Active Galactic Nuclei in Void Regions

Abstract

We present a comprehensive study of accretion activity in the most underdense environments in the universe, the voids, based on the SDSS DR2 data. Based on investigations of multiple void regions, we show that AGN's occurrence rate and properties differ from those in walls. AGN are more common in voids than in walls, but only among moderately luminous and massive galaxies (M_r < -20, log M_*/M_sun < 10.5), and this enhancement is more pronounced for the weakly accreting systems (i.e., L_[O III] < 10^39 erg/s). Void AGN hosted by moderately massive and luminous galaxies are accreting at equal or lower rates than their wall counterparts, show less obscuration than in walls, and similarly aged stellar populations. The very few void AGN in massive bright hosts accrete more strongly, are more obscured, and are associated with younger stellar emission than wall AGN. Thus, accretion strength is probably connected to the availability of fuel supply, and accretion and star-formation co-evolve and rely on the same source of fuel. Nearest neighbor statistics indicate that the weak accretion activity (LINER-like) is not influenced by the local environment. However, H IIs, Seyferts, and Transition objects prefer more grouped small scale structures, indicating that the rate at which galaxies interact with each other affects their activity. These trends support a potential H II -> Seyfert/Transition Object -> LINER evolutionary sequence that we show is apparent in many properties of actively line-emitting galaxies, in both voids and walls. The subtle differences between void and wall AGN might be explained by a longer, less disturbed duty cycle of these systems in voids.