Recent attempts to map Galactic neutral hydrogen in absorption against extragalactic continuum sources at very small scales have produced evidence of significant structure in the cold HI gas at scale from a few tens of AUs up to hundreds of AUs. Such structure is surprising since the predicted pressure in this gas is much higher that observed in the surrounding medium. We present the first results from a program to map the small scale Galactic neutral hydrogen structure using the Very Long Baseline Array (VLBA) over a range of angular size scales and along a variety of sight lines. We have mapped small scale structure towards two sources, 3C138, which has previously been observed in HI absorption, and CJ12216+416, a source in a direction in which the small scale HI structure has not been previously studied. We present the HI opacity maps as well as a discussion of relavent theoretical issues such as structure geometry, magnetic fields, and turbulence.
American Astronomical Society Meeting Abstracts #190
- Pub Date:
- May 1997