Dense Molecular Gas in the Nearby Low Metallicity Dwarf Starburst Galaxy IC 10

TL;DR

This study investigates how low metallicity affects dense molecular gas tracers like HCN and HCO+ in a nearby starburst galaxy, revealing that these tracers remain effective but the emitting regions are smaller, impacting high-redshift galaxy observations.

Contribution

First detailed analysis of dense molecular gas tracers in a low metallicity starburst galaxy, demonstrating their continued usefulness and implications for high-redshift galaxy studies.

Findings

HCN and HCO+ are detectable in low metallicity galaxy IC 10.

Dense gas fraction similar to spiral galaxies, lower than U/LIRGs.

Dense gas star formation efficiency is high, comparable to U/LIRGs.

Abstract

Dense molecular gas and star formation are correlated in galaxies. The effect of low metallicity on this relationship is crucial for interpreting observations of high redshift galaxies, which have lower metallicities than galaxies today. However, it remains relatively unexplored because dense molecular gas tracers like HCN and HCO+ are faint in low metallicity systems. We present Green Bank Telescope observations of HCN(1-0) and HCO+(1-0) on giant molecular cloud (34pc) scales in the nearby low metallicity ($12+\log({\rm O/H})=8.2$) starburst IC 10 and compare them to those in other galaxies. We detect HCN and HCO+ in one and three of five pointings, respectively. The $I_{\rm HCN}/I_{\rm HCO+}$ values are within the range seen in other galaxies, but are most similar to those seen in other low metallicity sources and in starbursts. The detections follow the fiducial $L_{\rm IR}$-$L_{\rm HCN}$ and $L_{\rm IR}$-$L_{\rm HCO+}$ relationships. These trends suggest that HCN and HCO+ can be used to trace dense molecular gas at metallicities of 1/4 $Z_\odot$, to first order. The dense gas fraction is similar to that in spiral galaxies, but lower than that in U/LIRGs. The dense molecular gas star formation efficiency, however, is on the upper end of those in normal galaxies and consistent with those in U/LIRGs. These results suggest that the CO and HCN/HCO+ emission occupy the same relative volumes as at higher metallicity, but that the entire emitting structure is reduced in size. Dense gas mass estimates for high redshift galaxies may need to be corrected for this effect.