A Compact Proto-group at $z\sim 5$: A Massive Galaxy Caught in Formation

Abstract

We report the discovery of SCGG-z5, a compact galaxy proto-group at $z=4.97$ in the MACS0416 field, identified from the SAPPHIRES Early Data Release. Six members are spectroscopically confirmed via H$\alpha$ emission, spanning $4.96\le z_{\mathrm{spec}}\le 4.98$ within a projected diameter of $\sim 16$ pkpc. Spectral energy distribution (SED) fitting yields individual stellar masses $8.4\le \log (M_{\ast }/M_{\odot })\le 9.8$, a total group stellar mass of $\log (M_{\ast }/M_{\odot })=10.07\pm 0.04$; three of the six members lie above or on the star-forming main sequence at $z\sim 5$, by up to $0.5$ dex. Pixel-by-pixel analysis reveals diverse resolved radial star-formation profiles: three members show declining specific SFR radial profiles and outward-rising stellar age gradients, consistent with inside-out stellar mass growth, while the most massive member shows a tentative inverted sSFR profile suggestive of reduced central star formation. The line-of-sight velocity dispersion over all six members is ${\sigma }_v=375_{-195}^{+55}$ km s${}^{-1}$. The projected mass estimator yields $\log (M_{\mathrm{PM}}/M_{\odot })\approx 12.30_{-0.25}^{+0.30}$, consistent with a dark-matter-dominated group halo. EAGLE simulations of structurally similar groups predict full coalescence by $z\sim 3$--$4$, with the merged remnant reaching $\log (M_{\ast }/M_{\odot })>11$ by $z\sim 1$, consistent with SCGG-z5 representing a rare pre-coalescence phase of early massive galaxy formation, possibly tracing the assembly of a future brightest group or cluster galaxy.

Accepted for publication in ApJL