CO observations toward HI-rich Ultra Diffuse Galaxies

TL;DR

This study reports the first detection of CO emission in an ultra-diffuse galaxy, revealing low molecular-to-atomic gas ratios and suggesting star formation inefficiency is due to poor molecule formation from atomic gas.

Contribution

First CO detection in an HI-rich ultra-diffuse galaxy, providing insights into molecular gas content and star formation processes in such galaxies.

Findings

CO detected in one UDG, indicating low molecular gas content.

Upper limits on molecular mass in other UDGs suggest low molecular-to-atomic gas ratios.

Star formation inefficiency likely caused by low molecular formation, not low star formation efficiency.

Abstract

We present CO observations toward a sample of six HI-rich Ultra-diffuse galaxies (UDGs) as well as one UDG (VLSB-A) in the Virgo Cluster with the IRAM 30-m telescope. CO 1-0 is marginally detected at 4sigma level in AGC122966, as the first detection of CO emission in UDGs. We estimate upper limits of molecular mass in other galaxies from the non-detection of CO lines. These upper limits and the marginal CO detection in AGC122966 indicate low mass ratios between molecular and atomic gas masses. With the star formation efficiency derived from the molecular gas, we suggest that the inefficiency of star formation in such HI-rich UDGs is likely caused by the low efficiency in converting molecules from atomic gas, instead of low efficiency in forming stars from molecular gas.