All stars are
formed deep inside molecular clouds. One of the current
enigmas lies in the formation and evolution of molecular clouds,
specifically the conversion from atomic to molecular gas. Until
recently, studies of the relationship between the atomic and molecular
gas were limited to observations of individual small clouds. The
enhanced capabilities of ALFA currently allow the observation of the
neutral hydrogen in and surrounding large molecular clouds spanning
several hundreds of square degrees on the sky. In July of 2005 GALFA
mapped about 300 square degrees of the Taurus Molecular Cloud (TMC)
complex yielding the largest high-resolution 21cm map of a star forming
region, while using less than 40 hours of observing time. This map
provides clues that will allow us to tackle long standing questions in
star formation theory.
The top map
shows the integrated intensity of the HI emission in Taurus
over a velocity range of just 4 - 6 km/sec. It is accompanied by a 13CO
map over the boxed region previously made at FCRAO (Goldsmith, Tang,
Brunt, Heyer, Li, Narayanan, and Snell in preparation) and an HI
spectrum towards a particular position. Many of the darker (blue)
regions of the HI map correlate very well with the molecular emmission
of 13CO. If we look at a sample spectrum we can see that there is a
clear absorption feature in HI. These absorption features are called HI
Narrow Self-Absorption (HINSA) and they correspond extremely well in
velocity and linewidth to the 13CO emission. HINSA is a recently
discovered feature; It is caused by extremely cold (10K) HI gas
embedded in the heart of the dark, dense molecular clouds which are
about to form stars. For the first time this map will allow us to
directly measure the HI content of Taurus thus giving us invaluable
clues as to the chemistry,
dynamics, and ages of stellar nurseries.
The fast
mapping capabilities of GALFA allow for such large maps as the
one shown here. Large scale, high resolution maps are important in that
they not only give us the details of our primary source but they also
help us to place that source in context with its environment. For
example we can clearly see the drop-off in the HI emission as we look
further away from the galactic plane. A cursory look at the HI
distribution in this region immediately shows us a wide variety of
environments ranging from diffuse gas, and pre-stellar dark clouds, to
newly formed stars. This shows us that the different portions of the
TMC complex are in various stages of evolution. In short, we now have
our first complete high-resolution, high-sensitivity head-to-toe guide
to star formation in a giant molecular cloud complex.
(Note: The
inherently narrow HINSA linewidths and the velocity
gradients in Taurus make it somewhat difficult to see all the HINSA in
any one range of channels. The vast majority of the 13CO emission in
Taurus is accompanied by HINSA even though it may not be clearly
visible in this particular range of channels).
