Comprehensive JWST+ALMA Study on the Extended Ly$\alpha$ Emitters, Himiko and CR7 at $z\sim 7$: Blue Major Merger Systems in Stark Contrast to Submillimeter Galaxies

Abstract

We present various properties of two bright extended Ly$\alpha$ objects, Himiko and CR7, at $z=6.6$ thoroughly investigated with JWST/NIRCam photometry, NIRSpec-IFU spectroscopy, and ALMA 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 $\Delta v<220\mathrm{km}{\mathrm{s}}^{-1}$ in the [OIII]$\lambda \lambda 4959,5007$ line maps, three of which exhibit stellar components with comparable stellar masses ranging in $\log (M_{\ast }/M_{\odot })=8.4$--$9.0$ ($8.3$--$8.8$), indicative of major merger systems that are consistent with our numerical simulations. The [CII]158$\mu$m and Ly$\alpha$ 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 [OIII] components (250--400$\mathrm{km}{\mathrm{s}}^{-1}$) in Himiko and CR7, likely tracing outflow and tidal features, while the central clump in Himiko presents a broad H$\alpha$ ($\sim 1000\mathrm{km}{\mathrm{s}}^{-1}$) line explained by an AGN with a low mass black hole of $M_{\mathrm{BH}}=10^{6.6}{M}_{\odot }$, which contribute to the extended and bright nature of Himiko and CR7. We find low metallicities of $12+\log (O/H)=$7.9--8.1 in Himiko and CR7 based on auroral [OIII]$\lambda 4363$ and strong lines that are consistent with no 1-mm continuum detection corresponding to the dust mass limits of $M_{\mathrm{dust}}\lesssim 9\times 10^6{M}_{\odot }$. Himiko and CR7 are metal- and dust-poor blue merger systems with stellar and dust masses $\gtrsim 2$ orders of magnitude smaller than the massive dust-rich merger systems represented by submillimeter galaxies.

37 pages, 22 figures, 5 tables, Accepted for publication in The Astrophysical Journal