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 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-H _ { 2 } conversion factor , we derive a molecular gas mass of M _ { gas } = 8.7 \times 10 ^ { 10 } M _ { \odot } , which places the host in a sequence of normal star-forming galaxies in a 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 \times 10 ^ { 8 } M _ { \odot } .
The spatially-resolving 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 \sim 2 .