Pre-Virialized Assembly at Cosmic Dawn: The Dynamics and Extreme Ionization of Compact Group CGG-z7 at $z\sim 7.04$

Abstract

We report the discovery of CGG-z7, the most compact galaxy group at $z\gtrsim 7$ 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 $7.8\times 5.7$ kpc${}^2$, four of which are spectroscopically confirmed via [O III] and H${\hat{I}}^2$ emission. The group exhibits a low line-of-sight velocity dispersion ($\approx 93.7$ km s${}^{-1}$) relative to its substantial stellar mass ($M_{\ast }\approx 10^{9.8}{M}_{\odot }$), yielding a stellar-to-dynamical mass ratio of $M_{\ast }/M_{\mathrm{vir}}\approx 0.15$. 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${\hat{I}}^2$ ratio of $\sim 18$, 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.