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

TL;DR

Using JWST NIRSpec IFU, NIRCam photometry, and archival ALMA Band 6 data, the study presents a detailed, multiwavelength view of Himiko and CR7 at $z\sim 6.6$. It finds multiple stellar clumps with small velocity offsets ($\Delta v<220$ km s$^{-1}$) and central AGN activity in Himiko, supported by broad emission lines and a low-mass black hole of $M_\mathrm{BH}=10^{6.6}\,M_\odot$. The metallicities are low ($12+\log(\mathrm{O/H})\approx7.9$–$8.1$) and dust masses $M_\mathrm{dust}\lesssim 9\times 10^{6}\,M_\odot$, with [CII]158 μm and Lyα emission distributed between clumps, indicating non-coincident gas–star distributions in mergers. These systems are blue, metal-poor major mergers at $z\sim 6.6$ that contrast with dusty submillimeter galaxies (SMGs), offering constraints on early merger assembly and feedback at $z\sim 7$. The results showcase the power of combining JWST spectroscopy with ALMA to resolve the internal structure and evolution of the first massive galaxies.

Abstract

We present various properties of two bright extended Ly$α$ 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 $Δv<220~\mathrm{km~s^{-1}}$ in the [OIII]$λ\lambda4959,5007$ line maps, three of which exhibit stellar components with comparable stellar masses ranging in $\log{(M_*/M_\odot)}=8.4$--$9.0$ ($8.3$--$8.8$), indicative of major merger systems that are consistent with our numerical simulations. The [CII]158$μ$m and Ly$α$ 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$~\mathrm{km~s^{-1}}$) in Himiko and CR7, likely tracing outflow and tidal features, while the central clump in Himiko presents a broad H$α$ ($\sim1000~\mathrm{km~s^{-1}}$) 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(\mathrm{O/H})=$7.9--8.1 in Himiko and CR7 based on auroral [OIII]$\lambda4363$ 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 $\gtrsim2$ orders of magnitude smaller than the massive dust-rich merger systems represented by submillimeter galaxies.

Figures (6)

• Figure 1: NIRCam images of Himiko and CR7 ($5\hbox{$^{\prime\prime}$} \times 5\hbox{$^{\prime\prime}$}$). The F090W, F115W, F150W, F200W, F277W, F356W, F410M, and F444W (F115W, F150W, F277W, and F444W) images are shown for Himiko (CR7). The scale bar is $1\hbox{$^{\prime\prime}$}$ (5.5 kpc at $z=6.6$). We can see the multiple components in both Himiko and CR7. The circle in the bottom-right corner represents the size of the PSF FWHM. In the F277W image of CR7, we show a possible chance-projected object, indicated by the gray arrow (see Section \ref{['subsec:morphology']}).
• Figure 2: Left: JWST NIRSpec IFU G395H images collapsed around [O iii]$\lambda5007$ emission line with a range of 750 km s$^{-1}$ for Himiko (top) and CR7 (bottom). Black squares show the selected region for the following moment map analysis (Figure \ref{['fig:moment-maps']}). White circles show the apertures for the extracted spectrum. North is up, and east is to the left side. We can find the five and four clumps for Himiko and CR7, respectively. Right: [O iii]$\lambda4959, 5007$ emission lines from the individual apertures taken with NIRSpec IFU G395H, for Himiko (top) and CR7 (bottom). The black histograms show the spectrum, and the gray shadings show 1$\sigma$ uncertainties. The red and gray lines show the total Gaussian fitting results and individual Gaussian components, respectively.
• Figure 3: Multiwavelength images of Himiko (left) and CR7 (right). The origins of the relative coordinates of Himiko and CR7 correspond to (R.A., Decl.) = (2$^\mathrm{h}$17$^\mathrm{m}$57.571$^\mathrm{s}$, $-05$° $08\hbox{$^\prime$}44\hbox{$^{\prime\prime}$}.816$) and (10$^\mathrm{h}$00$^\mathrm{m}$58.007$^\mathrm{s}$, $01$° $48\hbox{$^\prime$}14\hbox{$^{\prime\prime}$}.930$), respectively (J2000.0). The gray images show the NIRCam F115W, corresponding to rest-frame UV (beam size $\sim0\hbox{$.\!\!^{\prime\prime}$}07$). The green contours show the H$\alpha$ emission (2$\sigma$, 3$\sigma$, 5$\sigma$, 10$\sigma$, 20$\sigma$, 60$\sigma$) taken with the NIRSpec IFU PRISM (beam size $\sim0\hbox{$.\!\!^{\prime\prime}$}2$). The red contours show the [C ii] 158$\hbox{$\mu$m}$ emission (2$\sigma$, 3$\sigma$, 4$\sigma$, 5$\sigma$) obtained by the ALMA observations (beam size $\sim0\hbox{$.\!\!^{\prime\prime}$}9$). The red crosses indicate the [C ii] 158$\hbox{$\mu$m}$ peak position, where the bar scales correspond to their positional uncertainty. The blue contours show the Ly$\alpha$ emission (2$\sigma$, 3$\sigma$, 5$\sigma$, 7.5$\sigma$, 10$\sigma$, 15$\sigma$) taken with NB921 (beam size $\sim0\hbox{$.\!\!^{\prime\prime}$}8$). The scale bars show 3 kpc at $z=6.6$. In the figures, we do not conduct a PSF matching.
• Figure 4: Distributions of Ly$\alpha$ and H$\alpha$ emission from Himiko (left) and CR7 (right). The background image represents the Ly$\alpha$ emission observed with NIRSpec IFU PRISM. Only spaxels with S/N $> 3$ are shown. The blue and gray contours indicate the Ly$\alpha$ emission from NB921 and the H$\alpha$ emission from NIRSpec IFU PRISM, respectively. The contour levels and the relative coordinates are the same for Figure \ref{['fig:jwst-alma-subaru-image']}. The scale bars correspond to 3 kpc at $z=6.6$. A PSF matching is not applied in the figures.
• Figure 5: Integrated spectrum of Himiko taken with NIRSpec IFU PRISM ($R\sim100$; left) and G395H ($R\sim2700$; right). From top to bottom, the spectra of Himiko-A to Himiko-E are shown. The black lines show the spectra, and the blue shaded areas show their $1\sigma$ uncertainties. In the PRISM spectra of Himiko-A, there is a clear Ly$\alpha$ emission line.