The Stripe 82 1-2 GHz Very Large Array Snapshot Survey: host galaxy properties and accretion rates of radio galaxies

Abstract: A sample of 1161 radio galaxies with $0.01 < z < 0.7$ and $10^{21} < L_{1.4~\rm GHz} / \textrm{W Hz}^{-1} < 10^{27}$ is selected from the Stripe 82 1-2 GHz Karl G. Jansky Very Large Array Snapshot Survey, which covers 100 square degrees and has a 1$\sigma$ noise level of 88 $\mu$Jy beam$^{-1}$. Optical spectra are used to classify these sources as high excitation and low excitation radio galaxies (HERGs and LERGs), resulting in 60 HERGs, 149 LERGs and 600 `probable LERGs'. The host galaxies of the LERGs have older stellar populations than those of the HERGs, in agreement with previous results in the literature. We find that the HERGs tend to have higher Eddington-scaled accretion rates than the LERGs but that there is some overlap between the two distributions. We show that the properties of the host galaxies vary continuously with accretion rate, with the most slowly accreting sources having the oldest stellar populations, consistent with the idea that these sources lack a supply of cold gas. We find that 84 per cent of our sample release more than 10 per cent of their accretion power in their jets, showing that mechanical AGN feedback is significantly underestimated in many hydrodynamical simulations. There is a scatter of $\sim 2$ dex in the fraction of the accreted AGN power deposited back into the ISM in mechanical form, showing that the assumption in many simulations that there is a direct scaling between accretion rate and radio-mode feedback does not necessarily hold. We also find that mechanical feedback is significant for many of the HERGs in our sample as well as the LERGs.