A Compact Proto-group at z about 5: A Massive Galaxy Caught in Formation

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 at most z spec at most 4.98 within a projected diameter of about16 pkpc. Spectral energy distribution (SED) fitting yields individual stellar masses 8.4 at most (M*/M) at most 9.8, a total group stellar mass of (M*/M) = 10.07 plus or minus 0.04; three of the six members lie above or on the star-forming main sequence at z about 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+55-195 km s-1. The projected mass estimator yields (M PM/M) about 12.30+0.30-0.25, consistent with a dark-matter-dominated group halo. EAGLE simulations of structurally similar groups predict full coalescence by z about 3 -- 4, with the merged remnant reaching (M*/M) > 11 by z about 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.