2019ApJ...876...91H

We present the results of CO(1-0) and CO(4-3) observations of the host galaxy of a long-duration gamma-ray burst GRB 080207 at z = 2.0858 by using the Karl G. Jansky Very Large Array and the Atacama Large Millimeter/submillimeter Array. The host is detected in CO(1-0) and CO(4-3), becoming the first case for a gamma-ray burst (GRB) host with more than two CO transitions detected combined with CO(2-1) and CO(3-2) in the literature. Adopting a metallicity-dependent CO-to-H₂ conversion factor, we derive a molecular gas mass of M_gas = 8.7 x 10¹⁰ M_☉, which places the host in a sequence of normal star-forming galaxies in an M_gas-star formation rate (SFR) plane. A modified blackbody fit to the far-infrared-millimeter photometry results in a dust temperature of 37 K and a dust mass of M_dust = 1.5 x 10⁸ M_☉. The spatially resolved CO(4-3) observations allow us to examine the kinematics of the host. The CO velocity field shows a clear rotation and is reproduced by a rotation-dominated disk model with a rotation velocity of 350 km s^–1 and a half-light radius of 2.4 kpc. The CO spectral line energy distribution derived from the four CO transitions is similar to that of starburst galaxies, suggesting a high excitation condition. Comparison of molecular gas properties between the host and normal (main-sequence) galaxies at similar redshifts shows that they share common properties such as gas mass fraction, gas depletion timescale, gas-to-dust ratio, location in the M_gas-SFR (or surface density) relation, and kinematics, suggesting that long-duration GRBs can occur in normal star-forming environments at z ∼ 2.