Gas-to-dust ratios in massive star-forming galaxies at z ∼ 1.4

Abstract

We present results of ¹²CO(J = 2-1) observations toward four massive star-forming galaxies at z ∼ 1.4 with the Nobeyama 45 m radio telescope. The galaxies are detected with Spitzer/MIPS in 24 μm and Herschel/SPIRE in 250 μm and 350 μm, and they mostly reside in the main sequence. Their gas-phase metallicities derived by the N2 method using the Hα and [N II]λ 6584 emission lines are near the solar value. CO lines are detected toward three galaxies. The molecular-gas masses obtained are (9.6-35) × 10¹⁰ M_⊙ by adopting the Galactic CO-to-H₂ conversion factor and a CO(2-1)/CO(1-0) flux ratio of 3. The dust masses derived from the modified blackbody model (assuming a dust temperature of 35 K and an emissivity index of 1.5) are (2.4-5.4) × 10⁸ M_⊙. Resulting gas-to-dust ratios (not accounting for H I mass) at z ∼ 1.4 are 220-1450, which are several times larger than those in local star-forming galaxies. A dependence of the gas-to-dust ratio on the far-infrared luminosity density is not clearly seen.