Gaseous Galaxy Halos

Abstract

Galactic halo gas traces inflowing star formation fuel and feedback from a galaxy's disk and is therefore crucial to our understanding of galaxy evolution. In this review, we summarize the multi-wavelength observational properties and origin models of Galactic and low redshift spiral galaxy halo gas. Galactic halos contain multiphase gas flows that are dominated in mass by the ionized component and extend to large radii. The densest, coldest halo gas observed in neutral hydrogen (HI) is generally closest to the disk (< 20 kpc), and absorption line results indicate warm and warm-hot diffuse halo gas is present throughout a galaxy's halo. The hot halo gas detected is not a significant fraction of a galaxy's baryons. The disk-halo interface is where the multiphase flows are integrated into the star forming disk, and there is evidence for both feedback and fueling at this interface from the temperature and kinematic gradient of the gas and HI structures. The origin and fate of halo gas is considered in the context of cosmological and idealized local simulations. Accretion along cosmic filaments occurs in both a hot (> 10 5.5 K) and cold mode in simulations, with the compressed material close to the disk the coldest and densest, in agreement with observations. There is evidence in halo gas observations for radiative and mechanical feedback mechanisms, including escaping photons from the disk, supernova-driven winds, and a galactic fountain. Satellite accretion also leaves behind abundant halo gas. This satellite gas interacts with the existing halo medium, and much of this gas will become part of the diffuse halo before it can reach the disk. The accretion rate from cold and warm halo gas is generally below a galaxy disk's star formation rate, but gas at the disk-halo interface and stellar feedback may be important additional fuel sources.

Neutral Gas Accretion onto Nearby Galaxies

“…Distant dwarfs have M HI /L * ≈ 1 (Spekkens et al, 2014), so we can estimate the H I mass that has been deposited in the CGM through this stripping. For the dwarf galaxies currently known around the Milky Way, it amounts to M HI ≈ 3 × 10 7 M , an value almost identical to the estimated total H I mass of HVCs in each of the HVC systems of the Milky Way, M31, and M33 (Putman et al, 2012;Westmeier et al, 2008;Keenan et al, 2016). It is tempting to dismiss this as a coincidence, especially because the inferred mass of the "stripped" H I for the Milky Way is dominated by the most massive galaxy, the Sagittarius dSph (Spekkens et al, 2014).…”

“…The disk-halo gas extends to heights where it may mix with material accreting through or cooling from the CGM (Putman et al, 2012). Thus the presence of neutral gas many scale-heights away from the plane is not unexpected, and distinguishing between gas that is "recycled" as much of the disk-halo material must be, and gas that has never been in the disk, may not be straightforward in the absence of other information.…”

“…This does not mean that some of the more compact, more isolated HVCs might not be free-floating independent systems, but the total H I mass within the CGM of normal galaxies (aside from the products of tidal interaction) must be relatively small (Pisano et al, 2007(Pisano et al, , 2011. The HVC system of the Milky Way has been estimated to contain an H I mass of ∼ 3 × 10 7 M (Putman et al, 2012). The HVC population likely contains as much ionized gas as neutral gas (Shull et al, 2009(Shull et al, , 2011Lehner & Howk, 2011).…”

“…They adopt the stance that any significant deviation of a galaxy's H I from symmetry is evidence for interaction, and that interaction implies accretion. Accretion also has a prominent place in the recent review of gaseous galaxy halos by Putman et al (2012) which takes a thorough look at circumgalactic gas in all phases. I will refer to these reviews throughout this article for more complete discussion of some topics.…”

“…This gas may cool and condense, feeding the disk. There may also be cosmological accretion -cold flows -which may have to traverse the CGM to reach the disk and in that passage may be disrupted entirely (Putman et al, 2012). Then there is gas that has been stripped from one galaxy through interaction and ultimately ends up in the disk of another.…”