ALMA Imaging of the CO(7-6) Line Emission in the Submillimeter Galaxy
  LESS 073 at redshift 4.755$^\star$
  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 (ALMA).
At the achieved resolution of
$\sim$$1^{\prime\prime}.2\times0^{\prime\prime}.9$ ($8\times6$~kpc$^2$), the
CO(7-6) emission is largely unresolved (with a deconvolved size of
$1^{\prime\prime}.1(\pm0^{\prime\prime}.5) \times
0^{\prime\prime}.9(\pm0^{\prime\prime}.8)$), and the continuum emission is
totally unresolved. The CO(7-6) line emission has an integrated flux of
$0.86\pm0.08$~Jy km/s, and a line width of $343\pm40$ km/s. The continuum
emission has a flux density of 0.51 mJy. By fitting the observed far-infrared
(FIR) spectral energy distribution of LESS 073 with a single-temperature
modified blackbody function, we obtained a dust temperature $T_{\rm
dust}=57.6\pm3.5$ K, 60-to-100 $\mu$m flux density ratio
$f_{60}/f_{100}=0.86\pm0.08$, and total infrared luminosity $L_{\rm
IR}=(5.8\pm0.9) \times 10^{12}~L_\odot$. The SED-fit-based $f_{60}/f_{100}$ is
consistent with those estimated from various line ratios as advocated by our
earlier work, indicating that those proposed line-ratio-based method can be
used to practically derive $f_{60}/f_{100}$ for high-$z$ sources. The total
molecular gas mass of LESS 073 is $(3.3\pm1.7) \times10^{10}~M_\odot$, and the
inferred gas depletion time is about 43 Myr.
