The Weak Carbon Monoxide Emission In An Extremely Metal Poor Galaxy, Sextans A

TL;DR

This study detects extremely faint CO emission in the metal-poor galaxy Sextans A, revealing that CO is a weak tracer of molecular gas in such environments and challenging its role as a coolant.

Contribution

It provides the first deep CO observation in an extremely metal poor galaxy, establishing a much higher CO-to-H2 conversion factor than in the Milky Way.

Findings

CO emission is at least 700 times weaker than in the Milky Way.

The high conversion factor constrains CO's effectiveness as a molecular gas tracer.

CO's role as a coolant in metal-poor galaxies is likely limited.

Abstract

Carbon monoxide (CO) is one of the primary coolants of gas and an easily accessible tracer of molecular gas in spiral galaxies but it is unclear if CO plays a similar role in metal poor dwarfs. We carried out a deep observation with IRAM 30 m to search for CO emission by targeting the brightest far-IR peak in a nearby extremely metal poor galaxy, Sextans A, with 7% Solar metallicity. A weak CO J=1-0 emission is seen, which is already faint enough to place a strong constraint on the conversion factor (a_CO) from the CO luminosity to the molecular gas mass that is derived from the spatially resolved dust mass map. The a_CO is at least seven hundred times the Milky Way value. This indicates that CO emission is exceedingly weak in extremely metal poor galaxies, challenging its role as a coolant in these galaxies.