Hubble Space Telescope Images of submillimeter sources: Large irregular galaxies at high redshift

We present new Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) high-resolution optical imaging of a sample of 13 submillimeter luminous galaxies for which the optical emission has been pinpointed either through radio 1.4 GHz or millimeter interferometry. We find a predominance of irregular and complex morphologies in the sample, suggesting that mergers are likely common for submillimeter galaxies. The component separations in these objects are on average a factor 2 larger than in local galaxies with similarly high bolometric luminosities. The sizes and star formation rates of the submillimeter galaxies are consistent with the maximal star formation rate densities of 20 M_⊙ kpc^-2 in local starburst galaxies (Lehnert & Heckman). We derive quantitative morphological information for optical galaxies hosting submillimeter emission: total and isophotal magnitudes, Petrosian radius, effective radius, concentration, aspect ratio, surface brightness, and asymmetry. We compare these morphological indexes with those of other galaxies lying within the same STIS images. Most strikingly, we find ∼70% of the submillimeter galaxies to be extraordinarily large and elongated relative to the field population, regardless of optical magnitude. Comparison of the submillimeter galaxy morphologies with those of optically selected galaxies at z ∼ 2-3 reveals the submillimeter galaxies to be a morphologically distinct population, with generally larger sizes, higher concentrations, and more prevalent major-merger configurations.