Early-type galaxy speciation: elliptical (E) and ellicular (ES) galaxies in the Mbh–M*,sph diagram, and a merger-driven explanation for the origin of ES galaxies, antitruncated stellar discs in lenticular (S0) galaxies, and the Sérsicification of E galaxy light profiles

Abstract

In a recent series of papers, supermassive black holes were used to discern pathways in galaxy evolution. By considering the black holes’ coupling with their host galaxy’s bulge/spheroid, the progression of mass within each component has shed light on the chronological sequence of galaxy speciation. Offsets between the galaxy-morphology-dependent Mbh-M⋆, sph scaling relations trace a pattern of ‘punctuated equilibrium’ arising from merger-driven transitions between galaxy types, such as from spirals to dust-rich lenticulars and further to ‘ellicular’ and elliptical galaxies. This study delves deeper into the distinction between the ellicular galaxies — characterised by their intermediate-scale discs — and elliptical galaxies. Along the way, it is shown how some anti-truncated large-scale discs in lenticular galaxies can arise from the coexistence of a steep intermediate-scale disc and a relatively shallow large-scale disc. This observation undermines application of the popular exponential-disc plus Sérsic-bulge model for lenticular galaxies and suggests some past bulge mass measurements have been overestimated. Furthermore, it is discussed how merger-driven disc-heating and blending likely leads to the spheroidalisation of discs and the conglomeration of multiple discs leads to the (high-n) Sérsicification of light profiles. The ellicular and elliptical galaxy distribution in the Mbh-M⋆, sph diagram is explored relative to major-merger-built lenticular galaxies and brightest cluster galaxies. The (super-)quadratic Mbh-M⋆ relations, presented herein, for merger-built systems should aid studies of massive black hole collisions and the gravitational wave background. Finally, connections to dwarf compact elliptical and ultra-compact dwarf galaxies, with their 100-1000 times higher Mbh/M⋆, sph ratios, are presented.