Very Long Baseline Array Continuum and HiAbsorption Observations of the Ultraluminous Infrared Galaxy IRAS 17208−0014

We present phase-referenced VLBI observations of the radio continuum emissionfrom, and the neutral hydrogen 21 cm absorption toward, the Ultra-LuminousInfrared Galaxy IRAS 17208-0014. The observations were carried out at 1362 MHzusing the Very Long Baseline Array, including the phased Very Large Array as anelement. The high-resolution radio continuum images reveal a nuclear starburstregion in this galaxy, which is composed of diffuse emission approximately 670x 340 pc on the plane of the sky, and a number of compact sources. Thesesources are most likely to be clustered supernova remnants and/or luminousradio supernovae. Their brightness temperatures range over (2.2-6.6) x 10^{5}K, with radio spectral luminosities between (1-10) x 10^{21} W Hz^{-1}. Thetotal VLBI flux density of the starburst region is ~52 mJy, which is about 50%of the total flux density detected with the VLA at arcsecond resolution. Forthis galaxy, we derive a massive star formation rate of ~84pm13 M{_\odot}yr^{-1}, and a supernova rate of ~4pm1 yr^{-1}. H I absorption is detected inmultiple components with optical depths ranging between 0.3 and 2.5, andvelocity widths between 58 and 232 km s^{-1}. The derived column densities,assuming T_{s}=100 K, range over (10-26) x 10^{21} cm^{-2}. The H I absorptionshows a strong velocity gradient of 453 km s^{-1} across 0.36 arcsec (274 pc).Assuming Keplerian motion, the enclosed dynamical mass is about 2.3 x 10^9sin^{-2}i M{_\odot}, comparable to the enclosed dynamical mass estimated fromCO observations.