Differences in Halo-Scale Environments between Type 1 and Type 2 AGNs at Low Redshift

TL;DR

This study compares the environments of type 1 and type 2 AGNs at low redshift, revealing differences in small-scale surroundings and satellite distributions that challenge simple unification models.

Contribution

It provides new insights into the environmental differences between AGN types, emphasizing the role of galaxy interactions beyond orientation-based unification.

Findings

Type 2 AGNs have more neighbors within 100 kpc than type 1s.

Both AGN types reside in halos of similar mass.

Type 2 AGNs have more satellites and more centrally concentrated satellite distributions.

Abstract

Using low-redshift (z<0.09) samples of AGNs, normal galaxies and groups of galaxies selected from the Sloan Digital Sky Survey (SDSS), we study the environments of type 1 and type 2 AGNs both on small and large scales. Comparisons are made for galaxy samples matched in redshift, $r$-band luminosity, [OIII] luminosity, and also the position in groups (central or satellite). We find that type 2 AGNs and normal galaxies reside in similar environments. Type 1 and type 2 AGNs have similar clustering properties on large scales ($\gtrsim1$Mpc), but at scales smaller than 100 kpc, type 2s have significant more neighbors than type 1s ($3.09\pm0.69$ times more for central AGNs at $\lesssim30$kpc). These results suggest that type 1 and type 2 AGNs are hosted by halos of similar masses, as is also seen directly from the mass distributions of their host groups ($\sim10^{12}h^{-1} M_{\odot}$ for centrals and $\sim10^{13}h^{-1} M_{\odot}$ for satellites). Type~2s have significantly more satellites around them, and the distribution of their satellites is also more centrally concentrated. The host galaxies of both types of AGNs have similar optical properties, but their infrared colors are significantly different. Our results suggest that the simple unified model based solely on torus orientation is not sufficient, but that galaxy interactions in dark matter halos must have played an important role in the formation of the dust structure that obscures AGNs.