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标题：Tracers of Dense Gas in the Outer Galaxy
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作者：Sudeshna Patra ; Neal J. Evans II ; Kee-Tae Kim 等
期刊名称：The Astronomical journal
印刷版ISSN：0004-6256
电子版ISSN：1538-3881
出版年度：2022
卷号：164
期号：4
页码：1-15
DOI：10.3847/1538-3881/ac83af
语种：English
出版社：American Institute of Physics
摘要：We have mapped $\mathrm{HCN}$ and HCO+ (J = 1 → 0) line emission toward a sample of seven star-forming regions (with $12+\mathrm{log}[{\rm{O}}/{\rm{H}}]$ 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 (AV ≥ 8 mag), inferred from the 13CO line data. $\mathrm{HCN}$ 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 $\mathrm{HCN}$ luminosity from within the regions with AV ≥ 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, $\mathrm{HCN}$ and HCO+ trace dense gas in high column density regions better than does 13CO. HCO+ is less concentrated than $\mathrm{HCN}$ 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 (Mdense) and line luminosities of $\mathrm{HCN}$ and HCO+, when integrated over the whole cloud, is comparable to factors used in extragalactic studies.