Ben Oppenheimer - August 31, 2016

Virial Thermometers: A New Understanding of the Circumgalactic Medium

The circumgalactic medium, halo gas surrounding galaxies, is observed to be highly
enriched in the low-redshift (z~0.2) Universe. I will present new results of
hydrodynamical simulations confronting COS-Halos quasar absorption line observations
of highly ionized oxygen (O VI) within 150 kpcs of L* spirals and ellipticals.
Observations show a strong correlation between a galaxy's star formation rate and
the amount of O VI in the circumgalactic medium. While initial interpretations of
this correlation suggested galactic superwinds actively enriching extended halos,
I will argue for a different model. Most metals nucleosynthesized by stars are
ejected by superwinds into the circumgalactic gas. However, ionization corrections
determine what oxygen ion is observed. I will present a model where O VI happens
to trace the virial temperature of halos hosting star-forming spirals, while
passive elliptical live in more massive, more enriched halos where oxygen primarily
lives in higher ions. The results are much greater reservoirs of circumgalactic
metals than previously assumed. Thus the amount and state of oxygen serves as an
indicator of a halo's virial temperature.