Revealing the Warm and Hot Halo Baryons via Thomson Scattering of Quasar
  Light
  The baryonic content and physical properties of the warm and hot
($10^5\lesssim T\lesssim 10^7$ K) phases of the circumgalactic medium (CGM) are
poorly constrained, owing to the lack of observables probing the requisite
range of temperature, spatial scale, halo mass, and redshift. The radiation
from a luminous quasar produces a spatially extended emission halo resulting
from Thomson scattering off of free electrons in the CGM, which can be used to
measure the electron density profile, and therefore, the amount of warm and hot
baryonic matter present. We predict the resulting surface brightness profiles
and show that they are easily detectable in a three hour integration with the
James Webb Space Telescope (JWST), out to $\sim 100$ physical kpc from the
centers of individual hyper-luminous quasars. This electron scattering surface
brightness is redshift independent, and the signal-to-noise ratio depends only
very weakly on redshift, in principle allowing measurements of the warm and hot
CGM into the Epoch of Reionization at $z\sim 6.5$. We consider a litany of
potential contaminants, and find that for fainter quasars at $z\lesssim1$,
extended stellar halos might be of comparable surface brightness. At $z>2$,
JWST mid-IR observations start to probe rest-frame optical/UV wavelengths
implying that scattering by dust grains in the CGM becomes significant,
although multi-color observations should be able to distinguish these scenarios
given that Thomson scattering is achromatic.
