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]lambda lambda 3726.8,3728.9 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(star) = 7 +/- 2 x 108 M(circle dot), a dynamical mass of M(dyn) = 3 +/- 1 x 109csc2(i) M(circle dot) (within of 2 kpc) and a star formation rate of 42 +/- 8 M(circle dot) 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) similar to 108-9 M(circle dot) with sizes of similar to 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 Ly alpha and UV interstellar medium (ISM) lines and find that this galaxy is surrounded by a galactic scale outflow in which the Ly alpha appears redshifted by similar to 150 km s-1 and the UV-ISM lines blueshifted by similar to -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 (less than or similar to 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.