The Scuba-2 Cosmology Legacy Survey: Alma Resolves the Rest–frame Far-infrared Emission of Sub-millimeter Galaxies

We present high-resolution (0.3) ALMA 870μm imaging of 52 sub-millimeter galaxies (SMGs) in the Ultra Deep Survey (UDS) field and investigate the size and morphology of the sub–millimeter (sub– mm) emission on 2–10kpc scales. We derive a median intrinsic angular size of FWHM=0.30± 0.04 for the 23 SMGs in the sample detected at a signal-to-noise ratio (SNR) > 10. Using the photometric redshifts of the SMGs we show that this corresponds to a median physical half–light diameter of 2.4 ± 0.2 kpc. A stacking analysis of the SMGs detected at an SNR< 10 shows they have sizes consistent with the 870μm–bright SMGs in the sample. We compare our results to the sizes of SMGs derived from other multi–wavelength studies, and show that the rest–frame ∼ 250μm sizes of SMGs are consistent with studies of resolved CO (J =3–2 to 7–6) emission lines, but that sizes derived from 1.4GHz imaging appear to be approximately two times larger on average, which we attribute to cosmic ray diffusion. The rest–frame optical sizes of SMGs are around four times larger than the sub-millimeter sizes, indicating that the star formation in these galaxies is compact relative to the pre-existing stellar distribution. The size of the starburst region in SMGs is consistent with the majority of the star formation occurring in a central region, a few kpc in extent, with a median star formation rate surface density of 90± 30M⊙ yr −1 kpc, which may suggest that we are witnessing an intense period of bulge growth in these galaxies . Subject headings: galaxies: starburst, galaxies: high-redshift