We present JWST NIRCam imaging of B14-65666 ("Big Three Dragons"), a bright Lyman-break galaxy system ($M_\text{ UV}=-22.5$ 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 ($\beta_\text{ UV}=-2.2\pm0.1$). 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 \r{ A} 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 $\mathrm{ \mu m}$. 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, $\simeq1$ dex higher than the star-formation main sequence at the same redshift. The galaxy E is a dusty ($A_\text{ V}=1.2\pm0.1$ mag) starburst with a possible high dust temperature ($\ge63$-$68$ K). The galaxy W would have a low dust temperature ($\le27$-$33$ K) or patchy stellar-and-dust geometry, as suggested from the infrared excess (IRX) and $\beta_\text{ UV}$ diagram. The high optical-to-FIR [OIII] line ratio of the E-core shows its lower gas-phase metallicity ($\simeq0.2$-$0.4$ Z$_{ \odot}$) 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.
Categories: astro-ph.GA