Enriching the hot circumgalactic medium

Abstract

Simple models of galaxy formation in a cold dark matter universe predict that massive galaxies are surrounded by a hot, quasi-hydrostatic circumgalactic corona of slowly cooling gas, predominantly accreted from the intergalactic medium (IGM). This prediction is borne out by the recent cosmological hydrodynamical simulations of Crain et al., which reproduce observed scaling relations between the X-ray and optical properties of nearby disc galaxies. Such coronae are metal poor, but observations of the X-ray emitting circumgalactic medium (CGM) of local galaxies typically indicate enrichment to near-solar iron abundance, potentially signalling a shortcoming in current models of galaxy formation. We show here that, while the hot CGM of galaxies formed in the simulations is typically metal poor in a mass-weighted sense, its X-ray luminosity-weighted metallicity is often close to solar. This bias arises because the soft X-ray emissivity of a typical ∼0.1 keV corona is dominated by collisionally excited metal ions that are synthesized in stars and recycled into the hot CGM. We find that these metals are ejected primarily by stars that form in situ to the main progenitor of the galaxy, rather than in satellites or external galaxies. The enrichment of the hot CGM therefore proceeds in an ‘inside–out’ fashion throughout the assembly of the galaxy: metals are transported from the central galaxy by supernova-driven winds and convection over several Gyr, establishing a strong negative radial metallicity gradient. Whilst metal ions synthesized by stars are necessary to produce the X-ray emissivity that enables the hot CGM of isolated galaxies to be detected with current instrumentation, the electrons that collisionally excite them are equally important. Since our simulations indicate that the electron density of hot coronae is dominated by the metal-poor gas accreted from the IGM, we infer that the hot CGM observed via X-ray emission is the outcome of both hierarchical accretion and stellar recycling.