A molecular Einstein ring at z = 4.12: Imaging the dynamics of a quasar host galaxy through a cosmic lens

Abstract

We present high-resolution (0.3") Very Large Array imaging of the molecular gas in the host galaxy of the high-redshift quasar PSS J2322+1944 (z = 4.12). These observations confirm that the molecular gas (CO) in the host galaxy of this quasar is lensed into a full Einstein ring and reveal the internal gas dynamics in this system. The ring has a diameter of ~1.5" and thus is sampled over ~ 20 resolution elements by our observations. Through a model-based lens inversion, we recover the velocity gradient of the molecular reservoir in the quasar host galaxy of PSS J2322+1944. The Einstein ring lens configuration enables us to zoom in on the emission and to resolve scales down to ≾ 1 kpc. From the model-reconstructed source, we find that the molecular gas is distributed on a scale of 5 kpc and has a total mass of M(H_2) = 1.7 x 10^10 M⊙. A basic estimate of the dynamical mass gives M_(dyn) = 4.4 x 10^10 sin^-2 iM⊙, that is, only ~2.5 times the molecular gas mass and ~30 times the black hole mass (assuming that the dynamical structure is highly inclined). The lens configuration also allows us to tie the optical emission to the molecular gas emission, which suggests that the active galactic nucleus does reside within, but not close to the center of, the molecular reservoir. Together with the (at least partially) disturbed structure of the CO, this suggests that the system is interacting. Such interaction, possibly caused by a major "wet" merger, may be responsible for both feeding the quasar and fueling the massive starburst of 680 M⊙ yr^-1 in this system, in agreement with recently suggested scenarios of quasar activity and galaxy assembly in the early universe.

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