"Big Three Dragons": a z = 7.15 Lyman BreakGalaxy Detected in [OIII] 88
  $\mu$m, [CII] 158 $\mu$m, and Dust Continuum with ALMA
  We present new ALMA observations and physical properties of a Lyman Break
Galaxy at z=7.15. Our target, B14-65666, has a bright ultra-violet (UV)
absolute magnitude, $M_{\rm UV}\approx-22.4$, and has been spectroscopically
identified in Ly$\alpha$ with a small rest-frame equivalent width of $\approx4$
\AA. Previous HST image has shown that the target is comprised of two spatially
separated clumps in the rest-frame UV. With ALMA, we have newly detected
spatially resolved [OIII] 88 $\mu$m, [CII] 158 $\mu$m, and their underlying
dust continuum emission. In the whole system of B14-65666, the [OIII] and [CII]
lines have consistent redshifts of $7.1520\pm0.0003$, and the [OIII]
luminosity, $(34.4\pm4.1)\times10^{8}L_{\rm \odot}$, is about three times
higher than the [CII] luminosity, $(11.0\pm1.4)\times10^{8}L_{\rm \odot}$. With
our two continuum flux densities, the dust temperature is constrained to be
$T_{\rm d}\approx50-60$ K under the assumption of the dust emissivity index of
$\beta_{\rm d}=2.0-1.5$, leading to a large total infrared luminosity of
$L_{\rm TIR}\approx1\times10^{12}L_{\rm \odot}$. Owing to our high spatial
resolution data, we show that the [OIII] and [CII] emission can be spatially
decomposed into two clumps associated with the two rest-frame UV clumps whose
spectra are kinematically separated by $\approx200$ km s$^{-1}$. We also find
these two clumps have comparable UV, infrared, [OIII], and [CII] luminosities.
Based on these results, we argue that B14-65666 is a starburst galaxy induced
by a major-merger. The merger interpretation is also supported by the large
specific star-formation rate (defined as the star-formation rate per unit
stellar mass), sSFR$=260^{+119}_{-57}$ Gyr$^{-1}$, inferred from our SED
fitting. Probably, a strong UV radiation field caused by intense star formation
contributes to its high dust temperature and the [OIII]-to-[CII] luminosity
ratio.
