A spatially resolved map of the kinematics, star formation and stellar mass assembly in a star-forming galaxy at z = 4.9

We present a detailed study of the spatially resolved kinematics, star formation and stellar mass in a highly amplified galaxy at z = 4.92 behind the lensing cluster MS 1358+62. We use the observed optical, near- and mid-infrared imaging from Hubble Space Telescope ACS & NICMOS and Spitzer IRAC to derive the stellar mass and the Gemini/NIFS IFU to investigate the velocity structure of the galaxy from the nebular [O ii]λλ3726.8,3728.9 emission. Using a detailed gravitational lens model, we account for lensing amplification factor 12.5 ± 2.0 and find that this intrinsically L* galaxy has a stellar mass of M_* = 7 ± 2 × 10⁸ M _⊙, a dynamical mass of M_dyn = 3 ± 1 × 10⁹csc²(i) M_⊙ (within of 2 kpc) and a star formation rate of 42 ± 8 M_⊙ yr^-1. The source-plane UV/optical morphology of this galaxy is dominated by five discrete star-forming regions. Exploiting the dynamical information we derive masses for individual star-forming regions of M_cl ∼ 10^8-9 M _⊙ with sizes of ∼200 pc. We find that, at a fixed size, the star formation rate density within these H ii regions is approximately two orders of magnitude greater than those observed in local spiral/starburst galaxies, but consistent with the most massive H ii regions in the local Universe such as 30 Doradus. Finally, we compare the spatially resolved nebular emission-line velocity with the Lyα and UV interstellar medium (ISM) lines and find that this galaxy is surrounded by a galactic scale outflow in which the Lyα appears redshifted by ∼150 km s^-1 and the UV-ISM lines blueshifted by ∼ -200 km s^-1 from the (systemic) nebular emission. The velocity structure of the outflow mirrors that of the nebular emission suggesting the outflow is young (≲15 Myr), and has yet to burst out of the system. Taken together, these results suggest that this young galaxy is undergoing its first major epoch of mass assembly.