Minimal HCN emission from Molecular Clouds in M33

TL;DR

This study investigates HCN emission in molecular clouds of M33, finding it to be significantly weaker than in the Milky Way, likely due to low metallicity and evolutionary effects within GMCs, challenging its role as a star formation tracer.

Contribution

The paper provides the first search for HCN emission in M33's GMCs and analyzes the reasons for its weakness compared to the Milky Way, highlighting environmental effects.

Findings

No significant HCN emission detected in M33 GMCs.

High CO-to-HCN ratios suggest reduced HCN formation in low-metallicity environments.

Lower HCN-to-star formation rate ratios compared to the Milky Way.

Abstract

Since HCN emission has been shown to be a linear tracer of ongoing star formation activity, we have searched for HCN (J = 1->0) emission from known GMCs in the nearby galaxy M33. No significant HCN emission has been found along any of the lines of sight. We find two lines of sight where CO-to-HCN integrated intensity ratios up to 280, nearly a factor of 6 above what is found in comparable regions of the Milky Way. Star formation tracers suggest that the HCN-to-star formation rate ratio (L_HCN/M_SFR) is a factor of six lower than what is observed in the Milky Way (on average) and local extragalactic systems. Simple chemical models accounting for the sub-solar N/O ratio suggest that depletion cannot account for the high CO-to-HCN ratios. Given HCN formation requires high extinction (A_V > 4), low metallicity may yield reduced dust shielding and thus a high CO/HCN ratio. The turbulence and structure of GMCs in M33 are comparable to those found in other systems, so the differences are unlikely to result from different GMC properties. Since lower CO-to-HCN ratios are associated with the highest rates of star formation, we attribute the deficits in part to evolutionary effects within GMCs.