Tracers of Dense Gas in the Outer Galaxy

Abstract

We have mapped and HCO+ (J = 1 → 0) line emission toward a sample of seven star-forming regions (with ranging from 8.34 to 8.69) in the outer Milky Way (Galactocentric distance >9.5 kpc), using the 14 m radio telescope of the Taeduk Radio Astronomy Observatory. We compare these two molecular lines with other conventional tracers of dense gas, millimeter-wave continuum emission from dust and extinction thresholds (A V ≥ 8 mag), inferred from the 13CO line data. and HCO+ correlate better with the millimeter emission than with the extinction criterion. A significant amount of luminosity comes from regions below the extinction criterion and outside the millimeter clump for all the clouds. The average fraction of luminosity from within the regions with A V ≥ 8 mag is 0.343 ± 0.225; for the regions of millimeter emission, it is 0.478 ± 0.149. Based on a comparison with column density maps from Herschel, and HCO+ trace dense gas in high column density regions better than does 13CO. HCO+ is less concentrated than for outer Galaxy targets, in contrast with the inner Galaxy sample, suggesting that metallicity may affect the interpretation of tracers of dense gas. The conversion factor between the dense gas mass (M dense) and line luminosities of and HCO+, when integrated over the whole cloud, is comparable to factors used in extragalactic studies.