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In this review of gas accretion and the angular momentum of galaxies and galaxy halos, we began (§ 2) by reviewing the origin of angular momentum in dark matter halos via Tidal Torque Theory, where large scale tidal torques before halo turnaround set the initial angular momentum of a collapsing region based on the structure of large scale overdensities. This ultimately sets a distribution of halo spin parameters in dark matter halos, independent of halo mass, and with typical spins of λ = j / √2 Vvir Rvir ≃ 0.04.

Under the canonical galaxy formation model (§ 3), it is presumed that the angular momentum of inflowing gas matches that of the dark matter, shock–heats to the virial temperature of the halo, where the gas becomes well-mixed, before ultimately cooling out of the halo while conserving angular momentum to form a rotationally supported disk galaxy at the halo center. Under this picture, the hot gas halo is expected to have roughly the same spin distribution of the dark matter (λgas = λDM), such that the disk galaxy that eventually forms should have a disk size of roughly Rd ≃ λ Rvir. While this picture is a good approximation for estimating galactic disk sizes in practice, there are growing number of observations (outlined in § 4.1) both in the local and distant universe that demonstrate the presence of significantly higher angular momentum material at large distances from the centers of galaxies, which might seem difficult to explain under the above scenario.

Our main emphasis in this review (§ 4.2) has been a summation of recent findings from hydrodynamic cosmological simulations that suggest a modified picture for angular momentum acquisition, particular as it relates to predictions for the circumgalactic medium (CGM) of massive galaxies at z ≳ 1. This emerging picture, found in qualitative agreement among a variety of simulations — including large statistical samples of intermediate resolution simulations, smaller statistical samples of high resolution zoom-in simulations, and a wide range of code architectures and feedback physics implementations — is summarized as follows:

It is important to note that much of emphasis in this modified picture for angular momentum acquisition has been on the process by which cold filamentary gas transitions from the cosmic web through the CGM on its way to the galaxy, particularly that this cold gas has significantly higher angular momentum while in the CGM than either the dark matter halo or the baryons in the galaxy. One important clarification is that this factor of ∼ 4 enhancement in cold halo gas spin versus dark matter is a result of the coupling of cold CGM gas properties (gas that is freshly accreted to the halo, and that is of a filamentary origin) working together to produce this enhancement. The same level of spin enhancement should not hold for the cold gas in the galactic region, as the baryons near the galactic center (or indeed within the galaxies themselves) are more likely to probe a prolonged accretion history from multiple “modes” of accretion, and is thus more likely to mimic the spin of the dark matter halo, as expected from the canonical picture of galaxy formation.

For example, Figure 2 showed the spin parameter distribution for cold halo gas, but did not include the inner region (i.e. 0.1 < r / Rvir < 1). However, misalignment between the inspiraling cold streams and the baryons in the central region typically leads to significant vector cancellation, with a lower overall spin parameter for gas once the galactic region is included. Thus, while the mean spin parameter of cold halo gas shown in Figure 2 is ⟨λcold⟩ = 0.11, in a complementary panel in the same figure (Figure 6 from Danovich et al., 2015) the mean spin parameter for all cold gas within the virial radius of the halo (r < Rvir) is noticeably reduced: ⟨ λcold⟩ = 0.086 ∼ 2 ⟨λDM⟩. Thus, in angular momentum studies where all material within the virial radius is included (including the galactic region), only this factor of ∼2 enhancement of cold gas versus dark matter is expected. For example, Teklu et al. (2015) compared of the spin of all cold gas within r < Rvir to that of the dark matter at z = 2, finding that λcold = 0.074 ∼ 2λDM. Similarly, Zjupa & Springel (2016) recently studied the angular momentum of dark matter halos and their baryons for ∼ 320,000 moderately high resolution halos from the Illustris simulation (Vogelsberger et al., 2014) — comparing all gas within the virial radius (r < Rvir) and not making any distinction between cold versus hot gas components, finding that λgas ≃ 0.1 ∼ 2λDM, in agreement with other work reviewed here.

We also note that while the modifications suggested here for the standard picture of angular momentum acquisition in galaxy halos has strong implications for the angular momentum of baryons in the CGM, it is unclear at this time how this modified picture directly impacts of the angular momentum of the galaxies that form at the center of the halo. If cold gas accretion onto galaxies typically has higher spin than the dark matter, but that angular momentum is subsequently lost from the galactic disk by strong torques from inspiraling cold streams, or redistribution of angular momentum via subsequent mergers, diffuse accretion and/or outflows, it may be that the similar spins for galactic disks and dark matter halos are merely the result of coincidence. Alternatively, since the galaxy that ultimately forms at the center of a growing dark matter halo is the result of an extended, cumulative process, which must by its very nature account for misalignments in the angular momentum direction of accretion over cosmic timescales, it may not be surprising (or coincidental) that the specific angular momentum of galactic disks are similar to their dark matter halos. After all, the dark matter halo also probes the cumulative history of angular momentum acquisition over cosmic time.

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