1 research outputs found
Constraining Cosmic-ray Transport with Observations of the Circumgalactic Medium
Recent theoretical studies predict that the circumgalactic medium (CGM)
around low-redshift, galaxies could have substantial nonthermal
pressure support in the form of cosmic rays. However, these predictions are
sensitive to the specific model of cosmic-ray transport employed, which is
theoretically and observationally underconstrained. In this work, we propose a
novel observational constraint for calculating the lower limit of the
radially-averaged, effective cosmic-ray transport rate, . Under a wide range of assumptions (so long as cosmic rays do not lose a
significant fraction of their energy in the galactic disk, regardless of
whether the cosmic-ray pressure is important or not in the CGM), we demonstrate
a well-defined relationship between and three
observable galaxy properties: the total hydrogen column density, the average
star formation rate, and the gas circular velocity. We use a suite of FIRE-2
galaxy simulations with a variety of cosmic-ray transport physics to
demonstrate that our analytic model of is a robust
lower limit of the true cosmic-ray transport rate. We then apply our new model
to calculate for galaxies in the COS-Halos sample,
and confirm this already reveals strong evidence for an effective transport
rate which rises rapidly away from the interstellar medium to values
(corresponding to anisotropic streaming velocities of ) in the diffuse CGM, at impact parameters
larger than \,kpc. We discuss how future observations can provide
qualitatively new constraints in our understanding of cosmic rays in the CGM
and intergalactic medium.Comment: 9 pages, 2 figures, accepted to MNRA