Very Long Baseline Array continuum and H I 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 × 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) × 10⁵ K, with radio spectral luminosities of (1-10) × 10²¹ W Hz^-1. The total 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. For this galaxy, we derive a massive star formation rate of ∼84 ± 13 M_⊙ yr^-1 and a supernova rate of ∼4 ± l 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) ± 10²¹ 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 × 10⁹ (sin^-2 i) M_⊙, comparable to the enclosed dynamical mass estimated from CO observations.