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 ($6.3\pm0.7)\times10^{10} M_{\odot}$ within $r = 2.6\pm0.1$ 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, $1.6^{+1.9}_{-0.86}\times10^{10} M_{\odot}$, unaffected by strong differential magnification. The baryonic mass is is expected to be dominated by the molecular gas content ($f_{gas} \leq 80 \pm 20$ \%) 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 ($SFR = 190 \pm 10 M_{\odot}/yr$), implying a molecular gas depletion time of $0.25\pm0.08$ Gyr.