Diverse Properties of Molecular Gas in the Host Galaxies of Fast Radio Bursts

Abstract: We report the properties of molecular gas in a sample of six host galaxies of fast radio bursts (FRBs) obtained from CO observations with the Atacama Large Millimeter/submillimeter Array (FRBs 20180924B, 20190102C, and 20190711A) and results of one non-detection in a dwarf galaxy (FRB20121102A) and two events detected in M81 (FRB20200120E) and the Milky Way (FRB20200428A). The CO observations resulted in the detection of CO(3-2) emission in the FRB20180924B host and non-detections of CO(3-2) and CO(2-1) emission in the hosts of FRB20190102C and FRB20190711A, respectively. The derived molecular gas mass and 3$\sigma$ upper limit is $(2.4 \pm 0.2) \times 10^9$ $M_{\odot}$, $<3.8 \times 10^8$ $M_{\odot}$, and $<6.7 \times 10^9$ $M_{\odot}$ for the hosts of FRB20180924B, FRB20190102C, and FRB20190711A, respectively. We found diversity in molecular gas properties (gas mass, gas depletion time, and gas fraction to stellar mass) in the sample. Compared to other star-forming galaxies, the FRB20180924B host is gas-rich (the larger molecular gas fraction), and the hosts of FRB20190102C and FRB20200120E are gas-poor with a shorter depletion time for their stellar mass and star-formation rate. Our findings suggest that FRBs arise from multiple progenitors or single progenitors that can exist in a wide range of galaxy environments. Statistical analysis shows a significant difference in the distribution of molecular gas fraction between the FRB hosts and local star-forming galaxies. However, the difference is not substantial when an outlier, the FRB20200120E host, is excluded, and analysis with a larger sample is needed.