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

Abstract

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 SN Refsdal. Sp1149 lies at $z\simeq 1.49$, has a stellar mass $M_{\ast }\simeq 5\times 10^9{\mathrm{M}}_{\odot }$, a star-formation rate $\mathrm{SFR}\simeq 1-6{\mathrm{M}}_{\odot }/yr$ and represents a likely progenitor of a Milky-Way-like galaxy. All the four multiple images of Sp1149 in our data show strong OII-line emissions pointing to a clear rotation pattern. We take advantage of the gravitational lensing magnification effect ($\simeq 4\times$) on the OII emission of the least distorted image to fit 3D 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 $R\simeq 1$ kpc and then (initially) declines and flattens to an average $V_{\mathrm{flat}}=128_{-19}^{+29}$ km/s. The shape of the rotation curve is well determined but the actual value of $V_{\mathrm{flat}}$ 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 $27\pm 5$ km/s. This value is consistent with $z=0$ measurements in the ionized gas component and a factor 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 ($V/\ddot{I}ƒ\simeq 5$) spiral galaxy in a 4 Gyr old Universe.

10 pages, 5 figures. Accepted for publication in MNRAS