Constraint on the gas-to-dust ratio in massive star-forming galaxies at z ∼ 1.4

Abstract

We carried out ¹²CO (J = 2-1) observations toward three star-forming galaxies on the main sequence at z ∼ 1.4 with the Nobeyama 45 m radio telescope. These galaxies have been detected with Spitzer/MIPS in 24 μm, Herschel/SPIRE in 250 μm and 350 μm; their gas metallicity, derived from optical emission line ratios based on near-infrared spectroscopic observations, is close to the solar metallicity. Although weak signal-like features of CO were seen, we could not detect significant CO emission. The dust mass and the upper limits on the molecular gas mass are (3.4-6.7) × 10⁸ M_⊙ and (9.7-14) × 10¹⁰ (α_CO/4.36) M_⊙, respectively. The upper limits on the gas-to-dust ratios at z ∼ 1.4 are 150-410, which are comparable to the gas-to-dust ratios in local galaxies with similar gas metallicity. A line stacking analysis enables us to detect significant CO emission and to derive an average molecular gas mass of 1.3 × 10¹¹ M_⊙ and gas-to-dust ratio of 250. This gas-to-dust ratio is also near to that in local galaxies with solar metallicity. These results suggest that the gas-to-dust ratio in star-forming galaxies with solar metallicity does not evolve significantly up to z ∼ 1.4. By comparing to a theoretical calculation, a rapid increase of the dust mass in an earlier epoch of galaxy evolution is suggested.