Imaging the Molecular Gas in a z=3.9 Quasar Host Galaxy at 0.3" Resolution: A Central, Sub-Kiloparsec Scale Star Formation Reservoir in APM 08279+5255

TL;DR

This study maps the molecular gas in a high-redshift quasar host galaxy at high resolution, revealing a compact, near-AGN gas reservoir and revising gravitational lensing models, with implications for galaxy evolution.

Contribution

First high-resolution CO mapping of a z=3.9 quasar host showing the molecular gas is in a compact, near-AGN region and providing a revised lensing model.

Findings

Molecular gas is in two compact peaks near the AGN.

Lensing magnification of CO emission is only a factor of 4.

The stellar bulge mass is much lower than expected from local black hole-bulge relations.

Abstract

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 0.3" 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 <~0.4". Our observations reveal, for the first time, that the emission from cold molecular gas is virtually cospatial with the optical/near-infrared continuum emission of the central 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 <~25 deg, i.e., relatively close to face-on. From the CO luminosity, we derive a molecular gas mass of M_gas=1.3x10^11 M_sun for this galaxy. From the CO structure and linewidth, we derive a dynamical mass of M_dyn(sin i)^2=4.0x10^10 M_sun. Based on a revised mass estimate for the central black hole of M_BH=2.3x10^10 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-sigma_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.