Imaging The Molecular Gas in a z = 3.9 Quasar Host Galaxy at 0farcs3 Resolution: A Central, Sub-Kiloparsec Scale Star Formation Reservoir in APM 08279+5255

We have mapped the molecular gas content in the host galaxy of the strongly lensed high-redshift quasar APM 08279+5255 (z = 3.911) with the Very Large Array at 0farcs3 resolution. The CO(J = 1 → 0) emission is clearly resolved in our maps. The CO(J = 1 → 0) line luminosity derived from these maps is in good agreement with a previous single-dish measurement. In contrast to previous interferometer-based studies, we find that the full molecular gas reservoir is situated in two compact peaks separated by lsim0farcs4. Our observations reveal, for the first time, that the emission from cold molecular gas is virtually co-spatial with the optical/near-infrared continuum emission of the central active galactic nucleus (AGN) in this source. This striking similarity in morphology indicates that the molecular gas is situated in a compact region close to the AGN. Based on the high-resolution CO maps, we present a revised model for the gravitational lensing in this system, which indicates that the molecular gas emission is magnified by only a factor of 4 (in contrast to previously suggested factors of 100). This model suggests that the CO is situated in a circumnuclear disk of ~550 pc radius that is possibly seen at an inclination of lsim25°, i.e., relatively close to face-on. From the CO luminosity, we derive a molecular gas mass of M _gas = 1.3 × 10¹¹ M _sun for this galaxy. From the CO structure and linewidth, we derive a dynamical mass of M _dynsin² i = 4.0 × 10¹⁰ M _sun. Based on a revised mass estimate for the central black hole of M _BH = 2.3 × 10¹⁰ M _sun and the results of our molecular line study, we find that the mass of the stellar bulge of APM 08279+5255 falls short of the local M _BH-σ_bulge relationship of nearby galaxies by more than an order of magnitude, lending support to recent suggestions that this relation may evolve with cosmic time and/or change toward the high-mass end.