Speaker
Description
Relatively massive galaxies such as the Milky Way are expected from theoretical galaxy formation and cosmological simulations to contain a hot gas corona extending to roughly the virial radius. In the Milky Way the existence of this hot corona has been established mainly relatively close to the galactic disc through observations in absorption and X-ray emission. Most constraints on the density of the present day corona have been derived from the ram pressure stripping of satellite galaxies that have recently lost their gas. However, our knowledge of the evolution of this corona is very limited. Using GAIA proper motions for the Draco dwarf satellite galaxy we show that its first infall around redshift z=1.6 coincides with the steep decline in Draco's star formation history. Hence, this is consistent with Draco having been stripped by ram pressure from the early corona. We have simulated this first infall in 3D hydrodynamical simulations and are able to, for the first time, put a constraint on the density of the corona at high redshift. Our lower limit of n>8x10^-4 cm^-3 in the outer part of the corona is consistent with an upper limit derived from the cosmic baryon fraction. Comparing to the constraints on the present day corona in the literature we find that the density in the outer part of the corona must have dropped by about a factor of 10 since z=1.6. This evolution agrees well with simple analytical estimates as well as predictions from cosmological zoom-in simulations.
