Revealing the Warm and Hot Halo Baryons via Thomson Scattering of Quasar Light

Abstract

The baryonic content and physical properties of the warm and hot (10 5  ≲  T  ≲ 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 ∼100 physical kpc from the centers of individual hyperluminous 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  ∼ 6.5. We consider a litany of potential contaminants and find that for fainter quasars at z  ≲ 1, 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.