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

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), the results of one nondetection in a dwarf galaxy (FRB 20121102A), and two events detected in M81 (FRB 20200120E) and the Milky Way (FRB 20200428A). The CO observations resulted in the detection of CO(3–2) emission in the FRB 20180924B host and nondetections of CO(3–2) and CO(2–1) emission in the hosts of FRB 20190102C and FRB 20190711A, respectively. The derived molecular gas mass and 3σ upper limit is (2.4 ± 0.2) × 109 M⊙, <3.8 × 108 M⊙, and <6.7 × 109 M⊙ for the hosts of FRB 20180924B, FRB 20190102C, and FRB 20190711A, 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 FRB 20180924B host is gas-rich (the larger molecular gas fraction), and the hosts of FRB 20190102C and FRB 20200120E 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 FRB 20200120E host, is excluded, and analysis with a larger sample is needed.