Big Three Dragons: Molecular Gas in a Bright Lyman-break Galaxy at z = 7.15
Takuya Hashimoto, Akio Inoue, Yuma Sugahara, Yoshinobu Fudamoto, Seiji Fujimoto, K. K. Knudsen, Hiroshi Matsuo, Yoichi Tamura, S. Yamanaka, Yuichi Harikane, Nario Kuno, Yoshiaki Ono, Dragan Salak, Nozomi Ishii
Abstract
Abstract We report Atacama Large Millimeter/submillimeter Array Band 3 observations of CO(6−5), CO(7−6), and [C i ](2−1) in B14-65666 (“Big Three Dragons”), one of the brightest Lyman-break galaxies at z > 7 in the rest-frame ultraviolet continuum, far-infrared continuum, and emission lines of [O iii ] 88 μ m and [C ii ] 158 μ m. CO(6−5), CO(7−6), and [C i ](2−1), whose 3 σ upper limits on the luminosities are approximately 40 times fainter than the [C ii ] luminosity, are all not detected. The L [C II ] / L CO(6–5) and L [C II ] / L CO(7–6) ratios are higher than the typical ratios obtained in dusty star-forming galaxies or quasar host galaxies at similar redshifts, and they may suggest a lower gas density in the photodissociated region in B14-65666. By using the (1) [C ii ] luminosity, (2) dust mass-to-gas mass ratio, and (3) a dynamical mass estimate, we find that the molecular gas mass ( M mol ) is (0.05–11) × 10 10 M ⊙ . This value is consistent with the upper limit inferred from the nondetection of mid- J CO and [C i ](2−1). Despite the large uncertainty in M mol , we estimate a molecular gas-to-stellar mass ratio ( μ gas ) of 0.65–140 and a gas depletion time ( τ dep ) of 2.5–550 Myr; these values are broadly consistent with those of other high-redshift galaxies. B14-65666 could be an ancestor of a passive galaxy at z ≳ 4 if no gas is fueled from outside the galaxy.