Molecular Line Observations and Chemical Modelling of Edge Cloud 2
Abstract
Edge Cloud 2 (EC2) is a large molecular cloud with one of the largest galactocentric distances known to exist in the Milky Way. We present observations of 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) ∼ 104 cm-3. Based on our CO maps, we estimate the mass of EC2 at around 104 Msun 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 five relative to the solar neighbourhood (similar to dwarf irregular galaxies and damped Lyman alpha systems); very low extinction (AV < 4 mag) due to a very low dust-to-gas ratio; an enhanced cosmic ray ionisation 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 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.
- Publication:
-
The Evolving ISM in the Milky Way and Nearby Galaxies
- Pub Date:
- January 2009
- Bibcode:
- 2009eimw.confE..56R
- Keywords:
-
- astrochemistry;
- dust;
- extinction;
- ISM: clouds;
- ISM: individual (Edge Cloud 2);
- ISM: molecules;
- radio lines: ISM