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Lenticular galaxies are notoriously misclassified as elliptical galaxies and, as such, a (disc inclination) -dependent correction for dust is often not applied to the magnitudes of dusty lenticular galaxies. This results in overly red galaxy colours, impacting their distribution in the colour-magnitude diagram. It is revealed how this has led to an underpopulation of the `green valley' by hiding a `green mountain' of massive dust-rich lenticular galaxies - known to be built from gas-rich major mergers - within the `red sequence' of colour- (stellar mass) diagrams. Correcting for dust, a `green mountain' appears at M *, ₆₀₋10^11 M_, along with signs of an extension to lower masses producing a `green range' or `green ridge' on the green side of the `red sequence' and `blue cloud. ' The `red sequence' is shown to be comprised of two components: a red plateau defined by elliptical galaxies with a near-constant colour and by lower-mass dust-poor lenticular galaxies, which are mostly a primordial population but may include faded/transformed spiral galaxies. The quasi-triangular-shaped galaxy evolution sequence, previously called the `Triangal', is revealed in the galaxy colour- (stellar mass) diagram. It tracks the speciation of galaxies and their associated migration through the diagram. The connection of the `Triangal' to previous galaxy morphology sequences (Fork, Trident, Comb) is also shown herein. Finally, the colour- (black hole mass) diagram is revisited, revealing how the dust correction generates a blue-green sequence for the spiral and dust-rich lenticular galaxies that is offset from a green-red sequence defined by the dust-poor lenticular and elliptical galaxies.
Alister W. Graham (Wed,) studied this question.