RIOJA. Complex Dusty Starbursts in a Major Merger B14-65666 at z=7.15

We present JWST NIRCam imaging of B14-65666 ("Big Three Dragons"), a bright Lyman-break galaxy system (M_UV=-22.5 mag) at z=7.15. The high angular resolution of NIRCam reveals the complex morphology of two galaxy components: galaxy E has a compact core (E-core), surrounded by diffuse, extended, rest-frame optical emission, which is likely to be tidal tails; and galaxy W has a clumpy and elongated morphology with a blue UV slope (β_UV=-2.2±0.1). The flux excess, F356W-F444W, peaks at the E-core (1.05^+0.08_-0.09 mag), tracing the presence of strong [OIII] 4960,5008 Å emission. ALMA archival data show that the bluer galaxy W is brighter in dust continua than the redder galaxy E, while the tails are bright in [OIII] 88 μ m. The UV/optical and sub-mm SED fitting confirms that B14-65666 is a major merger in a starburst phase as derived from the stellar mass ratio (3:1 to 2:1) and the star-formation rate, ≃1 dex higher than the star-formation main sequence at the same redshift. The galaxy E is a dusty (A_V=1.2±0.1 mag) starburst with a possible high dust temperature (≥63-68 K). The galaxy W would have a low dust temperature (≤27-33 K) or patchy stellar-and-dust geometry, as suggested from the infrared excess (IRX) and β_UV diagram. The high optical-to-FIR [OIII] line ratio of the E-core shows its lower gas-phase metallicity (≃0.2 Z_⊙) than the galaxy W. These results agree with a scenario where major mergers disturb morphology and induce nuclear dusty starbursts triggered by less-enriched inflows. B14-65666 shows a picture of complex stellar buildup processes during major mergers in the epoch of reionization.