The Critical Role of Compaction into a Blue Nugget in Galaxy Evolution
Most high-z galaxies, when their stellar mass is ~10^10 Msun, undergo an event of dramatic consequences. Cosmological simulations reveal a gaseous shrinkage to a compact star-forming phase, a “blue nugget” (BN), followed by central quenching of star formation to a compact passive stellar core, a “red nugget”, once in a halo above a critical mass. These resemble the nuggets observed mostly at z~2-3, which seed today’s early-type galaxies. The compaction is associated with angular-momentum loss, triggered, e.g., by major or minor mergers, counter-rotating streams, or violent disk instability. The BN phase marks drastic transitions in the galaxy structural, compositional, kinematic and other physical properties. The compaction to a BN triggers inside-out quenching which is maintained by a hot massive halo aided by AGN feedback. These transitions are, for example, from star forming to quenched, from diffuse to compact with an extended disc and envelope, from prolate to oblate, from pressure to rotation support, from low to high metallicity, and from supernova to AGN feedback. The black-hole growth, first suppressed by supernova feedback below the critical mass, is boosted by the compaction event and keeps growing once the halo is massive enough to lock the supernova ejecta by its deep potential well and the hot CGM.
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