A Compact Protogroup at z ∼ 5: A Massive Galaxy Caught in Formation

Abstract We report the discovery of SCGG-z5 (nicknamed the Shirui Group), a compact galaxy protogroup at z = 4.97 in the MACS0416 field, identified from the SAPPHIRES Early Data Release. Six members are spectroscopically confirmed via H α emission, spanning 4.96 ≤ z spec ≤ 4.98 within a projected diameter of ∼16 pkpc. Spectral energy distribution fitting yields individual stellar masses 8.4 ≤ log ( M ∗ / M ⊙ ) ≤ 9.8 , a total group stellar mass of log ( M ∗ / M ⊙ ) = 10.07 ± 0.04 ; three of the six members lie above or on the star-forming main sequence at z ∼ 5, by up to 0.5 dex. Pixel-by-pixel analysis reveals diverse resolved radial star formation profiles; three members show declining specific star formation rate (sSFR) 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 σ v = 37 5 − 195 + 55 km s −1 . The projected mass estimator yields log ( M PM / M ⊙ ) ≈ 12.3 0 − 0.25 + 0.30 , consistent with a dark-matter-dominated group halo. EAGLE simulations of structurally similar groups predict full coalescence by z ∼ 3–4, with the merged remnant reaching log ( M ∗ / M ⊙ ) > 11 by z ∼ 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.