The Magic Scale of Galaxies

This talk will address the preferred mass and time for galaxy formation, in dark-matter haloes similar to the one that hosts the Milky way but when the Universe was only a few Gigayears old. It is proposed that this magic scale arises from the interplay between supernova explosions in low-mass galaxies and feedback from super-massive black holes in massive galaxies, associated with shock heating of the circum-galactic gas which suppresses cold gas supply for star formation in massive galaxies. Cosmological simulations reveal that the same mechanisms are responsible for a robust sequence of events in the history of typical galaxies, were galaxies undergo a dramatic gaseous compaction, sometimes caused by galaxy mergers, into a compact star-forming phase, termed “blue nugget”. This process triggers inside-out quenching of star formation, which is maintained by a hot massive halo aided by black-hole feedback, leading to todays passive elliptical galaxies. The blue-nugget phase is responsible for drastic transitions in the main galaxy structural, kinematic and compositional properties. In particular, the growth of the black hole in the galaxy center, first suppressed by supernova feedback when 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 halo. These events all occur near the same characteristic halo mass, giving rise to the highest efficiency of galaxy formation and black-hole growth at this magic mass and time.