Do Red Galaxies Form More Stars Than Blue Galaxies?

TL;DR

This paper proposes a new model where red galaxies can form stars efficiently without blue stellar populations, challenging the standard view that red galaxies are only old and quiescent.

Contribution

It introduces a novel scenario where red, high-metallicity galaxies can sustain star formation without blue stars, explaining observational puzzles about galaxy mass and IMF differences.

Findings

Red star-forming galaxies can form stars without blue stellar populations.

The model explains discrepancies in mass and IMF ratios between blue and red galaxies.

Predicts observable signatures to test the new galaxy formation scenario.

Abstract

A new model is proposed in which typical galaxies form most of their stellar mass in a phase with an intrinsically red stellar population. In the standard picture, galaxies with intrinsically red stellar populations are believed to have old stellar populations, so that only galaxies with blue stellar populations have significant star formation, and subsequent changes to the stellar population come from predominantly from aging and merging populations which have already formed. However, several observational puzzles have developed which are difficult to reconcile with this standard scenario. The most massive blue star-forming galaxies, presumed to be at the end of their stellar mass growth, are $\sim 1$ dex less massive, have a $\sim 1$ dex lower $M_*/M_{BH}$ ratio, and have a bottom-lighter IMF than local quiescent galaxies. Here, a new solution is proposed: at low temperature and high metallicity, galaxies can continue to form stars efficiently without being able to form O and B stars. These red star-forming galaxies would have many of the same properties of the population currently described as post-starburst galaxies, allowing a new interpretation of their origin. Finally, additional falsifiable observational predictions of this model are also discussed.