The dichotomy of Seyfert 2 galaxies: intrinsic differences and evolution

Abstract

We present a study of the local environment (<200 kpc/h) of 31 Hidden Broad Line Region (HBLR) and 43 non-HBLR Seyfert 2 galaxies (Sy2) in the nearby universe (z<0.04). To compare our findings, we constructed two control samples that match the redshift and the morphological type distribution of the HBLR and non-HBLR samples respectively. We used the NED (NASA extragalactic database) to find all neighboring galaxies within projected radius of 200 kpc/h around each galaxy, and radial velocity difference delta_u< 500 km/s. We find that, within a projected radius of at least 150 kpc/h around each Seyfert, the fraction of non-HBLR Sy2 galaxies with a close companion is significantly higher than that of their control sample, at the 96% confidence level. Interestingly, the difference is due to the high frequency of mergers in the non-HBLR sample, seven versus only one in the control sample, while also they present a high number of hosts with signs of peculiar morphology. In sharp contrast, the HBLR sample is consistent with its control sample and furthermore, the number of their host galaxies that present peculiar morphology, which probably implies some level of interactions or merging in the past, is the lowest in all four galaxy samples. Given that the HBLR Seyfert 2 galaxies are essentially Seyfert 1 (Sy1), with their broad line region (BLR) hidden because of the obscuration by the torus, while the non-HBLR Sy2 sample probably also includes intrinsically different objects, like "true" Sy2s that lack the BLR, and heavily obscured objects that prohibit even its indirect detection, our results are discussed within the context of an evolutionary sequence of activity triggered by close galaxy interactions and merging. We argue that the non-HBLR Sy2 galaxies may represent different stages of this sequence, possibly the beginning and the end of the nuclear activity.