"Big Three Dragons": a z = 7.15 Lyman BreakGalaxy Detected in [OIII] 88 μm, [CII] 158 μm, 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.