Enriching the hot circumgalactic medium. (arXiv:1304.4730v1 [astro-ph.CO]):
Models of galaxy formation in a CDM universe predict that massive galaxies
are surrounded by a hot, quasi-hydrostatic circumgalactic corona of slowly
cooling gas, predominantly accreted from the IGM. This prediction is borne out
by the cosmological hydrodynamical simulations of Crain et al., which reproduce
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 galaxy
formation models. We show here that, while the hot CGM of galaxies formed in
the simulations is 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 synthesised 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 SNe-driven winds and convection over
several gigayears, establishing a strong negative radial metallicity gradient.
Whilst metal ions synthesised by stars are necessary to produce the X-ray
emissivity that enables the hot CGM of isolated galaxies to be detected, 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.
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