Evidence of a Flat Outer Rotation Curve in a Star-bursting Disk Galaxy at z = 1.6

Observations of the baryon to dark matter fraction in galaxies through cosmic time are a fundamental test for galaxy formation models. Recent observational studies have suggested that some disk galaxies at z > 1 host declining rotation curves, in contrast with observations of low redshift disk galaxies where stellar or H i rotation curves flatten at large radii. We present an observational counterexample, a galaxy named DSFG850.95 at z = 1.555 (4.1 Gyr after the big bang) that hosts a flat rotation curve between radii of ∼6-14 kpc (1.2-2.8 disk scale lengths) and has a dark matter fraction of 0.44 0.08 at the H-band half light radius, similar to the Milky Way. We create position-velocity and position-dispersion diagrams using Keck/MOSFIRE spectroscopic observations of H and [N ii] emission features, which reveal a flat rotation velocity of Vflat = 285 12 km s−1 and an ionized gas velocity dispersion of 0 = 48 4 km s−1. This galaxy has a rotation-dominated velocity field with Vflat/ 0 ∼ 6. Ground-based H-band imaging reveals a disk with Sérsic index of 1.29 0.03, an edge-on inclination angle of 87° 2°, and an H-band half light radius of 8.4 0.1 kpc. Our results point to DSFG850.95 being a massive, rotationally supported disk galaxy with a high dark-matter-to-baryon fraction in the outer galaxy, similar to disk galaxies at low redshift.