The Galaxy End Sequence

TL;DR

This paper analyzes the distribution of galaxies in the SSFR versus stellar mass diagram, revealing a continuous, curved galaxy sequence rather than two distinct populations, and challenges the traditional dichotomy of galaxy types.

Contribution

It demonstrates that galaxies form a continuous, curved sequence in SSFR-stellar mass space, questioning the existence of a clear division between star-forming and passive galaxies.

Findings

Galaxies follow a strongly curved, extended GMS with a steep slope at high masses.

No clear morphological break exists along the galaxy sequence.

The red sequence results from color changes below a critical SSFR, not a distinct population.

Abstract

A common assumption is that galaxies fall in two distinct regions on a plot of specific star-formation rate (SSFR) versus galaxy stellar mass: a star-forming Galaxy Main Sequence (GMS) and a separate region of `passive' or `red and dead galaxies'. Starting from a volume-limited sample of nearby galaxies designed to contain most of the stellar mass in this volume, and thus being a fair representation of the Universe at the end of 12 billion years of galaxy evolution, we investigate the distribution of galaxies in this diagram today. We show that galaxies follow a strongly curved extended GMS with a steep negative slope at high galaxy stellar masses. There is a gradual change in the morphologies of the galaxies along this distribution, but there is no clear break between early-type and late-type galaxies. Examining the other evidence that there are two distinct populations, we argue that the `red sequence' is the result of the colours of galaxies changing very little below a critical value of the SSFR, rather than implying a distinct population of galaxies, and that Herschel observations, which show at least half of early-type galaxies contain a cool interstellar medium, also imply continuity between early-type and late-type galaxies. This picture of a unitary population of galaxies requires more gradual evolutionary processes than the rapid quenching processes needed to to explain two distinct populations. We challenge theorists to reproduce the properties of this `Galaxy End Sequence'.