Do Hot Halos Around Galaxies Contain the Missing Baryons?

Abstract

Galaxies are missing most of their baryons, and many models predict these baryons lie in a hot halo around galaxies. We establish observationally motivated constraints on the mass and radii of these haloes using a variety of independent arguments. First, the observed dispersion measure of pulsars in the Large Magellanic Cloud allows us to constrain the hot halo around the Milky Way: if it obeys the standard NFW profile, it must contain less than 4-5% of the missing baryons from the Galaxy. This is similar to other upper limits on the Galactic hot halo, such as the soft X-ray background and the pressure around high velocity clouds. Second, we note that the X-ray surface brightness of hot haloes with NFW profiles around large isolated galaxies is high enough that such emission should be observed, unless their haloes contain less than 10-25% of their missing baryons. Third, we place constraints on the column density of hot haloes using nondetections of OVII absorption along AGN sightlines: in general they must contain less than 70% of the missing baryons or extend to no more than 40 kpc. Flattening the density profile of galactic hot haloes weakens the surface brightness constraint so that a typical L * galaxy may hold half its missing baryons in its halo, but the OVII constraint remains unchanged, and around the Milky Way a flattened profile may only hold 6 − 13% of the missing baryons from the Galaxy (2 − 4 × 10 10 M ⊙ ). We also show that AGN and supernovae at low to moderate redshift -the theoretical sources of winds responsible for driving out the missing baryons -do not produce the expected correlations with the baryonic Tully-Fisher relationship and so are insufficient to explain the missing baryons from galaxies. We conclude that most of missing baryons from galaxies do not lie in hot haloes around the galaxies, and that the missing baryons never fell into the potential wells of protogalaxies in the first place. They may have been expelled from the galaxies as part of the process of galaxy formation.

How can star formation be sustained?

“…The above results agree nicely with recent indirect measures of the corona of the Milky Way (Anderson & Bregman 2010). Gatto et al (2013), using ram-pressure stripping from dwarf galaxies around the Milky Way estimated that the mass of the Milky Way's corona could be around 10 − 20% of the baryons missing from the Galaxy potential well.…”

“…These atmospheres (cosmological coronae) are expected to contain a large fraction of the baryons associated to a galactic potential well (Fukugita & Peebles 2006) and yet their detection proved elusive (Rasmussen et al 2009). In the last couple of years the first convincing detections were obtained, all coming from massive late-type galaxies: NGC 1961 (Anderson & Bregman 2011;Bogdan et al 2012), NGC 12591 (Dai et al 2012), and NGC 6753 (Bogdan et al 2012). The typical extent of these coronae are ∼ 50−80 kpc from the centre of the galaxy and the total masses are calculated by extrapolating to the virial radius, with obvious uncertainties.…”

An optical and H i study of the dwarf Local Group galaxy VV124 = UGC4879

“…Using the numbers relevant for VV124, we estimate that the density of the ambient medium should be n IGM ∼ 5 × 10 −4 cm −3 , which is probably too large at the distance of VV124 from the centre of the Local Group (e.g. Anderson & Bregman 2010). For example, this value is larger than the density required to produce the Magellanic Stream (at mere 50 kpc from the MW) in the model by Mastropietro et al (2005).…”

The Baryonic Tully–fisher Relation of Gas-Rich Galaxies as a Test of Λcdm and Mond

“…Such a component is expected in some galaxy formation scenarios. While some hot gas is certainly present, it appears that its density is too low to make a substantial contribution here (Anderson & Bregman 2010). …”