HIGH-SENSITIVITY ARRAY OBSERVATIONS OF THEz= 1.87 SUBMILLIMETER GALAXY GOODS 850-3

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 47 × 0 34 (3.9 × 2.9 kpc) resolution shows a marginally resolved continuum structure with a peak flux density of 148 ± 38 μJy beam −1 , 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) × <0 23, or 2.3(±1.0) × <1.9 kpc, and the derived intrinsic brightness temperature is >(5 ± 2) × 10 3 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 (13.2 × 7.2 mas, 111 × 61 pc), with a 4σ point source upper limit of 26 μJy beam −1 , or an upper limit to the intrinsic brightness temperature of 4.7 × 10 5 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 −1 . 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.