Very Long Baseline Array continuum and HI absorption observations of the ultraluminous infrared galaxy IRAS 17208-0014

We present phase-referenced VLBI observations of the radio continuum emission from, and the neutral hydrogen 21 cm absorption toward, the ultraluminous infrared galaxy IRAS 17208-0014. The observations were carried out at 1362 MHz using the Very Long Baseline Array, including the phased Very Large Array (VLA) as an element. The high-resolution radio continuum images reveal a nuclear starburst region in this galaxy that is composed of diffuse emission approximately 670 x 340 pc on the plane of the sky and a number of compact sources. These sources are most likely to be clustered supernova remnants and/or luminous radio supernovae. Their brightness temperatures range over (2.2-6.6) x 10(5) K, with radio spectral luminosities of (1-10) x 10(21)W Hz(-1). The total VLBI flux density of the starburst region is similar to52 mJy, which is about 50% of the total flux density detected with the VLA at arcsecond resolution. For this galaxy, we derive a massive star formation rate of similar to84 +/- 13 M-. yr(-1) and a supernova rate of similar to4 +/- 1 yr(-1). H I absorption is detected in multiple components with optical depths ranging between 0.3 and 2.5 and velocity widths between 58 and 232 km s(-1). The derived column densities, assuming T-s = 100 K, range over (10-28) x 10(21) cm(-2). The H I absorption shows a strong velocity gradient of 453 km s(-1) across 0.36 (274 pc). Assuming Keplerian motion, the enclosed dynamical mass is about 2.3 x 10(9) (sin(-2) i) M-., comparable to the enclosed dynamical mass estimated from CO observations.