The Boston University-FCRAO Milky Way Galactic Ring Survey

James M. Jackson
Institute for Astrophysical Research
Boston University

The Milky Way's dominant star-forming structure, the 5 kpc ring, remains
largely unexplored.  Using the new SEQUOIA multi-pixel array receiver on the
FCRAO 14 m telescope, we are conducting a new molecular line survey of the
inner Galaxy, the Boston University-Five College Radio Astronomy Observatory
Galactic Ring Survey (GRS).  The GRS will map 13CO 1-0 emission in the inner
Galaxy from l=15 to 52 degrees and b=-1 to 1 degree.  Compared with previous
molecular line surveys of the inner Galaxy, the GRS offers excellent
sensitivity  (0.4 K), better spectral resolution (0.21 km/s), the same or
better angular resolution (46"), better sampling (22"), and the use of 13CO
(1-0), a better column density tracer than 12CO.

Theoretical modelling suggests that all molecular clouds
contain significant column densities of cold atomic hydrogen.
We have found that 21 cm H I self-absorption features toward GRS molecular
clouds caused by this cold H I allow us to resolve the long-standing
near/far kinematic distance ambiguity.  Because we can measure the distances
to clouds and their embedded infrared young stellar objects and star
clusters, we can for the first time establish their masses, sizes,
distributions, and luminosities.

We have also studied MSX infrared dark clouds. These clouds are easily
detected in 13CO emission. We find that many are in fact cores of larger
giant molecular clouds.  Because we can establish their kinematic
distances, we can deduce their masses, sizes, and distributions.
Their masses (~few thousand solar masses) and sizes (~few pc) suggest
that these are the intial condensations that will ultimately become
OB clusters or associations.  Their radial Galactic distribution
peaks in the 5 kpc ring.