Galactic Edge Clouds. I. Molecular Line Observations and Chemical Modeling of Edge Cloud 2

Abstract

Edge Cloud 2 (EC2) is a molecular cloud, about 35 pc in size, with one of the largest galactocentric distances known to exist in the Milky Way. We present observations of a peak CO emission region in the cloud and use these todetermineitsphysicalcharacteristics.Wecalculateagastemperatureof20Kandadensityofn(H2) � 10 4 cm � 3 . Based on our CO maps, we estimate the mass of EC2 at around 10 4 Mand continuum observations suggest a dust- to-gas mass ratio as low as 0.001. Chemical models have been developed to reproduce the abundances in EC2, and they indicate that heavy element abundances may be reduced by a factor of 5 relative to the solar neighborhood (similar to dwarf irregular galaxies and damped Lysystems), very low extinction (AV < 4 mag) due to a very low dust-to-gas mass ratio, an enhanced cosmic-ray ionization rate, and a higher UV field compared to local interstellar values. The reduced abundances may be attributed to the low level of star formation in this region and are probably alsorelatedtothecontinuinginfallofprimordial(orlow-metallicity)halogassincetheMilky Wayformed.Finally, we note that shocks from the old supernova remnant GSH 138Y01Y94 may have determined the morphology and dynamics of EC2. Subject headings: astrochemistry — dust, extinction — ISM: clouds — ISM: individual (Edge Cloud 2) — ISM: molecules — radio lines: ISM