Cross-correlation of Luminous Red Galaxies with ML-selected AGN in HSC-SSP: Unobscured AGN residing in more massive halos

Abstract

Active galactic nuclei (AGN) are the signposts of black hole growth, and likely play an important role in galaxy evolution. An outstanding question is whether AGN of different spectral types indicate different evolutionary stages in the coevolution of black holes and galaxies. We present the angular correlation function between an AGN sample selected from the Hyper Suprime Camera Subaru Strategic Program (HSC-SSP) optical + Wide-field Infrared Survey Explorer (WISE) mid-IR photometry, and a luminous red galaxy (LRG) sample from HSC-SSP. We investigate AGN clustering strength as a function of their luminosity and spectral features across three independent HSC fields totaling $\sim600\,{\rm deg^{2}}$, for $z\in0.6-1.2$ and AGN with $L_{6\mu m}>3\times10^{44}{\rm\,erg\,s^{-1}}$. There are $\sim28,500$ AGN and $\sim1.5$ million LRGs in our primary analysis. We determine the inferred average halo mass for the full AGN sample ($M_h \approx 10^{12.9}h^{-1}M_\odot$), and note that it does not evolve significantly as a function of redshift (over this narrow range) or luminosity. We find that, on average, unobscured AGN ($M_h \approx10^{13.3}h^{-1}M_\odot$) occupy $\sim4.5\times$ more massive halos than obscured AGN ($M_h \approx10^{12.6}h^{-1}M_\odot$), at $5\sigma$ statistical significance using 1-D uncertainties, and at $3\sigma$ using the full covariance matrix, suggesting a physical difference between unobscured and obscured AGN, beyond the line-of-sight viewing angle. Furthermore, we find evidence for a halo mass dependence on reddening level within the Type I AGN population, which could support the existence of a previously claimed dust-obscured phase in AGN-host galaxy coevolution. However, we also find that even quite small systematic shifts in the redshift distributions of the AGN sample could plausibly explain current and previously observed differences in $M_{h}$.