2020ApJ...892..145Z

In this paper we present our imaging observations on the CO (7-6) line and its underlying continuum emission of the young submillimeter galaxy LESS 073 at redshift 4.755, using the Atacama Large Millimeter/submillimeter Array. At the achieved resolution of ∼1′′ / . 2×0′′ / . 9 (8 x 6 kpc²), the CO (7-6) emission is largely unresolved (with a deconvolved size of 1′′ / . 1(±0′′ / . 5)×0′′ / . 9(±0′′ / . 8)), and the continuum emission is totally unresolved. The CO (7-6) line emission has an integrated flux of 0.86 ± 0.08 Jy km s^–1, and a line width of 343 ± 40 km s^–1. The continuum emission has a flux density of 0.51 mJy. By fitting the observed far-infrared spectral energy distribution (SED) of LESS 073 with a single-temperature modified blackbody function, we obtained a dust temperature T_dust=57.6±3.5 K, 60-100 µm flux density ratio f₆₀/f₁₀₀=0.86±0.08, and total infrared luminosity L_IR=(5.8±0.9)×10¹²L_☉. The SED-fit-based f₆₀/f₁₀₀ is consistent with those estimated from various line ratios as advocated by our earlier work, indicating that the proposed line-ratio-based method can be used to practically derive f₆₀/f₁₀₀ for high-z sources. The total molecular gas mass of LESS 073 is (3.3±1.7)×10¹⁰M_☉, and the inferred gas depletion time is about 43 Myr.