"Big Three Dragons": a z = 7.15 Lyman BreakGalaxy Detected in [OIII] 88 $\hat{I}¼$m, [CII] 158 $\hat{I}¼$m, and Dust Continuum with ALMA

Abstract

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, $M_{\mathrm{UV}}\approx -22.4$, and has been spectroscopically identified in Ly$\hat{I}\pm$ with a small rest-frame equivalent width of $\approx 4$ Ã . 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 $\hat{I}¼$m, [CII] 158 $\hat{I}¼$m, and their underlying dust continuum emission. In the whole system of B14-65666, the [OIII] and [CII] lines have consistent redshifts of $7.1520\pm 0.0003$, and the [OIII] luminosity, $(34.4\pm 4.1)\times 10^8{L}_{\odot }$, is about three times higher than the [CII] luminosity, $(11.0\pm 1.4)\times 10^8{L}_{\odot }$. With our two continuum flux densities, the dust temperature is constrained to be $T_{\mathrm{d}}\approx 50-60$ K under the assumption of the dust emissivity index of ${\hat{I}}_{\mathrm{d}}^2=2.0-1.5$, leading to a large total infrared luminosity of $L_{\mathrm{TIR}}\approx 1\times 10^{12}{L}_{\odot }$. 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 $\approx 200$ 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$=260_{-57}^{+119}$ 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.

24 pages, 13 figures, 9 tables. Accepted for publication in PASJ. In addition to the dust continuum detection in ALMA Band 6 reported in the original manuscript, we newly report the dust continuum detection in ALMA Band 8 based on our new ALMA Cycle 5 observations