The dichotomy of Seyfert 2 galaxies: intrinsic differences and evolution

Abstract

We present a study of the local environment (≤200 h −1 kpc) of 31 hidden broad line region (HBLR) and 43 non-HBLR Seyfert 2 (Sy2) galaxies 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. We used the NASA Extragalactic Database (NED) to find all neighboring galaxies within a projected radius of 200 h −1 kpc around each galaxy, and a radial velocity difference δu ≤ 500 km s −1 . Using the digitized Schmidt survey plates (DSS) and/or the Sloan Digital Sky Survey (SDSS), when available, we confirmed that our sample of Seyfert companions is complete. We find that, within a projected radius of at least 150 h −1 kpc 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 they also present a high number of hosts with signs of peculiar morphology. In sharp contrast, the HBLR sample is consistent with its control sample. Furthermore, the number of the HBLR 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 Sy2 galaxies are essentially Seyfert 1 (Sy1) with their broad line region (BLR) hidden because of the obscuring torus, while the non-HBLR Sy2 galaxies probably also include true Sy2s that lack the BLR as well as heavily obscured objects that prevent even the indirect detection of the BLR, 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.

The XMM spectral catalog of SDSS optically selected Seyfert 2 galaxies

“…Interestingly, a number of Compton-thick sources exhibit low accretion rates. This agrees with the evolutionary scheme of AGN proposed by Koulouridis (2014), where a fraction of Compton-thick sources are predicted to emerge shortly after a galaxy interaction or merging event that causes the inflow of gas and dust toward the central region of the galaxy, enhances circumnuclear star formation and triggers the AGN. During this phase the accretion rate is expected to be low and the BLR absent.…”

“…The total population of non-HBLR Sy2s is probably a mixture of objects with low accretion rate and/or high obscuration. Koulouridis (2014) argue that both of the above scenarios agree with an AGN evolutionary scheme (Krongold et al 2002;Koulouridis et al 2006aKoulouridis et al , b, 2013, where a low accretion rate is predicted at the beginning and the end of the Seyfert duty cycle, without ruling out the possibility that some HBLR Sy2s could also be created by minor disturbances or even secular processes.…”

“…Theoretical models attributed the absence of a BLR to either a low Eddington ratio (Nicastro 2000) or to low luminosity (Elitzur & Shlossman 2006). Many studies propose an evolutionary model where a fraction of Sy2 represents the first or the last phase in the life of an AGN (Hunt & Malkan 1999;Dultzin-Hacyan 1999;Krongold et al 2002;Levenson et al 2001;Koulouridis et al 2006aKoulouridis et al ,b, 2013Koulouridis 2014;Elitzur et al 2014). This was supported by studies of the local environment of Seyfert galaxies, which showed that Sy2s reside in richer environments compared to Sy1s (e.g., Villaroel & Korn 2014).…”

The XXL Survey

“…This kind of perturbation can occur in interactions and merging between two galaxies, which results in the feeding of the black hole and the activation of the AGN phase (e.g. Umemura 1998; Kawakatu et al 2006;Koulouridis et al 2006a,b;Koulouridis 2014;Ellison et al 2011;Silverman et al 2011;Villforth et al 2012;Hopkins & Quataert 2011). Thus, the cluster environment, where the concentration of galaxies is very high relative to the field, would also seem favourable to AGN.…”

Large-Scale Environment Properties of Narrow-Line Seyfert 1 Galaxies at z < 0.4

“…To effectively exploit this novel perspective the NLS1 environment studies should be extended to concern smaller-scale environment; the group and cluster scales, and host galaxy properties, both of which are likely to induce changes in AGN properties at shorter time-scales. The local environment studies (e.g., Dultzin-Hacyan et al, 1999;Krongold et al, 2001;Koulouridis et al, 2006Koulouridis et al, , 2013Koulouridis, 2014;Villarroel and Korn, 2014) indicate that Sy1 and Sy2 galaxies differ in their smaller-scale environments and host galaxies, but comprehensive studies including NLS1 galaxies are still needed. Extensive studies at various scales will clarify how the environment affects the AGN evolution, and help us establish the big picture of AGN.…”