2007/08/28 by P. M. E. Ruffle, T. J. Millar, H. Roberts +7
Earth and Planetary Sciences · Physics and Astronomy · #Astronomy #Astrophysics #Astrophysics and Star Formation Studies #Atmospheric Ozone and Climate #Extinction (optical mineralogy) #Galaxy #Halo #Interstellar medium #Ionization #Metallicity #Milky Way #Molecular cloud #Physics #Star formation #Stars #Stellar, planetary, and galactic studies #Supernova #astro-ph
paper · pdf · doi:10.1086/522775
Accepted by ApJ 7 August 2007. 29 pages, 9 figures, 10 tables. PMR now at NRAO, Green Bank, WV, USA. TJM now at Queen's University Belfast, UK. GB now at Yale University, CT, USA
arxiv created 2007/08/28 · openalex publication_date 2007/12/20 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
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 to determine its physical characteristics. We calculate a gas temperature of 20 K and a density of n (H 2 ) ∼ 10 4 cm −3 . Based on our CO maps, we estimate the mass of EC2 at around 10 4 M ☉ and 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 Lyα systems), very low extinction ( A V < 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 also related to the continuing infall of primordial (or low-metallicity) halo gas since the Milky Way formed. Finally, we note that shocks from the old supernova remnant GSH 138-01-94 may have determined the morphology and dynamics of EC2.