The Halo Occupation Distribution of Obscured Quasars: Revisiting the Unification Model

TL;DR

This study models the clustering of obscured and unobscured quasars at z~1 using a unified Halo Occupation Distribution approach, revealing differences in host halo mass and challenging simple unification theories.

Contribution

It demonstrates that a single HOD model can describe various AGN populations, suggesting a universal relationship between AGN and dark matter halos.

Findings

Obscured quasars reside in more massive halos than unobscured ones.

Satellite fractions increase from optically bright to obscured quasars.

Results challenge the 'orientation only' unification model of AGN.

Abstract

We model the projected angular two-point correlation function (2PCF) of obscured and unobscured quasars selected using the Wide-field Infrared Survey Explorer (WISE), at a median redshift of $z \sim 1$ using a five-parameter Halo Occupation Distribution (HOD) parameterization, derived from a cosmological hydrodynamic simulation by Chatterjee et al. The HOD parameterization was previously used to model the 2PCF of optically selected quasars and X-ray bright active galactic nuclei (AGN) at $z \sim 1$. The current work shows that a single HOD parameterization can be used to model the population of different kinds of AGN in dark matter halos suggesting the universality of the relationship between AGN and their host dark matter halos. Our results show that the median halo mass of central quasar hosts increases from optically selected ($4.1^{+0.3}_{-0.4} \times 10^{12} \; h^{-1} \; {M_{sun}}$) and infra-red (IR) bright unobscured populations ($6.3^{+6.2}_{-2.3} \times 10^{12} \; h^{-1} \; {M_{sun}}$) to obscured quasars ($10.0^{+2.6}_{-3.7} \times 10^{12} \; h^{-1} \; {M_{sun}}$), signifying an increase in the degree of clustering. The projected satellite fractions also increase from optically bright to obscured quasars and tend to disfavor a simple `orientation only' theory of active galactic nuclei unification. Our results also show that future measurements of the small-scale clustering of obscured quasars can constrain current theories of galaxy evolution where quasars evolve from an IR- bright obscured phase to the optically bright unobscured phase.