Opacities of dense gas tracers in galactic massive star-forming regions

Abstract

Optical depths of dense molecular gas are commonly used in Galactic and extragalactic studies to constrain the dense gas mass of the clouds or galaxies. The optical depths are often obtained based on spatially unresolved data, especially in galaxies, which may affect the reliability of such measurements. We examine such effects in spatially resolved Galactic massive star forming regions. Using the 10-m SMT telescope, we mapped HCN and H13CN 3-2, HCO+ and H13CO+ 3-2 toward 51 Galactic massive star forming regions, 30 of which resulted in robust determination of spatially-resolved optical depths. Conspicuous spatial variations of optical depths have been detected within each source. We first obtained opacities for each position and calculated an optical-thick line intensity-weighted average, then averaged all the spectra and derived a single opacity for each region. The two were found to agree extremely well, with a linear least square correlation coefficient of 0.997 for the whole sample.

Dense Outflowing Molecular Gas in Massive Star-forming Regions

“…The map of H 13 CO + or H 13 CN 3-2 is gridded to match the center and each position of the spatial resolved HCO + or HCN 3-2 data. The more detailed information can be found in Liu et al (2023). The antenna temperature T A * is converted to main beam brightness temperature T mb using the equation…”