High Sensitivity Array Observations of the z=1.87 Sub-Millimeter Galaxy
  GOODS 850-3
  We present sensitive phase-referenced 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 x 0.34 arcsec
(3.9 x 2.9 kpc) resolution shows a marginally resolved continuum structure with
a peak flux density of 148 \pm 38 uJy/beam, and a total flux density of 168 \pm
73 uJy, consistent with previous VLA and MERLIN measurements. The derived
intrinsic brightness temperature is > 5 \pm 2 x 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 HST optical image, indicating a dusty
source of nearly 9 kpc in diameter. No continuum emission is detected at the
full VLBI resolution (13.2 x 7.2 mas, 111 x 61 pc), with a 4-sigma point source
upper limit of 26 uJy/beam, or an upper limit to the intrinsic brightness
temperature of 4.7 x 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_sun/yr.
Moreover, the absence of any continuum emission at the full resolution of the
VLBI observations indicates the lack of a compact radio AGN source in this
z=1.87 SMG.
