The "cosmic Seagull": A Highly Magnified Disk-like Galaxy at z ≃ 2.8 behind the Bullet Cluster

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) × 10¹⁰ M _o 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, , 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 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 _o yr^-1), implying a molecular gas depletion time of 0.25 ± 0.08 Gyr.