SPLASH: The Southern Parkes Large-Area Survey in Hydroxyl - First
  Science from the Pilot Region
  SPLASH (the Southern Parkes Large-Area Survey in Hydroxyl) is a sensitive,
unbiased and fully-sampled survey of the Southern Galactic Plane and Galactic
Centre in all four ground-state transitions of the hydroxyl (OH) radical. The
survey provides a deep census of 1612-, 1665-, 1667- and 1720-MHz OH absorption
and emission from the Galactic ISM, and is also an unbiased search for maser
sources in these transitions. We present here first results from the SPLASH
pilot region, which covers Galactic longitudes 334 to 344 degrees and latitudes
of -2 to +2 degrees. Diffuse OH is widely detected in all four transitions,
with optical depths that are always small (averaged over the Parkes beam), and
with departures from LTE common even in the 1665- and 1667-MHz main lines. To a
3$\sigma$ sensitivity of 30 mK, we find no evidence of OH envelopes extending
beyond the CO-bright regions of molecular cloud complexes, and conclude that
the similarity of the OH excitation temperature and the level of the continuum
background is at least partly responsible for this. We detect masers and maser
candidates in all four transitions, approximately 50 per cent of which are new
detections. This implies that SPLASH will produce a substantial increase in the
known population of ground-state OH masers in the Southern Galactic Plane.
