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Register a new userThe Nature of CO Emission From z~6 Quasars
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Desika Narayanan
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We investigate the nature of CO emission from z~6 quasars by combining non-LTE radiative transfer calculations with merger-driven models of z~6 quasar formation that arise naturally in LCDM cosmological simulations. We consider four model quasars formed in 10^12-10^13 M_sun halos from different merging histories. Our main results follow. Owing to massive starbursts and funneling of dense gas into the nuclear regions of merging galaxies, the CO is highly excited and the flux density peaks between J=5-8. The CO morphology of z~6 quasars often exhibits multiple emission peaks which arise from H2 concentrations which have not yet fully coalesced. Quasars at z~6 display a large range of sightline dependent line widths such that the lines are narrowest when the rotating H2 gas associated with the quasar is viewed face-on (when L_B is largest), and broadest when the gas is seen edge-on (when L_B is lowest). Thus for all models selection effects exist such that quasars selected for optical luminosity are preferentially face-on which may result in detected CO line widths narrower than the median. The sightline averaged line width is reflective of the circular velocity (V_c) of the host halo, and ranges from sigma~300-650 km/s. For optically selected QSOs, 10-25% (halo-mass dependant) of sightlines have narrow line widths compatible with the sole CO detection at z>6, J1148+5251. When accounting for both the temporal evolution of CO line widths, as well as the redshift evolution of halo V_c, these models self-consistently account for the CO line widths of both z~2 sub-mm galaxies and QSOs. Finally, the dynamical mass derived from the sightline averaged line widths provides a good estimate of the total mass, and allows for a stellar bulge and SMBH consistent with the local M_BH-M_bulge relation. [abridged]
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High-redshift quasars are currently the only probes of the growth of supermassive black holes and potential tracers of structure evolution at early cosmic time. Here we present our candidate selection criteria from the Panoramic Survey Telescope & Rapid Response System 1 and follow-up strategy to discover quasars in the redshift range 5.7<z<6.2. With this strategy we discovered eight new 5.7<z<6.0 quasars, increasing the number of known quasars at z>5.7 by more than 10%. We additionally recovered 18 previously known quasars. The eight quasars presented here span a large range of luminosities (-27.3 < M_{1450} < -25.4; 19.6 < z_ps1 < 21.2) and are remarkably heterogeneous in their spectral features: half of them show bright emission lines whereas the other half show a weak or no Ly$alpha$ emission line (25% with rest-frame equivalent width of the Ly$alpha$ + Nv line lower than 15{AA}). We find a larger fraction of weak-line emission quasars than in lower redshift studies. This may imply that the weak-line quasar population at the highest redshifts could be more abundant than previously thought. However, larger samples of quasars are needed to increase the statistical significance of this finding.
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