2010ApJ...718L..73V

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 Hβ, Hγ, and Hδ 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 {&x2248;}0.5 Gyr. The mass-to-light ratio inferred from the spectrum implies a stellar mass of (4±1)x10¹¹ M_☉. We determine the morphology of the galaxy from a deep WFC3 H₁₆₀image. Similar to other massive galaxies at 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, 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 Hβ 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 ∼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.