Faint Submillimeter Galaxies: [ITAL]Hubble Space Telescope[/ITAL] Morphologies and Colors

We present deep optical imaging from the Hubble Space Telescope ( HST ) and from the ground for galaxies selected from the first submillimeter survey of the distant universe. Our survey comprises 850 μm continuum maps of seven massive lensing clusters and covers a total area of 0.01 deg 2 to 1 σ noise levels of ≤2 mJy beam -1 . We detect 17 sources brighter than the 50% completeness limits (10 brighter than the 80% limit). Of the sources that lie within our optical fields, counterparts are identified for 14 of the 16 sources in the f 50% sample (nine of 10 in the f 80% sample). The morphologies of those galaxies for which we have HST imaging fall into three broad categories: faint disturbed galaxies and interactions, faint galaxies too compact to classify reliably, and dusty, bright galaxies at intermediate redshifts. The disturbed and interacting galaxies constitute the largest class, which suggests that interactions remain an important mechanism for triggering star formation in ultraluminous galaxies in the distant universe. The faint, compact galaxies may represent a later evolutionary stage in these mergers or more centrally concentrated starbursts. It is likely that some of these will host active galactic nuclei. Analysis of the colors of our sample allow us to place a crude limit on the redshift distribution: ≳75% have z ≲5.5, while ≳50% lie at z ≲4.5, suggesting that the luminous submillimeter population is broadly coeval with the more modestly star-forming galaxies selected by UV/optical surveys of the distant universe.