Molecular Gas Properties in the Host Galaxy of GRB080207
  We present the results of CO(1-0) and CO(4-3) observations of the host galaxy
of a long-duration gamma-ray burst GRB080207 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-H2 conversion
factor, we derive a molecular gas mass of Mgas = 8.7 x 10^10 Modot, which
places the host in a sequence of normal star-forming galaxies in a
Mgas-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 Mdust = 1.5 x 10^8 Modot. 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 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 Mgas-SFR (or surface density) relation, and kinematics,
suggesting that long-duration GRBs can occur in normal star-forming
environments at z ~ 2.
