The Halo Occupation Distribution of obscured quasars: Revisiting the unification model

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 ~ 1 using a five parameter Halo Occupation Distribution (HOD) parametrization, derived from a cosmological hydrodynamic simulation by Chatterjee et al. The HOD parametrization was previously used to model the 2PCF of optically selected quasars and X-ray bright active galactic nuclei (AGNs) at z ~ 1. The current work shows that a single HOD parametrization can be used to model the population of different kinds of AGN in dark matter haloes suggesting the universality of the relationship between AGN and their host dark matter haloes. Our results show that the median halo mass of central quasar hosts increases from optically selected (4.1-0.4+0.3 × 1012 h-1M⊙) and infra-red (IR) bright unobscured populations (6.3-2.3+6.2 × 1012 h-1M⊙) to obscured quasars (10.0+2.6 -3.7 × 1012 h -1M⊙), signifying an increase in the degree of clustering. The projected satellite fractions also increase from optically bright to obscured quasars and tend to disfavour 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.