7. SUMMARY AND FUTURE PROSPECTS

Significant progress has been made over the last decade in characterizing the cool circumgalactic gas in massive halos of M_h > 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:

1. Chemically-enriched cool gas of T ∼ 10⁴ K is present in massive quiescent halos at z ∼ 0.5, with a declining annular average of covering fraction from ⟨ κ ⟩ ≳ 15% at d < 120 kpc to ⟨ κ ⟩ ≈ 5% out to the virial radius R_vir. The improved statistics help rule out definitively an absence of cool gas in massive quiescent halos at z ∼ 0.5, extending observations of cold gas in local early-type galaxies (e.g. Young et al., 2014) to those at intermediate redshifts.
2. Strong Mg II absorbers of W_r(2796) > 1 Å produced by photo-ionized cool gas are not uncommon throughout quiescent halos from d < 100 kpc to the virial radius d ≈ R_vir and the observed Mg II absorbing strength in these halos does not depend on either galaxy luminosity or mass. The lack of correlation between W_r(2796) and galaxy properties in quiescent halos suggests that the observed cool gas is likely to originate in infalling materials from the IGM, rather than outflowing gas from these early-type galaxies.
3. The velocity dispersion of Mg II absorbing gas around the majority (≈ 90%) of massive quiescent galaxies is suppressed, at ≈ 60% of what is expected from the virial motion. Dissipation is expected if these Mg II absorbers originate in cool clumps condensed out of the hot halo through thermal instabilities and the clumps decelerate due to ram-pressure while moving through the hot halo. In this simple cloud model, the volume filling factor of the clumps is small in these massive halos with a mean number of n_clump ∼ 4 per sightline in order to explain the large scatter found in the W_r(2796) versus d distribution, and a mean clump mass of m_cl ≈ 5 × 10⁴ M_⊙ in order to explain the suppression in velocity distribution.
4. While gas metallicity alone is insufficient for distinguishing between infalling and outflowing gas due to an unknown degree of chemical mixing in the CGM, the observed chemical composition of the gas offers important clues for the chemical enrichment history. The chemical composition of cool halo gas at d ≲ 100 kpc from massive quiescent galaxies displays an elevated iron abundance level that differs from an α-element enhancement typically found in star-forming galaxies and in the IGM. The observed Fe/Mg ratio implies a fractional contribution of SNe Ia to the total (Type Ia and core-collapse combined) of f_Ia ≈ 15−20% in these inner massive halos. Beyond d ≈ 100 kpc, the observed Fe/Mg ratio recovers the typical α-element enhanced level.
5. There exists a strong correlation between the cool halo gas covering fraction κ in quasar host halos and quasar bolometric luminosity L_bol, leading to a surge of cool gas in halos about luminous quasars at both low and high redshifts. The strong κ–L_bol correlation suggests a physical link between cool gas content on scales of 100 kpc and quasar activities on sub-parsec scales, but interpreting this strong correlation remains challenging. The primary difficulty lies in the relatively short quasar lifetime of ≈ 10−100 Myr in comparison to the long dynamical time necessary to move gaseous clouds over a large distance of ≈ 200 kpc. Direct imaging of quasar-driven outflows and observations of highly-ionized gas associated with cool gas at d ≈ 200 kpc are necessary to establish direct connections between outflows and cold gas detected at large distances.

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 L_bol 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 M_star ≳ 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.