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

Abstract

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 gas = (0.2–6) × 1010 M ⊙ assuming metallicity-dependent CO-to-H2 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 (μ gas) and a shorter gas depletion timescale (t depl) as compared with other SFGs at similar redshifts, especially at z ≲ 1. This could be a common property of GRB hosts or an effect introduced by the selection of targets that are typically above the main-sequence line. To eliminate the effect of selection bias, we analyzed μ gas and t depl as a function of the distance from the main-sequence line (δMS). We find that the GRB hosts follow the same scaling relations as other SFGs, where μ gas increases and t depl decreases with increasing δMS. No molecular gas deficit is observed when compared to other SFGs of similar star formation rate and stellar mass. These findings suggest that the same star formation mechanism is expected to be happening in GRB hosts as in other SFGs.