Evidence for Extended, Obscured Starbursts in Submillimeter Galaxies

Abstract

We compare high-resolution optical and radio imaging of 12 luminous submillimeter (submm) galaxies at a median z = 2.2 ± 0.2 observed with Hubble Space Telescope (HST) and the MERLIN and VLA radio interferometers at comparable spatial resolution, ∼ 0.3 ′′ (∼ 2 kpc). The radio emission is used as a tracer of the likely far-infrared morphology of these dusty, luminous galaxies. In ∼30% of the sample the radio emission appears unresolved at this spatial scale, suggesting that the power source is compact and may either be an obscured AGN or a compact nuclear starburst. However, in the majority of the galaxies, ∼70% (8/12), we find that the radio emission is resolved by MERLIN/VLA on scales of ∼ 1 ′′ (∼10 kpc). For these galaxies we also find that the radio morphologies are often broadly similar to their restframe UV emission traced by our HST imaging. To assess whether the radio emission may be extended on even larger scales, ≫ 1 ′′ , resolved out by the MERLIN+VLA synthesized images, we compare VLA B-array (5 ′′ beam) to VLA A-array (1.5 ′′ beam) fluxes for a sample of 50 µJy radio sources, including 5 submm galaxies. The submm galaxies have comparable fluxes at these resolutions and we conclude that the typical radio emitting region in these galaxies are unlikely to be much larger than ∼ 1 ′′ (∼10 kpc). We discuss the probable mechanisms for the extended emission in these galaxies and conclude that their luminous radio and submm emission arises from a large, spatially-extended starburst. The median star formation rates for these galaxies are ∼ 1700 M ⊙ yr −1 (M> 0.1M ⊙) occuring within regions with typical sizes of ∼40 kpc 2 , giving a star formation density of 45 M ⊙ yr −1 kpc −2. Such vigorous and extended starburst appear to be uniquely associated with the submm population. A more detailed comparison of the distribution of UV and radio emission in these systems shows that the broad similarities on large scales are not carried through to smaller scales, where there is rarely a one-to-one correspondance between the structures seen in the two wavebands. We interpret these differences as resulting from highly structured internal obscuration within the submm galaxies, suggesting that their vigorous activity is producing wind-blown channels through their obscuring dust clouds. If correct this underlines the difficulty of using UV morphologies to understand structural properties of this population and also may explain the surprising frequency of Lyα emission in the spectra of these very dusty galaxies.