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We present HST/WFC3 grism spectroscopy of the brightest galaxy at z>1.5 inthe GOODS-South WFC3 Early Release Science grism pointing, covering thewavelength range 0.9-1.7 micron. The spectrum is of remarkable quality andshows the redshifted Balmer lines Hbeta, Hgamma, and Hdelta in absorption atz=1.902, correcting previous erroneous redshift measurements from therest-frame UV. The average rest-frame equivalent width of the Balmer lines is8+-1 Angstrom, which can be produced by a post-starburst stellar populationwith a luminosity-weighted age of ~0.5 Gyr. The M/L ratio inferred from thespectrum implies a stellar mass of ~4x10^11 Msun. We determine the morphologyof the galaxy from a deep WFC3 F160W image. Similar to other massive galaxiesat z~2 the galaxy is compact, with an effective radius of 2.1+-0.3 kpc.Although most of the light is in a compact core, the galaxy has two red, smoothspiral arms that appear to be tidally-induced. The spatially-resolvedspectroscopy demonstrates that the center of the galaxy is quiescent and thesurrounding disk is forming stars, as it shows Hbeta in emission. The galaxy isinteracting with a companion at a projected distance of 18 kpc, which alsoshows prominent tidal features. The companion has a slightly redder spectrumthan the primary galaxy but is a factor of ~10 fainter and may have a lowermetallicity. It is tempting to interpret these observations as "smoking gun"evidence for the growth of compact, quiescent high redshift galaxies throughminor mergers, which has been proposed by several recent observational andtheoretical studies. Interestingly both objects host luminous AGNs, asindicated by their X-ray luminosities, which implies that these mergers can beaccompanied by significant black hole growth. This study illustrates the powerof moderate dispersion, low background near-IR spectroscopy at HST resolution,which is now available with the WFC3 grism.

Hubble Space Telescope WFC3 Grism Spectroscopy and Imaging of a Growing Compact Galaxy at z=1.9