The Cos-Halos Survey: Physical Conditions and Baryonic Mass in the Low-Redshift Circumgalactic Medium

Abstract

We analyze the physical conditions of the cool, photoionized (T ∼ 10 4 K) circumgalactic medium (CGM) using the COS-Halos suite of gas column density measurements for 44 gaseous halos within 160 kpc of L ∼ L * galaxies at z ∼ 0.2. These data are well described by simple photoionization models, with the gas highly ionized (n HII /n H 99%) by the extragalactic ultraviolet background (EUVB). Scaling by estimates for the virial radius, R vir , we show that the ionization state (tracked by the dimensionless ionization parameter, U) increases with distance from the host galaxy. The ionization parameters imply a decreasing volume density profile n H = (10 −4.2±0.25 )(R/R vir ) −0.8±0.3 . Our derived gas volume densities are several orders of magnitude lower than predictions from standard two-phase models with a cool medium in pressure equilibrium with a hot, coronal medium expected in virialized halos at this mass scale. Applying the ionization corrections to the H I column densities, we estimate a lower limit to the cool gas mass M cool CGM > 6.5 × 10 10 M ⊙ for the volume within R < R vir . Allowing for an additional warm-hot, OVI-traced phase, the CGM accounts for at least half of the baryons purported to be missing from dark matter halos at the 10 12 M ⊙ scale.

Astrophysical origin of high-energy cosmic neutrinos

“…The circumgalactic density suggested from observations of dozens of nearby galaxies indicating a decreasing density profile [72], which is also consistent with theoretical expectations. Then, for s ν ∼ 2, we typically expect i (E 2 ν ν i ) ∼ 7 × 10 −9 GeV cm −2 s −1 sr −1 , for a typical enhancement factor of past star formation [23].…”

The Properties of the Circumgalactic Medium in Red and Blue Galaxies: Results From the Cos-Gass+cos-Halos Surveys

“…Current constraints on the size of these structures (which are based on models in which the gas is photoionized by the metagalactic background) are rather weak, but characteristic pathlengths are of-order 1-10 kpc (Stocke et al 2013;Werk et al 2014).…”

“…Therefore, it is likely that most of these highly ionized absorbers are different from those detected in various other CGM and QSO-absorption studies (Chen et al 2001a;Wakker & Savage 2009;Tumlinson et al 2011;Borthakur et al 2013;Bordoloi et al 2014, and references therein). However, as discussed in detail by Werk et al (2014), Fox et al (2013), Meiring et al (2013), Lehner et al (2013), Tripp et al (2011), and references therein, O VI may represent a different phase of gas that differs from the ones traced by lower ionization transitions. Because the COS-GASS data do not cover the O VI line, we refrain from further discussion of O VI.…”

An Introduction to Gas Accretion onto Galaxies

“…While simulations require many of the metals created by a galaxy are ejected from the system (see panel c of Figure 3), there is a large mass of metals detected in galaxy halos that remains bound (Tumlinson et al, 2011;Werk et al, 2014). Westmeier et al, 2007), and NGC 891 (top right; Oosterloo et al, 2007).…”