Helium Reionization Simulations. I. Modeling Quasars as Radiation
  Sources
  We introduce a new project to understand helium reionization using fully
coupled $N$-body, hydrodynamics, and radiative transfer simulations. This
project aims to capture correctly the thermal history of the intergalactic
medium (IGM) as a result of reionization and make predictions about the
Lyman-$\alpha$ forest and baryon temperature-density relation. The dominant
sources of radiation for this transition are quasars, so modeling the source
population accurately is very important for making reliable predictions. In
this first paper, we present a new method for populating dark matter halos with
quasars. Our set of quasar models include two different light curves, a
lightbulb (simple on/off) and symmetric exponential model, and
luminosity-dependent quasar lifetimes. Our method self-consistently reproduces
an input quasar luminosity function (QLF) given a halo catalog from an $N$-body
simulation, and propagates quasars through the merger history of halo hosts.
After calibrating quasar clustering using measurements from BOSS, we find that
the characteristic mass of quasar hosts is $M_h \sim 2.5 \times 10^{12}
M_\odot$ $h^{-1}$ for the lightbulb model, and $M_h \sim 2.3 \times 10^{12}
M_\odot$ $h^{-1}$ for the exponential model. In the exponential model, the peak
quasar luminosity for a given halo mass is larger than that in the lightbulb
model, typically by a factor of 1.5-2. The effective lifetime for quasars in
the lightbulb model is 59 Myr, and in the exponential case, the effective time
constant is about 15 Myr. We include semi-analytic calculations of helium
reionization, and discuss how to include these quasars as sources of ionizing
radiation for full hydrodynamics with radiative transfer simulations in order
to study helium reionization.
