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

Abstract

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 $M_{\mathrm{gas}}=(0.2-6)\times 10^{10}$ $M_{\odot }$ 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 ($\hat{I}{¼}_{\mathrm{gas}}$) and a shorter gas depletion timescale ($t_{\mathrm{depl}}$) 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 $\hat{I}{¼}_{\mathrm{gas}}$ and $t_{\mathrm{depl}}$ as a function of the distance from the main-sequence line ($\hat{I}´$MS). We find that the GRB hosts follow the same scaling relations as other SFGs, where $\hat{I}{¼}_{\mathrm{gas}}$ increases and $t_{\mathrm{depl}}$ decreases with increasing $\hat{I}´\mathrm{MS}$. 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.

26 pages, 12 figures, accepted for publication in ApJ