HUBBLE SPACE TELESCOPE WFC3 GRISM SPECTROSCOPY AND IMAGING OF A GROWING COMPACT GALAXY AT z=1.9

We present HST/WFC3 grism near-IR spectroscopy of the brightest galaxy at z > 1.5 in the GOODS-South WFC3 ERS grism pointing. The spectrum is of remarkable quality and shows the redshifted Balmer lines H beta, H gamma, and H delta in absorption at z = 1.902 +/- 0.002. The absorption lines can be produced by a post-starburst stellar population with a luminosity-weighted age of approximate to 0.5 Gyr. The mass-to-light ratio inferred from the spectrum implies a stellar mass of (4 +/- 1) x 10(11) M circle dot. We determine the morphology of the galaxy from a deep WFC3 H(160) image. Similar to other massive galaxies at z similar to 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, smooth spiral arms that appear to be tidally induced. The spatially resolved spectroscopy demonstrates that the center of the galaxy is quiescent whereas the surrounding disk is forming stars, as it shows H beta in emission. The galaxy interacts with a companion at a projected distance of 18 kpc, which also shows prominent tidal features. The companion is a factor of similar to 10 fainter than the primary galaxy and may have a lower metallicity. It is tempting to interpret these observations as evidence for the growth of compact, quiescent high-redshift galaxies through minor mergers, which has been proposed by several recent observational and theoretical studies. Interestingly both objects host luminous active galactic nuclei, which implies that these mergers can be accompanied by significant black hole growth.