The Extended Distribution of Baryons around Galaxies

Abstract

Abstract We summarize and reanalyze observations bearing on 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 × 109 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 1.7R 200–3R 200, the ratio of baryon to dark matter 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–Zel’dovich (SZ) effect toward stacked luminous galaxies (primarily early type) indicate that most of their baryons are hot, are 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 * > 1011.3 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 H i observations and in simulations.