2010AJ....139.1622M

We present sensitive phase-referenced Very Long Baseline Interferometry (VLBI) results on the radio continuum emission from the z = 1.87 luminous submillimeter galaxy (SMG) GOODS 850-3. The observations were carried out at 1.4 GHz using the High-Sensitivity Array (HSA). Our sensitive tapered VLBI image of GOODS 850-3 at 0".47x0".34 (3.9x2.9 kpc) resolution shows a marginally resolved continuum structure with a peak flux density of 148 ±38 µJy/beam, and a total flux density of 168±73 µJy, consistent with previous Very Large Array and MERLIN measurements. The deconvolved size of the source is 0".27(±0".12)x<0".23, or 2.3(±1.0)x<1.9 kpc, and the derived intrinsic brightness temperature is >(5±2)x10³ K. The radio continuum position of this galaxy coincides with a bright and extended near-infrared source that nearly disappears in the deep Hubble Space Telescope optical image, indicating a dusty source of nearly 9 kpc in diameter. No continuum emission is detected at the full VLBI resolution (13.2x7.2 mas, 111x61 pc), with a 4σ point source upper limit of 26 µJy/beam, or an upper limit to the intrinsic brightness temperature of 4.7x10⁵ K. The extent of the observed continuum source at 1.4 GHz and the derived brightness temperature limits are consistent with the radio emission (and thus presumably the far-infrared emission) being powered by a major starburst in GOODS 850-3, with a star formation rate of ∼2500 M_☉/yr. Moreover, the absence of any continuum emission at the full resolution of the VLBI observations indicates the lack of a compact radio-loud active galactic nucleus in this z = 1.87 SMG.