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

TL;DR

This study investigates how spatial resolution affects optical depth measurements of dense gas tracers in Galactic star-forming regions, revealing significant spatial variations and validating averaged measurements against spatially resolved data.

Contribution

It provides the first detailed comparison of spatially resolved and unresolved optical depth measurements in Galactic star-forming regions, highlighting the reliability of averaged values.

Findings

Significant spatial variations in optical depths within regions.

High correlation (0.997) between spatially resolved and averaged optical depths.

Averaged optical depths reliably represent spatially resolved measurements.

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 towards 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.