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 Hα and [N II] emission features, which reveal a flat rotation velocity of V _flat = 285 ± 12 km s^-1 and an ionized gas velocity dispersion of σ ₀ = 48 ± 4 km s^-1. This galaxy has a rotation-dominated velocity field with V _flat/σ 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.