gas in massive halos of Mh > 1012 M⊙ at z ≈ 0.2−2 using absorption spectroscopy. This progress is facilitated by the unprecedentedly large galaxy and quasar samples available in the SDSS spectroscopic archive. Both massive galaxies and luminous quasars are rare. As a result, finding a background quasar in close projected distances for absorption-line studies of these rare objects requires a large survey volume. The large galaxy and quasar spectroscopic archive helps the assembly of statistically significant samples of close quasar and quiescent galaxy pairs and projected quasar pairs. These pair samples have enabled systematic studies of low-density gas beyond the nearby universe. Key findings from various studies can be summarized as follows:
Together, these findings suggest that infalling clouds from external sources are likely a dominant source of cool gas detected at d ≳ 100 kpc from massive quiescent galaxies. The origin of the gas closer in is currently less certain, but SNe Ia driven winds appear to contribute significantly to cool gas found at d < 100 kpc. In contrast, cool gas observed at d ≲ 200 kpc from luminous quasars appears to be intimately connected to the on-going quasar activities. The observed strong correlation between cool gas covering fraction in quasar host halos and quasar bolometric luminosity remains a puzzle.
With new instruments and new survey data becoming available, continuing progress is expected in a number of areas over the next few years for a better understanding of the CGM in massive halos. In particular, spatially-resolved observations of quasar outflows in the inner 10−30 kpc region, combined with absorption-line kinematics at ∼ 100 kpc from the quasar, will provide key insights into the strong correlation between κ and Lbol in quasar host halos. Integral field unit (IFU) spectrographs available on large ground-based telescopes provide a powerful tool for imaging quasar outflows based on observations of high-ionization lines.
In addition, while measurements of chemical compositions provide a unique constraint for the physical origin of chemically-enriched gas in massive quiescent halos, measurements of N(HI) are necessary for direct comparisons between observations and state-of-the-art cosmological simulations. The Cosmic Origins Spectrograph (COS; Green et al., 2012) on board the Hubble Space Telescope provides the spectral coverage necessary for N(HI) measurements at z ≲ 1. With an increasing number of z ≈ 0.5 LRGs found near the sightline of a UV bright QSO, there will soon be a statistical sample of massive quiescent halos with known N(HI) at different projected distances for testing simulation predictions.
Furthermore, understanding the roles of satellites and satellite interactions in producing chemically-enriched cool clumps in low-density halos requires deep galaxy survey data in quasar fields. With several wide-field integral field spectrographs being installed on ground-based telescopes, deep galaxy survey data in a large number of quasar fields will soon be available for systematic studies of the galaxy environments of kinematically complex absorbers.
Finally, little is known regarding the CGM properties and galactic environments of massive starburst galaxies with Mstar ≳ 1011 M⊙ (e.g. Borthakur et al., 2013). Although these galaxies are very rare, contributing to roughly 10% of the massive galaxy population (with the rest being quiescent LRGs), they are intrinsically UV luminous and massive stars in these galaxies serve as the backlight for probing the internal star-forming ISM in front of the massive young stars. With numerous large-scale galaxy surveys expected in the coming years (e.g. Zhu et al., 2015), combining intrinsic absorption-line observations with absorption spectroscopy along transverse sightlines (e.g. Rubin et al., 2010, Kacprzak et al., 2014) will be feasible for a statistically significant sample of massive starburst galaxies. These new data will offer an important empirical understanding of the impact of starbursts on the CGM in massive halos.
Acknowledgements The author wishes to thank Sean Johnson, Rebecca Pierce, Michael Rauch, 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.