Cross-correlation of Luminous Red Galaxies with Machine Learning Selected Active Galactic Nuclei 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 Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) optical photometry and Wide-field Infrared Survey Explorer mid-IR photometry and a luminous red galaxy (LRG) sample from HSC-SSP. We investigate AGN clustering strength as a function of luminosity and spectral features across three independent HSC fields totaling ∼600 deg 2 , for z ∈ 0.6 −1.2 and AGN with L 6 μ m > 3 × 10 44 erg s −1 . There are ∼28,500 AGN and ∼1.5 million LRGs in our primary analysis. We determine the average halo mass for the full AGN sample ( M h ≈ 10 12.9 h −1 M ⊙ ), 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 ≈ 10 13.3 h −1 M ⊙ ) occupy ∼4.5× more massive halos than obscured AGN ( M h ≈ 10 12.6 h −1 M ⊙ ), at 5 σ statistical significance using 1D uncertainties, and at 3 σ 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 dust-obscured phase. However, we also find that quite small systematic shifts in the redshift distributions of the AGN sample could explain current and previously observed differences in M h .