In the standard Lambda Cold Dark Matter (LCDM) paradigm, galaxies form at the center of extended dark matter halo. These halos grow hierarchically through halo mergers (including embedded galaxies) as well as by diffuse accretion of dark matter and gas from the cosmic web. Diffuse infalling gas is expected to shock–heat to the virial temperature of the halo, mixing within the halo until it virializes, with gas eventually cooling out of this hot gaseous halo, sinking to the center of the halo and onto the central galaxy (e.g. Binney, 1977, Rees & Ostriker, 1977, Silk, 1977, White & Rees, 1978, White & Frenk, 1991, Maller & Bullock, 2004). Under this (admittedly simplified) picture of galaxy formation, it is expected that the inflowing gas (and thus the virialized hot gaseous halo) should share the same angular momentum distribution as the inflowing dark matter. Conserving angular momentum, the galaxy that ultimately forms should also have specific angular momentum similar to that of the dark matter halo, resulting in a rotationally supported disk galaxy (in many cases) with a spin proportional to the dark matter halo (Mestel, 1963, Fall & Efstathiou, 1980, Mo et al., 1998), the statistical properties of which have been well–studied via dissipationless cosmological N-body simulations (e.g. Bullock et al., 2001, Vitvitska et al., 2002, Maller et al., 2002, Avila-Reese et al., 2005, D'Onghia & Navarro, 2007, Bett et al., 2010, Muñoz-Cuartas et al., 2011, Ishiyama et al., 2013, Trowland et al., 2013, Kim et al., 2015, Zjupa & Springel, 2016).
However, in recent years, advances in galaxy formation theory (both in analytic work and via cosmological hydrodynamic simulations) have begun to emphasize the importance of the filamentary nature of gas accretion onto massive galaxies, particularly at high redshift when cosmic filaments are significantly narrower and denser than in the local universe. Filamentary gas accretion, though diffuse, may be dense enough to allow cold streams to maintain cooling times shorter than the compression time to establish a stable shock, leading to what has been referred to as “cold flows” or “cold mode” gas accretion that can quickly penetrate from the virial radius of a dark matter halo all the way to the inner galactic region of the halo e.g. Kereš et al. (2005), Dekel & Birnboim (2006), Ocvirk et al. (2008), Brooks et al. (2009), Dekel et al. (2009), Faucher-Giguère & Kereš (2011), Faucher-Giguère et al. (2011), van de Voort et al. (2011), Hobbs et al. (2015), van de Voort et al. (2015).
While there has been some contention in recent years as to whether or not these cold streams are truly capable of delivering unshocked gas directly onto the galaxy, without heating in the inner regions of the halo (e.g., Torrey et al., 2012, Nelson et al., 2013, Nelson et al., 2016), the importance of distinguishing between these dense filamentary forms of gas accretion to galaxy halos (verses isotropic “hot mode” gas accretion) remains a crucial one for understanding galaxy formation. In particular — as it relates to this chapter — under this developing paradigm of filamentary versus isotropic gas accretion, halo gas (and particularly gas accreted in the cold mode) tends to show considerably higher specific angular momentum than the dark matter in the halo (Chen et al., 2003, Sharma & Steinmetz, 2005, Kereš et al., 2009, Kereš & Hernquist, 2009, Agertz et al., 2009, Brook et al., 2011, Stewart et al., 2011b, Kimm et al., 2011, Stewart et al., 2013, Codis et al., 2015, Danovich et al., 2015, Prieto et al., 2015, Teklu et al., 2015, Tillson et al., 2015, Stewart et al., 2016). In this picture, the resulting angular momentum of simulated stellar disks may be significantly different than that of the accreted gas, in part because feedback effects preferentially expel low angular momentum gas from galaxies (e.g. Maller & Dekel, 2002, Governato et al., 2010, Brook et al., 2011, Guedes et al., 2011), such that the total cumulative spin of a growing galactic disk may not be expected to match the cumulative spin of accreted dark matter or gas to the virial radius of the halo.
As a result, this emerging picture of galaxy grown seems to be in tension with the canonical picture in which the spin of the accreted gas (and ultimately, the galaxy) mimics the dark matter. Thus, a modified picture of angular momentum acquisition that takes into account these distinctions between filamentary and isotropic gas accretion seems to be developing in the literature. While this picture is by no means fully developed, nor universally agreed upon, one of the main goals of this review is to serve as a synthesis for a number of recent theoretical works (primarily utilizing the results from hydrodynamic cosmological simulations) in order to present a consistent new picture for angular momentum acquisition to galaxies, as well as the expected angular momentum content of gaseous halos around galaxies.
The outline of this review is as follows. We will begin in section 2 by briefly reviewing the origin of angular momentum in dark matter halos within the framework of Lambda Cold Dark Matter cosmology (LCDM), including a discussion of tidal torque theory (TTT), the role of mergers, and studies of dark matter halos from cosmological dissipationless N-body simulations. In section 3 we will review the canonical model for galaxy formation in LCDM, which builds upon these properties of dark matter halos as a means of understanding the process by which gas within dark matter halos shock–heats, dissipates energy, and ultimately sinks to the center of the halo's gravitational potential to form stars (galaxies) there. We will also discuss some of the historic challenges in simulating galaxies with realistic angular momentum in hydrodynamic cosmological simulations. The most in-depth portion of this review is section 4, which begins by briefly discussing some of the observational challenges to this canonical picture — namely, a number of recent observations of coherent co-rotation and high angular momentum gas in galaxy halos. This is followed by a deeper discussion of recent studies of hydrodynamic cosmological simulations that have begun to demonstrate a need to update the canonical picture of gas accretion onto galaxies (in large part by emphasizing the importance of “cold flow” filamentary gas accretion) and how these modifications are in better alignment with recent observations. We summarize and conclude in section 5.