2025ApJ...981..135S

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 ($1.0{5}_{-0.09}^{+0.08}$ mag), tracing the presence of strong [O III]λλ4960,5008 emission. Atacama Large Millimeter/submillimeter Array archival data show that the bluer galaxy W is brighter in dust continua than the redder galaxy E, while the tails are bright in [O III] 88 μm. The UV/optical and submillimeter spectral energy distribution (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. Galaxy E is a dusty (A_V = 1.2 ± 0.1 mag) starburst with a possible high dust temperature (≥63-68 K). Galaxy W would have a low dust temperature (≤27-33 K) or patchy stellar-and-dust geometry, as suggested by the IR excess and β_UV diagram. The high optical-to-far-IR [O III] line ratio of the E-core shows its lower gas-phase metallicity (≃0.2-0.4 Z_⊙) than 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.