Pre-Virialized Assembly at Cosmic Dawn: The Dynamics and Extreme Ionization of Compact Group CGG-z7 at $z\sim7.04$

Abstract

We report the discovery of CGG-z7, the most compact galaxy group at $z\gtrsim7$ identified to the north of the GOODS-North field, observed by the JWST POPPIES program. The system consists of at least six members within a projected size of $7.8\times5.7$ kpc$^2$, four of which are spectroscopically confirmed via [O III] and H$β$ emission. The group exhibits a low line-of-sight velocity dispersion ($\approx93.7$ km s$^{-1}$) relative to its substantial stellar mass ($M_* \approx 10^{9.8} M_{\odot}$), yielding a stellar-to-dynamical mass ratio of $M_*/M_{\mathrm{vir}} \approx 0.15$. This ratio, exceeding typical values for virialized halos by a factor of $3$, indicates that the system is highly likely not in dynamical equilibrium. Instead, we interpret CGG-z7 as a pre-virialized structure, likely a major merger caught near apocenter -- capturing the rapid, chaotic formation of a massive "Red Nugget". Spectroscopic analysis reveals extreme ionization conditions and low metallicity across the group. In particular, the central galaxy reaches an extraordinarily high [O III]/H$β$ ratio of $\sim18$, which is likely indicative of an obscured AGN. CGG-z7 thus serves as a unique laboratory for the physics of pre-virialized galaxy assembly, bridging the gap between turbulent high-$z$ assembly and the quiescent galaxies seen at cosmic noon.

Figures (5)

• Figure 1: Multiband image of CGG-z7 (zscale). The black bar in the bottom left marks a $0.5"$ scale (2.59 kpc). The black circle in the bottom right denotes the FWHM of PSFs.
• Figure 2: NIRCam F444W grism spectra of G0--G3 obtained through Cycle 3 PID 5398 (POPPIES). The F444W images here are rotated the observed PA of $218^\circ$ and flipped to match the dispersion direction of Module B of the JWST/NIRCam. For each source, clear detections of the H$\beta$ and [O iii] doublet emission lines are visible. The 3$\sigma$ and 5$\sigma$ source contours extracted from the direct image are overlaid onto the 2D spectra. These contours have been translated to the spectral plane using the measured spectroscopic redshift and are centered at the expected location of the [O iii]$\lambda$5007 line.
• Figure 3: Multi-band morphological analysis of CGG-z7. The structure of the compact galaxy group CGG-z7 is modeled using pysersic with a multi-band joint fitting technique. Each row displays the results for one bandpass (F444W, F200W, and F115W, from top to bottom). Columns show, from left to right: the science image, the best-fit model, and the residual ($\chi=(\mathrm{data}-\mathrm{model})/\sigma_{\mathrm{data}}$) map. The inset in each residual panel shows the normalized histogram of $\chi$.
• Figure 4: SEDs of G0--G3 in CGG-z7. The observed photometry means the model fluxes derived by pysersic. The best-fit models (blue lines) are obtained by jointly fitting the photometry and the F444W grism spectra; the segments highlighted in green indicate the wavelength ranges constrained by the grism data.
• Figure 5: F444W grism spectra (black) and the corresponding model spectra from pysersic (colored lines). The green, red, and blue lines show the 1D model spectra for the regions between the pairs of horizontal dashed lines of the same color.