Origins of Scatter in the Relationship Between HCN 1-0 and Dense Gas Mass in the Galactic Center

Abstract

We investigate the correlation of HCN 1-0 with dense gas mass in the Galactic center. We find that in general (on the ~10 pc size scale of individual molecular cloud cores) HCN 1-0 is well correlated with the dense molecular gas mass using a standard log-log relationship. However individual clouds in this environment show systematic deviations from this relationship that contribute to around 0.75 dex of scatter. Most notably, Sgr B2, the most massive cloud in the Galactic center, has an integrated HCN 1-0 intensity of cloud less than half its mass, and several other clouds including GCM-0.02-0.07 show an enhancement of HCN 1-0 by a factor of 2-3 relative to clouds of comparable mass. We also find that HCN 1-0 is more intense for a given mass in the large-scale diffuse emission in the central 300 parsecs compared to the compact emission from individual cloud cores. We identify the two primary sources of scatter in the relationship between HCN 1-0 and dense gas mass to be self-absorption (which reduces the observed HCN 1-0 intensity of Sgr B2) and variations in HCN abundance, which appear to increase the brightness of HCN 1-0 in GCM-0.02-0.07 and other clouds. We find that these sources of scatter would only contribute to an ~10% error in the dense gas mass inferred from the HCN 1-0 intensity in the Galactic center. However, the implied order of magnitude HCN abundance variations between Galactic center clouds, and the systematic nature of these variations warn of potential biases in the use of HCN as dense gas mass tracer in more extreme environments, such as AGN and shock-dominated regions. Finally, we also investigate other tracers having transitions near 3 mm, finding that HNCO, HNC and HCO+ largely behave like HCN, while HC3N, and CH3CN are higher fidelity tracers of the amount of gas in clouds like Sgr B2 that suffer from strong self-absorption of the HCN 1-0 line. [abridged]