CARMA CO Observations of Three Extremely Metal-Poor, Star-Forming Galaxies

TL;DR

This study uses CARMA observations to set upper limits on CO emission in three extremely metal-poor dwarf galaxies, providing insights into molecular gas content and star formation efficiency at low metallicities.

Contribution

First sensitive CO (J=1-0) upper limits in three metal-poor dwarf galaxies, constraining molecular gas estimates and informing models of star formation in low-metallicity environments.

Findings

No CO emission detected in the three galaxies.

Stringent upper limits on CO luminosity were established.

Results suggest either a rising CO-to-H2 conversion factor or highly efficient star formation with little molecular gas.

Abstract

We present sensitive CO (J = 1 - 0) emission line observations of three metal-poor dwarf irregular galaxies Leo P (Z ~ 3% Z_Solar), Sextans A (Z ~ 7.5% Z_Solar), and Sextans B (Z ~ 7.5% Z_Solar), all obtained with the Combined Array for Millimeter-wave Astronomy (CARMA) interferometer. While no CO emission was detected, the proximity of the three systems allows us to place very stringent (4 sigma) upper limits on the CO luminosity (L_CO) in these metal-poor galaxies. We find the CO luminosities to be L_CO < 2900 K km/s pc^2 for Leo P, L_CO < 12400 K km/s pc^2 for Sextans A, and L_CO < 9700 K km/s pc^2 for Sextans B. Comparison of our results with recent observational estimates of the factor for converting between L_CO and the mass of molecular hydrogen, as well as theoretical models, provides further evidence that either the CO-to-H_2 conversion factor increases sharply as metallicity decreases, or that stars are forming in these three galaxies very efficiently, requiring little molecular hydrogen.