Abstract We investigate the rest-frame optical size–stellar mass relation of galaxies at 0.5 M p , an increasing fraction of SFGs decouple from halo growth and become compact, likely associated with rapid bulge (and black hole) growth in M h ≳ 10 12 M ⊙ halos. These compact SFGs are promising progenitors of massive QGs, as evidenced by their similar masses, surface brightness profiles, and morphologies. Their high number densities can account for the observed buildup of massive QGs at z > 2, suggesting that the compaction pathway, rather than major mergers of extended SFGs, dominates the formation of high- z massive QGs.
Chen et al. (2026) studied this question.