A New Model for Dark Matter Halos Hosting Quasars
  A new model for quasar-hosting dark matter halos, meeting two physical conditions, is put forth. First, significant interactions are taken into consideration to trigger quasar activities. Second, satellites in very massive halos at low redshift are removed from consideration, due to their deficiency of cold gas. We analyze the {\em Millennium Simulation} to find halos that meet these two conditions and simultaneously match two-point auto-correlation functions of quasars and cross-correlation functions between quasars and galaxies at $z=0.5-3.2$. %The found halos have some distinct properties worth noting. The masses of found quasar hosts decrease with decreasing redshift, with the mass thresholds being $[(2-5)\times 10^{12}, (2-5)\times 10^{11}, (1-3)\times 10^{11}]\msun$ for median luminosities of $\sim[10^{46}, 10^{46}, 10^{45}]$erg/s at $z=(3.2, 1.4, 0.53)$, respectively, an order of magnitude lower than those inferred based on halo occupation distribution modeling. In this model quasar hosts are primarily massive central halos at $z\ge 2-3$ but increasingly dominated by lower mass satellite halos experiencing major interactions towards lower redshift. But below $z=1$ satellite halos in groups more massive than $\sim 2\times 10^{13}\msun$ do not host quasars. Whether for central or satellite halos, imposing the condition of significant interactions substantially boosts the clustering strength compared to the total population with the same mass cut. The inferred lifetimes of quasars at $z=0.5-3.2$ of $3-30$Myr are in agreement with observations. Quasars at $z\sim 2$ would be hosted by halos of mass $\sim 5\times 10^{11}\msun$ in this model, compared to $\sim 3\times 10^{12}\msun$ previously thought, which would help reconcile with the observed, otherwise puzzling high covering fractions for Lyman limit systems around quasars. 
Renyue Cen (Princeton), Mohammadtaher Safarzadeh (Johns Hopkins)
astro-ph.CO astro-ph.GA