Elliptical (E) and ellicular (ES) galaxies in the M_bh-M_*,sph diagram, and a merger-driven explanation for the origin of ES galaxies, anti-truncated stellar discs in lenticular galaxies, and the Sersicification 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 M_bh-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 S\'ersic-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) Sersicification of light profiles. The ellicular and elliptical galaxy distribution in the $M_{\rm bh}$-$M_{\rm\star,sph}$ diagram is explored relative to major-merger-built lenticular galaxies and brightest cluster galaxies. The (super-)quadratic $M_{\rm bh}$-$M_{\rm\star}$ 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 $M_{\rm bh}/M_{\rm\star,sph}$ ratios, are presented.