Kinematics of the SN Refsdal host revealed by MUSE: a regularly rotating spiral galaxy at z ≃ 1.5

We use Multi Unit Spectroscopic Explorer (MUSE) observations of the galaxy cluster MACS J1149.5+2223 to explore the kinematics of the grand-design spiral galaxy (Sp1149) hosting the supernova `Refsdal'. Sp1149 lies at z ≃ 1.49, has a stellar mass M* ≃ 5 × 109 M⊙, has a star formation rate (SFR) ∼eq 1-6 M_{⊙} yr^{-1}, and represents a likely progenitor of a Milky Way-like galaxy. All the four multiple images of Sp1149 in our data show strong [O II}-line emissions pointing to a clear rotation pattern. We take advantage of the gravitational lensing magnification effect (≃4×) on the [O II} emission of the least distorted image to fit three-dimensional kinematic models to the MUSE data cube and derive the rotation curve and the velocity dispersion profile of Sp1149. We find that the rotation curve steeply rises, peaks at R ≃ 1 kpc, and then (initially) declines and flattens to an average {V_flat}= 128^{+29}_{-19} km s-1. The shape of the rotation curve is well determined, but the actual value of Vflat is quite uncertain because of the nearly face-on configuration of the galaxy. The intrinsic velocity dispersion due to gas turbulence is almost constant across the entire disc with an average of 27 ± 5 km s-1. This value is consistent with z = 0 measurements in the ionized gas component and a factor of 2-4 lower than other estimates in different galaxies at similar redshifts. The average stellar-to-total mass fraction is of the order of one-fifth. Our kinematic analysis returns the picture of a regular star-forming, mildly turbulent, rotation-dominated (V/σ ≃ 5) spiral galaxy in a 4-Gyr-old Universe.