Molecular Gas Properties in the Host Galaxy of GRB 080207

Hatsukade, Bunyo and Hashimoto, Tetsuya and Kohno, Kotaro and Nakanishi, Kouichiro and Ohta, Kouji and Tóth, László Viktor (2019) Molecular Gas Properties in the Host Galaxy of GRB 080207. ASTROPHYSICAL JOURNAL, 876 (2). ISSN 1538-4357

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Abstract

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 × 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 × 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.

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