The Extended Distribution of Baryons around Galaxies

Abstract

We summarize and reanalyze observations bearing upon missing galactic baryons, where we propose a consistent picture for halo gas in L L* galaxies. The hot X-ray emitting halos are detected to 50-70 kpc, where typically, M hot (< 50 kpc) ∼ 5 × 10 9 M , and with density n ∝ r −3/2 . When extrapolated to R 200 , the gas mass is comparable to the stellar mass, but about half of the baryons are still missing from the hot phase. If extrapolated to 1.9-3R 200 , the baryon to dark matter ratio approaches the cosmic value. Significantly flatter density profiles are unlikely for R < 50 kpc and they are disfavored but not ruled out for R > 50 kpc. For the Milky Way, the hot halo metallicity lies in the range 0.3-1 solar for R < 50 kpc. Planck measurements of the thermal Sunyaev-Zeldovich effect toward stacked luminous galaxies (primarily early-type) indicate that most of their baryons are hot, near the virial temperature, and extend beyond R 200 . This stacked SZ signal is nearly an order of magnitude larger than that inferred from the X-ray observations of individual (mostly spiral) galaxies with M * > 10 11.3 M . This difference suggests that the hot halo properties are distinct for early and late type galaxies, possibly due to different evolutionary histories. For the cooler gas detected in UV absorption line studies, we argue that there are two absorption populations: extended halos; and disks extending to ∼ 50 kpc, containing most of this gas, and with masses a few times lower than the stellar masses. Such extended disks are also seen in 21 cm HI observations and in simulations.

GASP XXIII: A Jellyfish Galaxy as an Astrophysical Laboratory of the Baryonic Cycle

“…The latter condition is likely to be verified everywhere but, perhaps, the central kpc or so (e.g. Bregman et al (2018)). We also tested the hot halo stripping hypothesis with a simple model adopting a mass model for the galaxy including a stellar bulge, a stellar disk and a dark matter halo (see Appendix for details).…”

“…b) The second hypothesis concerns the galaxy hot, X-ray emitting halo (see Bregman et al (2018) for a review of hot circumgalactic gas). Before being stripped, we may expect JW100 to have possessed a hot, Xray-emitting halo, that could have been stretched during the stripping, producing the observed extra-planar diffuse emission (Bekki 2009).…”

Massive Warm/Hot Galaxy Coronae. II. Isentropic Model

“…However, P13 note that a single power-law is not a formally acceptable fit to the mea-suredỸ 500 − M 500 relation. This may be a result of the gas distributions in galaxies differing from those in clusters (see also Bregman et al 2018). Thus, the actual SZ signal for MW-mass galaxies may be different from the extrapolated value.…”

An X‐ray view of the hot circum‐galactic medium

“…The hot CGM could contain a significant fraction of the “missing baryons” (e.g., Anderson & Bregman ; Bregman ; Bregman et al ). As the soft X‐ray emission from hot gas is dominated by metal lines whose strength is proportional to both the gas density and metallicity, the detected soft X‐ray emission around nearby galaxies is largely biased to the highly metal‐enriched CGM around actively star‐forming (SF) galaxies.…”

“…Furthermore, because the origin of gas at larger radii may be quite different (more gas with external instead of internal origin, thus lower metallicity), the poor constraint of gas distribution at r ≳ (10 − 20)% r 200 prevents us from both searching for the accreted hot CGM and measuring the total mass and energy contained in the CGM. It is not impossible that the hot CGM extends beyond the virial radius of the dark matter halo; thus it contains a larger fraction of the missing baryons (Bregman et al ), but better observations are needed to constrain the hot gas distribution at large radii.…”