Tracing Dense Gas in Six Resolved GMCs of the Andromeda Galaxy

TL;DR

This study uses NOEMA observations of six GMCs in Andromeda to analyze dense gas tracers, compare continuum measurements, and measure dust-mass-to-light ratios, providing insights into dense gas properties in an external galaxy.

Contribution

First direct measurement of dust-mass-to-light ratios for dense gas tracers in GMCs of an external galaxy, using combined molecular line and continuum data.

Findings

HCN and HCO$^+$ emissions are spatially coincident with dust emission

At 3 mm, free-free contamination is high, making 1 mm more reliable for dust mass estimates

HCN and HCO$^+$ trace dense gas similarly to dust emission

Abstract

We present dense-gas--tracing molecular observations of six resolved Giant Molecular Clouds (GMCs) in the Andromeda Galaxy (M31). Using the NOEMA interferometer, we observed the transitions of HCN(1-0), HCO$^+$(1-0), and HNC(1-0), as well as $^{13}$CO(1-0) and 100 GHz continuum emission. This complements our earlier work with the Submillimeter Array (SMA), including resolved dust continuum detections of these clouds at 230 GHz. In this work, we first compare different continuum measurements to conclude that the average free-free contamination of the observed flux is 71% at 3 mm but only 13% at 1 mm, confirming that emission at 3 mm is less reliable than that at 1 mm for calculating dust masses of star-forming clouds. While the $^{13}$CO emission is more extended than both HCN and HCO$^+$ emission, which in turn is more extended than HNC emission, we find that both HCN and HCO$^+$ are spatially coincident with, and similarly extended as, the 230 GHz dust emission. This suggests that both the 230 GHz dust continuum and most importantly the HCN emission traces the dense gas component of these GMCs. From comparison of the molecular emission with dust masses derived from the 230 GHz continuum emission, we obtain the first direct measurements of the dust-mass-to-light ratios ($\alpha^\prime_{HCN}$ and $\alpha^\prime_{HCO^+}$) in GMCs of an external galaxy. For HCN, the result is broadly similar to a measurement in the local Perseus cloud suggesting that these are indeed dense gas conversion factors. A larger cloud sample will be required to assess whether HCN is tracing comparable cloud-scale density regimes across the environments of M31.