2018ApJ...863L..16M

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 µm (rest-frame) data at one of the highest differential magnifications ever recorded at submillimeter wavelengths (µ 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 ± 0.7) x 10¹⁰ M_☉ within r = 2.6 ± 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^+1.9×10¹⁰M_☉, unaffected by strong differential magnification. The baryonic mass is expected to be dominated by the molecular gas content (f_gas <= 80 ± 20%) based on an M_H2_ mass estimated from the difference between dynamical and stellar masses. The star formation rate (SFR) is estimated via the spectral energy distribution (SFR = 190 ± 10 M_☉ yr^–1), implying a molecular gas depletion time of 0.25 ± 0.08 Gyr.