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In the standard picture of galaxy formation and evolution, primordial gas first cools and condenses within dark matter halos to form stars (e.g. White & Rees, 1978, Blumenthal et al., 1984). Gas in high-mass halos with dark matter halo mass exceeding Mh ≈ 1012 M is expected to be shock-heated to high temperatures (e.g. Birnboim & Dekel, 2003, Kereš et al., 2005, Kereš et al., 2009, Dekel & Bernboim, 2006). Within the hot gas halos, thermal instabilities can induce the formation of pressure-supported cool clouds (e.g. Mo & Miralda-Escudé, 1996, Maller & Bullock, 2004, Sharma et al., 2012, Voit et al., 2015). In lower mass halos, cool filaments from the intergalactic medium can reach deep into the center of the halo without being shock-heated. Both condensed cool clouds of hot halos and cool filaments can, in principle, supply the fuels necessary to sustain star formation in galaxies. As new stars form and evolve, the surrounding interstellar and circumstellar gas is expected to be heated and enriched by heavy elements ejected from massive stars, regulating subsequent star formation.

Several semi-analytic studies have been carried out in searching for a general prescription that connects galaxies found in observations to dark matter halos formed in theoretical frameworks. These studies seek to establish a mean relation between galaxy stellar mass (Mstar) and host halo mass (Mh) by matching the observed space density of galaxies with the expected abundance of dark matter halos as a function of mass (e.g. Vale & Ostriker, 2004, Shankar et al., 2006). The mean galaxy mass and halo mass ratio is found to peak at Mh ∼ 1012 M with Mstar / Mh ≈ 0.04 and declines rapidly both toward higher and lower masses (e.g. Moster et al., 2010, Guo et al., 2010, Behroozi et al., 2010). The declining Mstar / Mh indicates a reduced star formation efficiency in both low- and high-mass halos. Different feedback mechanisms are invoked in theoretical models in order to match the observed low star formation efficiency in low- and high-mass halos. As supernova-driven winds are thought to suppress star formation efficiency in low-mass dwarf galaxies (e.g. Larson, 1974, Dekel & Silk, 1986), feedback due to active galactic nuclei (AGN) powered by supermassive black holes is invoked to quench star formation in high-mass halos, resulting in massive quiescent galaxies (e.g. Bower et al., 2006, Croton et al., 2006, Dubois et al., 2013). While blueshifted broad absorption and emission lines are commonly seen in luminous quasars, indicating the presence of high-speed outflows, direct observational evidence of AGN feedback on large scales (∼ 10−100 kpc) remains scarce (e.g. Alexander et al., 2010, Greene et al., 2012, Maiolino et al., 2012) and see also Fabian (2012) for a review.

Observations of the circumgalactic medium (CGM) in massive halos offer complementary and critical constraints for the extent of feedback and gas accretion (e.g. Somerville & Davé, 2015). In particular, the circumgalactic space within the halo radius, Rvir, lies between galaxies, where star formation takes place, and the intergalactic medium (IGM), where 90 percent of all baryonic matter in the universe resides (Rauch et al., 1997). As a result, CGM properties are shaped by the complex interactions between IGM accretion and outflows driven by energetic feedback processes in the galaxies.

Imaging observations of the cool (T ∼ 104 K) CGM are only feasible in 21 cm surveys at z ≲ 0.2, because, with few exceptions (e.g. Cantalupo et al., 2014, Fernández et al., 2016, Hayes et al., 2016), the gas density is typically too low to be detected in emission. Absorption-line spectroscopy of background quasars provides a powerful, alternative tool for studying this tenuous gas in the distant universe based on the absorption features imprinted in the quasar spectra (e.g. Lanzetta et al., 1995, Bowen et al., 1995, Steidel et al., 2002). But, because both quasars and massive galaxies are rare, close pairs of massive galaxies and background quasars by chance projection are even rarer. Studying the CGM around massive galaxies using quasar absorption spectroscopy therefore requires a large spectroscopic sample of galaxies and quasars over a substantial volume in order to assemble a statistical sample of massive galaxy and quasar pairs. The Sloan Digital Sky Survey (SDSS; York et al., 2000) has produced a large spectroscopic archive of distant galaxies and quasars, facilitating the assembly of a statistically significant sample of close massive galaxy and quasar pairs, as well as a large sample of projected quasar pairs. These pair samples have enabled systematic studies of halo gas beyond the nearby universe using absorption spectroscopy.

This chapter presents a review of the current state of knowledge on the cool CGM properties in massive halos of Mh ≈ 1012−14 M at z ≈ 0.2−2. Specifically, the review will focus on massive quiescent galaxies with Mstar ≳ 1011 M at z ≲ 1, with additional coverage on quasar host halos. Empirical studies of cool/warm gas in galaxy groups and clusters of Mh ≳ 1014 M have been carried out for a small sample (e.g. Lopez et al., 2018, Yoon et al., 2012, Andrews et al., 2013, Stocke et al., 2014), but a detailed understanding of the gas phase in galaxy cluster and group environments relies primarily on x-ray studies of the hot plasma. Extensive reviews on the x-ray properties of intragroup and intracluster gas can be found in Mulchaey (2000), Mathews & Brighenti (2003), Kravtsov & Borgani (2012).

The emphasis on quiescent galaxies in high-mass halos is motivated by the empirical finding that more than 90% of massive galaxies with Mstar ≳ 1011 M in the local universe contain primarily evolved stellar populations with little on-going star formation (e.g. Peng et al., 2010, Tinker et al, 2013). It is therefore expected that a general understanding of the CGM properties in massive halos can be established based on observations of massive quiescent galaxies. An added bonus in studying massive quiescent galaxies is the unique opportunity to explore other feedback mechanisms for quenching star formation in massive halos, in the absence of complicated starburst driven winds.

The emphasis on quasar host halos is motivated by two factors. First, while determining quasar host mass is difficult due to uncertain host galaxy properties, the large observed clustering amplitude indicates that the mean halo mass of quasar hosts is high, Mh ≳ 1012.5 M (Porciani et al., 2004, White et al., 2012, Shen et al., 2013b). In addition, observations of the CGM properties in quasar host halos directly address the issues concerning the extent of AGN feedback (e.g. Shen et al., 2013a, Fumagalli et al., 2014, Rahmati et al., 2015, Faucher-Giguère et al., 2015, Faucher-Giguère et al., 2016). Most nearby elliptical galaxies are found to host a supermassive black hole at the center (Kormendy & Richstone, 1995, Ho, 2008), and therefore high-redshift quasar hosts are likely the progenitors of these nearby massive quiescent galaxies. Studies of the CGM in quasar host halos may provide important clues for how the CGM properties are shaped while the galaxy undergoes an active quasar phase.

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