CO observations toward HI-rich Ultra Diffuse Galaxies
  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.
