The weak carbon monoxide emission in an extremely metal-poor galaxy, Sextans A

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 marginal signal of CO J = 1 - 0 emission is seen, which is already faint enough to place a strong constraint on the conversion factor (αCO) from the CO luminosity to the molecular gas mass that is derived from the spatially resolved dust-mass map. The αCO is at least seven hundred times the Milky Way value. This indicates that CO emission is exceedingly weak in Sextans A, challenging its role as a coolant in extremely metal-poor galaxies.