Galaxy Morphology Evolution Stall

The morphology-density relation — the tendency for elliptical and lenticular galaxies to dominate dense environments while spiral disks are more common in the field — was established by z~1 and shows surprisingly little evolution since then, while ΛCDM merger simulations predict ongoing morphological transformation through continued merging activity at low redshift. The stall in morphological evolution implies that the processes driving morphological change — mergers, tidal stripping, ram pressure stripping — have been largely ineffective at transforming galaxy morphologies since z~1 despite continued halo growth. Successive Collision Theory explains this stall through the angular momentum inheritance framework. By z~1, the most efficient phase of angular momentum redistribution within galaxies — driven by the initial rapid post-collision accretion and the high gas fractions of early galaxies — had already determined the fundamental morphological class of each galaxy. Once a galaxy has established its angular momentum state and converted most of its gas into stars or expelled it through early feedback, its morphological class is largely locked in.

The low-redshift universe in SCT is dominated by gas-poor galaxies whose morphologies were set in the high-redshift phase of active angular momentum-driven evolution. Subsequent mergers do occur, but the low gas fractions of the merging galaxies mean that the resulting merger products quickly relax to configurations determined by the total angular momentum of the merger, which for equal-mass gas-poor mergers tends to produce ellipticals regardless of the progenitor disk morphology. This self-reinforcing morphological stability — where the gas fraction drives both the morphological class and the resistance to further morphological change — naturally produces the stall observed since z~1 without requiring that mergers have somehow stopped. The ongoing halos mergers predicted by ΛCDM do occur in SCT, but their morphological consequences are muted by the low gas fractions that result from the early star formation burst at collision nodes.