Sensitive Very Long Baseline Interferometry Studies of the OH Megamaser Emission from IRAS 17208–0014

We present phase-referenced VLBI results on the radio continuum and the OH 18 cm megamaser emission from the ultraluminous infrared galaxy, IRAS 17208-0014. The observations were carried out at 1599 MHz using the Very Long Baseline Array, the phased VLA, and the Green Bank Telescope. The highest resolution radio continuum results show several compact sources with brightness temperatures on the order of 10⁶ K. These sources are more likely to be clustered supernova remnants and/or luminous radio supernovae. However, the agreement between the number of observed and expected compact sources above the 5 σ level supports the possibility that each one of the compact sources could be dominated by a recently detonated luminous radio supernova. The continuum results suggest that there are no radio-loud AGNs in the nuclear region of this galaxy. The OH 18 cm megamaser emission in IRAS 17208-0014 is detected at various angular resolutions. It has an extent of 170 pc × 110 pc and is mostly localized in two regions separated by 61 pc. The structure and dynamics of the maser emission seem to be consistent with a clumpy, rotating, ringlike geometry, with the two dominant maser regions marking the tangential points of the proposed rotating-ring distribution. Assuming Keplerian motion for the rotating maser ring, the enclosed dynamical mass and the mass density within a radius of 30.5 pc are about 3 × 10⁷( i) M_☉ and 281( i) M_☉ pc^-3, respectively.