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

TL;DR

This paper proposes using Thomson scattering of quasar light to detect and measure the warm and hot baryonic gas in the circumgalactic medium, predicting detectable signals with JWST across a wide redshift range.

Contribution

It introduces a novel method to observe the warm/hot CGM via Thomson scattering, enabling measurements of baryonic content across cosmic time.

Findings

Surface brightness profiles are detectable with JWST within 100 kpc.

The signal is redshift independent, allowing studies up to z~6.5.

Dust scattering becomes relevant at high redshift, but can be distinguished.

Abstract

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.