Tracers of Dense Gas in the Outer Galaxy

We have mapped HCN and HCO+ (J = 1 -> 0) line emission toward a sample of seven star-forming regions (with 12 + log [O/H] 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 (CO)-C-13 line data. HCN 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 HCN 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, HCN and HCO+ trace dense gas in high column density regions better than does (CO)-C-13. HCO+ is less concentrated than HCN 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 HCN and HCO+ , when integrated over the whole cloud, is comparable to factors used in extragalactic studies.

Automated summary

In this study, we mapped HCN and HCO+ (J = 1 -> 0) line emission towards star-forming regions in the outer Milky Way. We found that HCN and HCO+ correlate better with millimeter-wave emission than with the extinction criterion. Furthermore, we observed that HCN and HCO+ trace dense gas in high column density regions better than (CO)-C-13. The concentration of HCO+ is less in outer Galaxy targets compared to inner Galaxy samples, suggesting a possible influence of metallicity on dense gas tracers.