The `Cosmic Seagull': a highly magnified disk-like galaxy at z~2.8 behind the Bullet Cluster

Abstract

We present Atacama Large Millimeter/submillimeter Array measurements of the `Cosmic Seagull', a strongly magnified galaxy at z=2.7779 behind the Bullet Cluster. We report CO(3-2) and continuum 344~$\mu$m (rest-frame) data at one of the highest differential magnifications ever recorded at submillimeter wavelengths ($\mu$ up to ~50), facilitating a characterization of the kinematics of a rotational curve in great detail (at ~620 pc resolution in the source plane). We find no evidence for a decreasing rotation curve, from which we derive a dynamical mass of ($6.3\pm 0.7)\times 10^{10}{M}_{\odot }$ within $r=2.6\pm 0.1$ kpc. The discovery of a third, unpredicted, image provides key information for a future improvement of the lensing modeling of the Bullet Cluster and allows a measure of the stellar mass, $1.6_{-0.86}^{+1.9}\times 10^{10}{M}_{\odot }$, unaffected by strong differential magnification. The baryonic mass is is expected to be dominated by the molecular gas content ($f_{gas}\le 80\pm 20$ \%) based on an $M_{H_2}$ mass estimated from the difference between dynamical and stellar masses. The star formation rate is estimated via the spectral energy distribution ($SFR=190\pm 10M_{\odot }/yr$), implying a molecular gas depletion time of $0.25\pm 0.08$ Gyr.

14 pages LaTeX using ApJLetter macros, author final version accepted, 4 figures