Big Three Dragons: Molecular Gas in a Bright Lyman-Break Galaxy at $z=7.15$

Abstract

We report ALMA Band 3 observations of CO(6-5), CO(7-6), and [CI](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 [OIII] 88 $\hat{I}¼$m and [CII] 158 $\hat{I}¼$m. CO(6-5), CO(7-6), and [CI](2-1), whose $3\ddot{I}ƒ$ upper limits on the luminosities are approximately 40 times fainter than the [CII] luminosity, are all not detected. The $L_{[\mathrm{CII}]}$/$L_{\mathrm{CO}(6-5)}$ and $L_{[\mathrm{CII}]}$/$L_{\mathrm{CO}(7-6)}$ 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) [CII] luminosity, (2) dust mass-to-gas mass ratio, and (3) a dynamical mass estimate, we find that the molecular gas mass ($M_{\mathrm{mol}}$) is $(0.05-11)\times 10^{10}$ $M_{\odot }$. This value is consistent with the upper limit inferred from the nondetection of mid-$J$ CO and [CI](2-1). Despite the large uncertauinty in $M_{\mathrm{mol}}$, we estimate a molecular gas-to-stellar mass ratio ($\hat{I}{¼}_{\mathrm{gas}}$) of $0.65-140$ and a gas depletion time ($\ddot{I}{„}_{\mathrm{dep}}$) 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 $z\gtrsim 4$ if no gas is fueled from outside the galaxy.

15 pages, 6 figures, 4 tables. Accepted for publication in ApJ