ALMA CO Observations of the Host Galaxies of Long-duration Gamma-ray Bursts. I: Molecular Gas Scaling Relations

We present the results of CO observations toward 14 host galaxies of long-duration gamma-ray bursts (GRBs) at z = 0.1-2.5 by using the Atacama Large Millimeter/submillimeter Array. We successfully detected CO(3-2) or CO(4-3) emission in eight hosts (z = 0.3-2), which more than doubles the sample size of GRB hosts with CO detection. The derived molecular gas mass is $M_{\rm gas} = (0.2-6) \times 10^{10}$ $M_{\odot}$ assuming metallicity-dependent CO-to-H$_2$ conversion factors. By using the largest sample of GRB hosts with molecular gas estimates (25 in total, of which 14 are CO-detected) including results from the literature, we compared molecular gas properties with those of other star-forming galaxies (SFGs). The GRB hosts tend to have a higher molecular gas mass fraction ($\mu_{\rm gas}$) and a shorter gas depletion timescale ($t_{\rm depl}$) as compared with other SFGs at similar redshifts especially at $z \lesssim 1$. This could be a common property of GRB hosts or an effect introduced by the selection of targets which are typically above the main-sequence line. To eliminate the effect of selection bias, we analyzed $\mu_{\rm gas}$ and $t_{\rm depl}$ as a function of the distance from the main-sequence line ($\delta$MS). We find that the GRB hosts follow the same scaling relations as other SFGs, where $\mu_{\rm gas}$ increases and $t_{\rm depl}$ decreases with increasing $\delta {\rm MS}$. No molecular gas deficit is observed when compared to other SFGs of similar SFR and stellar mass. These findings suggest that the same star-formation mechanism is expected to be happening in GRB hosts as in other SFGs.