GALFA HI: Candidate Sites for H2 Formation in Cold HI Emission and Other Tracers

Details GALFA HI: Candidate Sites for H2 Formation in Cold HI Emission and Other Tracers GALFA HI: Candidate Sites for H2 Formation in Cold HI Emission and Other Tracers

Jan 2012

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012aas...21934924n&link_type=abstract

American Astronomical Society, AAS Meeting #219, #349.24

Astronomy and Astrophysics

Astronomy

Scientific paper

Interstellar gas has a variety of temperature phases, but only the coldest clouds are dense enough to collapse gravitationally and form stars. How do such clouds form? A key step in this process is the transition from neutral atomic hydrogen (HI) to molecular hydrogen (H2). To identify candidate sites where this HI-to-H2 transition may be underway, we have developed a method of fitting isolated HI 21cm emission features to constrain their spin temperature and other properties vs. position 21cm-line data cubes. Our method uses the Nelder-Meade `amoeba' method to solve the relevant radiative transfer equation by identifying the absolute chi-squared minimum in the parameter space. As other investigators have noted, this approach requires a very high signal-to-noise ratio, so we are using sensitive Arecibo L-band Feed Array (ALFA) observations, starting with narrow-line HI emission clouds in the inner-Galaxy ALFA (I-GALFA) survey, and we have also tested the reliability of our method with a large suite of model spectra. Cold HI clouds confirmed by the fit will be compared to tracers of molecular gas, including CO lines and FIR dust emission.
The I-GALFA survey is part of the Galactic ALFA HI data set obtained with the Arecibo 305m telescope. Arecibo Observatory is part of the National Astronomy and Ionosphere Center, operated sequentially by Cornell University and Stanford Research Institute under Cooperative Agreement with the U.S. National Science Foundation.

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