The Radio Galaxy Population in the SIMBA SImulations
  We examine the 1.4GHz radio luminosities of galaxies arising from star
formation and active galactic nuclei (AGN) within the state-of-the-art
cosmological hydrodynamic simulation Simba. Simba grows black holes via
gravitational torque limited accretion from cold gas and Bondi accretion from
hot gas, and employs AGN feedback including jets at low Eddington ratios. We
define a population of radio loud AGN (RLAGN) based on the presence of ongoing
jet feedback. Within RLAGN we define high and low excitation radio galaxies
(HERGs and LERGs) based on their dominant mode of black hole accretion: torque
limited accretion representing feeding from a cold disk, or Bondi representing
advection-dominated accretion from a hot medium. Simba predicts good agreement
with the observed radio luminosity function (RLF) and its evolution, overall as
well as separately for HERGs and LERGs. Quiescent galaxies with AGN-dominated
radio flux dominate the RLF at > $\sim 10^{22-23}$ W Hz$^{-1}$, while star
formation dominates at lower radio powers. Overall, RLAGN have higher black
hole accretion rates and lower star formation rates than non-RLAGN at a given
stellar mass or velocity dispersion, but have similar black hole masses. Simba
predicts a LERG number density of 8.53 Mpc$^{-3}$, $\sim 10\times$ higher than
for HERGs, broadly as observed. While LERGs dominate among most massive
galaxies with the largest black holes and HERGs dominate at high specific star
formation rates, they otherwise largely populate similar-sized dark matter
halos and have similar host galaxy properties. Simba thus predicts that deeper
radio surveys will reveal an increasing overlap between the host galaxy
demographics of HERGs and LERGs.
