Galactic Edge Clouds I: Molecular Line Observations and Chemical Modelling of Edge Cloud 2

TL;DR

This study investigates Edge Cloud 2, a distant molecular cloud in the Milky Way, through molecular line observations and chemical modeling, revealing its physical properties, low metallicity, and possible influence of supernova shocks.

Contribution

It provides detailed physical and chemical characterization of EC2, including its low metallicity and the impact of supernova remnants, which is novel for such a distant galactic cloud.

Findings

Gas temperature of 20 K and density of 10^4 cm^-3

Mass of approximately 10^4 solar masses

Reduced heavy element abundances and low dust-to-gas ratio

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 to determine its physical characteristics. We calculate a gas temperature of 20 K and a density of n(H2) ~ 10^4 cm^-3. Based on our CO maps, we estimate the mass of EC2 at around 10^4 M_sun 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 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.