The Effect of Environment on AGN Activity: The Properties of Radio and Optical AGN in Void, Isolated, and Group Galaxies

The evolution of galaxies depends on their environments. In this work, active galactic nucleus (AGN) activity in different environments has been studied. The fractions of radio and optical AGN in four different environments have been compared using samples of void, isolated, group member, and the brightest group galaxies (BGGs). Galaxies in voids show significantly lower stellar ages, concentrations, colours and surface mass densities, and they experience more one-on-one interactions compared to the isolated galaxies and galaxies in groups. In order to study pure environmental effects, the biases caused by the stellar mass and galaxy type quantified by 4000$\AA$ break strength have been removed. While the results confirm no dependence of the optical AGN activity on environment in blue galaxies and with lower significance in green galaxies, a higher fraction of optical AGN has been observed for the massive red galaxies in voids compared to the galaxies in dense environments. This may be related to the higher amount of one-on-one interaction observed in the void galaxies, or it may reflect more fundamental differences in the host galaxies or environments of the voids. The radio-mode AGN activity increases in dense environment for red galaxies. No changes in the radio-loud AGN fraction have been observed for the blue and green galaxies. This shows that the effect of environment on AGN activity is not significant in the presence of cold gas in galaxies. We also discuss whether the efficiency of gas accretion depends on the properties of the host galaxy.
Comment: The Astronomical Journal, 160, 227, (2020)