Repainting the colour-mass diagrams by unearthing the green mountain: dust-rich S0 galaxies in the colour-(galaxy stellar mass) diagram, and the colour-(black hole mass) relations for dust-poor versus dust-rich galaxies

TL;DR

This paper uncovers a hidden 'green mountain' of dust-rich lenticular galaxies in the colour-mass diagram by correcting for dust effects, revealing new galaxy evolutionary sequences and relations with black hole mass.

Contribution

It introduces a dust correction method for galaxy classification, revealing a previously hidden population of dust-rich lenticular galaxies and clarifying galaxy evolution sequences.

Findings

Discovery of a 'green mountain' of dust-rich lenticular galaxies at high stellar mass.

Identification of a two-component 'red sequence' comprising ellipticals and dust-poor lenticulars.

Revealed distinct colour-black hole mass relations for dust-rich and dust-poor galaxies.

Abstract

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 $M_{\rm *,gal}\sim10^{11}$ M$_\odot$, 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.