Confirmation of the Compactness of a z = 1.91 Quiescent Galaxy with Hubble Space Telescope's Wide Field Camera 3

Abstract

We present very deep Wide Field Camera 3 (WFC3) photometry of a massive, compact galaxy located in the Hubble Ultra Deep Field. This quiescent galaxy has a spectroscopic redshift z = 1.91 and has been identified as an extremely compact galaxy by Daddi et al. We use new H _F160W imaging data obtained with Hubble Space Telescope/WFC3 to measure the deconvolved surface brightness profile to H ≈ 28 mag arcsec^-2. We find that the surface brightness profile is well approximated by an n = 3.7 Sérsic profile. Our deconvolved profile is constructed by a new technique which corrects the best-fit Sérsic profile with the residual of the fit to the observed image. This allows for galaxy profiles which deviate from a Sérsic profile. We determine the effective radius of this galaxy: r_e = 0.42 ± 0.14 kpc in the observed H _F160W band. We show that this result is robust to deviations from the Sérsic model used in the fit. We test the sensitivity of our analysis to faint "wings" in the profile using simulated galaxy images consisting of a bright compact component and a faint extended component. We find that due to the combination of the WFC3 imaging depth and our method's sensitivity to extended faint emission we can accurately trace the intrinsic surface brightness profile, and that we can therefore confidently rule out the existence of a faint extended envelope around the observed galaxy down to our surface brightness limit. These results confirm that the galaxy lies a factor ~10 off from the local mass-size relation.