The Angular Clustering of Infrared-Selected Obscured and Unobscured Quasars

TL;DR

This study measures the clustering of infrared-selected obscured and unobscured quasars, revealing that obscured quasars reside in only slightly more massive halos than unobscured ones, challenging simple unification models.

Contribution

It provides improved bias and halo mass measurements for WISE-selected quasars, showing a smaller difference between obscured and unobscured types than previously reported.

Findings

Obscured quasars have a bias of 2.67, halo mass ~2×10^{13} M_sun/h

Unobscured quasars have a bias of 2.04, halo mass ~6×10^{12} M_sun/h

Obscured phase lasts roughly twice as long as unobscured phase (~200 Myr)

Abstract

Recent studies of luminous infrared-selected active galactic nuclei (AGN) suggest that the reddest, most obscured objects display a higher angular clustering amplitude, and thus reside in higher-mass dark matter halos. This is a direct contradiction to the prediction of the simplest unification-by-orientation models of AGN and quasars. However, clustering measurements depend strongly on the "mask" that removes low-quality data and describes the sky and selection function. We find that applying a robust, conservative mask to WISE-selected quasars yields a weaker but still significant difference in the bias between obscured and unobscured quasars. These findings are consistent with results from previous Spitzer surveys, and removes any scale dependence of the bias. For obscured quasars with $\langle z \rangle = 0.99$ we measure a bias of $b_q = 2.67 \pm 0.16$, corresponding to a halo mass of $\log (M_h / M_{\odot} h^{-1}) = 13.3 \pm 0.1$, while for unobscured sources with $\langle z \rangle = 1.04$ we find $b_q = 2.04 \pm 0.17$ with a halo mass $\log (M_h / M_{\odot} h^{-1} )= 12.8 \pm 0.1$. This improved measurement indicates that WISE-selected obscured quasars reside in halos only a few times more massive than the halos of their unobscured counterparts, a reduction in the factor of $\sim$10 larger halo mass as has been previously reported using WISE-selected samples. Additionally, an abundance matching analysis yields lifetimes for both obscured and unobscured quasar phases on the order of a few 100 Myr ($\sim$ 1\% of the Hubble time) --- however, the obscured phase lasts roughly twice as long, in tension with many model predictions.