Authors: A. Graham
The `Triangal' galaxy evolution schema is utilised to assess whether the Tuning Fork (bar strength) or the van~den~Bergh Trident and ATLAS3D Comb (spiral strength) provide greater evolutionary insight. A new catalogue of quantitative bar strengths (the bar-to-total luminosity ratio, P), refined galaxy morphologies, and dust bin classifications is presented for 138 galaxies with multi-component decompositions and directly measured black hole masses, Mbh. By placing these galaxies within the Mbh–(M*,sph, M*,gal) parameter space, an evolutionary reference frame reflecting integrated growth is established.
Galaxies with varying bar strengths (including double bars) do not occupy preferred locations in this space, highlighting that bars are products of secular evolution—and can be transient or recurrent phenomena—that track neither hierarchical mass assembly nor galaxy speciation. In contrast, three physically distinct formation channels for S0/a galaxies are identified: (primeval S0)-to-S transitions; faded spirals; and, most commonly, wet-major-merger-built dust-rich S0 galaxies (located on the `green mountain'). The few galaxies with particularly strong spirals are found on the right-hand side of the spiral galaxy distribution in the Mbh–M*,gal diagram.
Beyond the Fork and Trident, a `Dust Attrition/Retention' sequence sees S0 (and compact massive ES,b) galaxies with dusty nuclear discs reside between the dust-poor and dust-rich S0 galaxies. Additionally, the Mbh–M*,sph diagram reveals a `Disc Down-sizing' sequence. The presence of dusty nuclear discs in E galaxies, potentially formed through `damp' mergers, bridges the ES,e (ellicular) galaxies with intermediate-scale stellar discs and the gas-poor pure E galaxies on the red sequence.