The Nature of CO Emission From z~ 6 Quasars (original) (raw)

2008, The Astrophysical …

We investigate the nature of molecular gas emission from z ∼ 6 quasars via the commonly observed tracer of H 2 , carbon monoxide (CO). We achieve these means by combining non-local thermodynamic equilibrium (LTE) radiative transfer calculations with merger-driven models of z ∼ 6 quasar formation that arise naturally in Λ-cold dark matter (ΛCDM) structure formation simulations. Motivated by observational constraints, we consider four representative z ∼ 6 quasars formed in the halo mass range ∼ 10 12 − 10 13 M ⊙ from different merging histories. Our main results are as follows. We find that, owing to massive starbursts and funneling of dense gas into the nuclear regions of merging galaxies, the CO is highly excited during both the hierarchical buildup of the host galaxy and the quasar phase and the CO flux density peaks between J=5-8. The CO morphology of z ∼ 6 quasars often exhibits multiple CO emission peaks which arise from molecular gas concentrations which have not yet fully coalesced. Both of these results are found to be consistent with the sole CO detection at z ∼ 6, in quasar J1148+5251. Quasars which form at z ∼ 6 display a large range of sightline-dependent line widths. The sightline dependencies are such that the narrowest line widths are when the rotating molecular gas associated with the quasar is viewed face-on (when the L B is largest), and broadest when the quasar is seen edge on (and the L B is lowest). Thus, we find that for all models selection effects exist such that quasars selected for optical luminosity are preferentially seen to be face-on which may result in CO detections of optically luminous quasars at z ∼ 6 having line widths narrower than the median. The mean sightline-averaged line width is found to be reflective of the circular velocity of the host halo, and thus scales with halo mass. For example, the mean line width for the ∼ 10 12 M ⊙ halo is σ ∼ 300 km s −1 , while the median for the ∼ 10 13 M ⊙ quasar host is σ ∼ 650 km s −1 . Depending on the host halo mass, approximately 2-10% of sightlines in our modeled quasars are found to have narrow line widths compatible with observations of J1148+5251. When considering the aforementioned selection effects, these percentages increase to 10-25% for quasars selected for optical luminosity. When accounting for both temporal evolution of CO line widths in galaxies, as well as the redshift evolution of halo circular velocities, these models can self-consistently account for the observed line widths of both submillimeter galaxies and quasars at z ∼ 2. Finally, we find that the dynamical mass derived from the mean sightline-averaged line widths provide a good estimate of the total mass, and allows for a massive molecular reservoir, supermassive black hole, and stellar bulge, consistent with the local M BH -M bulge relation.