A Method to Constrain Cosmic Ray Properties Using Observations of the Circumgalactic Medium

The circumgalactic medium (CGM), the gas surrounding galaxies, is critical to galaxy evolution since it supplies cold gas to fuel star formation. However, we still do not understand many of the fundamental properties of the CGM, including the main source of pressure around low-redshift L* galaxies. Recent simulations of galaxy formation predict that cosmic rays may be an important, or even the main source of pressure in the CGM, but these predictions are extremely sensitive to the invoked cosmic-ray transport model. In this work, we present a simple analytical model to constrain the cosmic ray transport rate. Assuming the cosmic-ray pressure in the CGM is a delayed measure of the recent star formation rate, we make predictions for the lower limit of the effective cosmic ray diffusivity as a function of observable CGM properties. We demonstrate the power of this simple scaling relation in the FIRE cosmological zoom-in simulations and compare them with observations.