The effects of environment on radio-loud AGN activity in Stripe 82

Abstract

We investigate the link between environment and radiative accretion efficiency using a sample of 8946 radio-loud AGN detected at 1 − 2 GHz in the SDSS Stripe 82 region. We quantify their environments using the surface-density parameter, ƩN, which measures galaxy density based on distances to Nth nearest neighbours. Comparing Ʃ2 and Ʃ5 between AGN and control galaxies, we obtain relative densities that quantify the degree of galaxy clustering around each AGN. Using this, we examine the relation between density and the HERG-LERG dichotomy (accretion-modes) classified using a 1.4 GHz luminosity (L1.4GHz) threshold. Our results indicate that, in the low-redshift interval (0.1 < z < 0.2), LERGs occupy environments denser than the field. At intermediate redshifts (0.2 < z < 1.2), both LERGs and HERGs occupy regions denser than the field. Spearman’s rank tests show that correlations between density and L1.4GHz in both redshift intervals are weak. We conclude that the absence of a strong correlation is confirmation of the idea that galaxy density plays a more secondary role on AGN activity and also, accretion-mode classification (both measured using L1.4GHz). It is likely that the rate of gas accretion or properties of galactic-scale magnetic fields correlate more strongly with L1.4GHz, hence being primarily influential.