2025ApJ...995..150K

We present various properties of two bright extended Lyα objects, Himiko and COSMOS Redshift 7 (CR7), at z = 6.6 thoroughly investigated with JWST/NIRCam photometry, NIRSpec-Integral Field Unit spectroscopy, and Atacama Large Millimeter/submillimeter Array data, uncovering their physical origins. Himiko (CR7) shows at least five (four) clumps with small separations of 2.4-7.3 kpc and velocity offsets of Δv < 220 km s^–1 in the [O III]λλ4959, 5007 line maps, three of which exhibit stellar components with comparable stellar masses ranging in ${\mathrm{log}}({M}_{* }/{M}_{\odot })=8.4$-9.0 (8.3-8.8), indicative of major merger systems that are consistent with our numerical simulations. The [C II] 158 μm and Lyα lines are found in the middle of two clumps (the brightest clump) in Himiko (CR7), suggesting that the distribution of neutral gas does not always coincide with that of ionized gas or stars in merging processes. We find that some of the clumps have broad [O III] components (250-400 km s^–1) in Himiko and CR7, likely tracing outflow and tidal features, while the central clump in Himiko presents a broad Hα (∼1000 km s^–1) line explained by an active galactic nucleus with a low-mass black hole of M_BH = 10^6.6M_⊙, which contribute to the extended and bright nature of Himiko and CR7. We find low metallicities of $12+{\mathrm{log}}({\rm{O}}/{\rm{H}})=$7.9-8.1 in Himiko and CR7 based on auroral [O III]λ4363 and strong lines that are consistent with no 1 mm continuum detection corresponding to the dust-mass limits of M_dust ≲ 9 × 10⁶M_⊙. Himiko and CR7 are metal- and dust-poor blue merger systems with stellar and dust masses ≳2 orders of magnitude smaller than the massive dust-rich merger systems represented by submillimeter galaxies.