Resequencing the Hubble sequence and the quadratic (black hole mass)–(spheroid stellar mass) relation for elliptical galaxies

Abstract

ABSTRACT One of the most protracted problems in astronomy has been understanding the evolution of galaxy morphology. Much discussion has surrounded how lenticular galaxies may form a bridging population between elliptical and spiral galaxies. However, with recourse to a galaxy’s central black hole mass, accretion-built spiral galaxies have emerged as the bridging population between low-mass lenticular galaxies and the dusty merger-built lenticular galaxies contiguous with elliptical galaxies and ‘brightest cluster galaxies’ in the black hole/galaxy mass diagram. Spiral galaxies, including the Milky Way, appear built from gas accretion and minor mergers onto what were initially lenticular galaxies. These connections are expressed as a new morphology sequence, dubbed the ‘Triangal’, which subsumes elements of the Hubble sequence and the van den Bergh trident and reveals the bridging nature of the often overlooked ellicular galaxies. Furthermore, a quadratic black hole/galaxy mass relation is found to describe ordinary elliptical galaxies. The relation is roughly parallel to the quadratic-like relations observed for the central spheroidal component of spiral galaxies, dust-rich lenticular galaxies, and old dust-poor lenticular galaxies. The brightest cluster galaxies are offset according to expectations from an additional major merger. The findings have implications for feedback from active galactic nuclei, mapping morphology into simulations, and predicting gravitational wave signals from colliding supermassive black holes. A new galaxy speciation model is presented. It disfavours the ‘monolithic collapse’ scenario for spiral, dusty lenticular, and elliptical galaxies. It reveals substantial orbital angular momentum in the Universe’s first galaxies and unites dwarf and ordinary ‘early-type’ galaxies.