GALFA Eye on Taurus

M. Krco & P. F. Goldsmith



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).