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

TL;DR

This paper reports the discovery of a highly compact, pre-virialized galaxy group at z~7, exhibiting extreme ionization, low metallicity, and signs of a major merger, providing insights into early galaxy assembly.

Contribution

It presents the first detailed analysis of a pre-virialized galaxy group at cosmic dawn, revealing its dynamical state, extreme ionization, and potential AGN activity, advancing understanding of early galaxy formation.

Findings

Most compact galaxy group at z~7 identified to date.

Group exhibits low velocity dispersion and high stellar-to-dynamical mass ratio.

Central galaxy shows an extremely high [O III]/Hβ ratio of ~18, suggesting obscured AGN.

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$\beta$ 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$\beta$ 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.