Paper detail

Galaxy And Mass Assembly: Deconstructing Bimodality - I. Red ones and blue ones

We measure the mass functions for generically red and blue galaxies, using a z < 0.12 sample of log M* > 8.7 field galaxies from the Galaxy And Mass Assembly (GAMA) survey. Our motivation is that, as we show, the dominant uncertainty in existing measurements stems from how &#39;red&#39; and &#39;blue&#39; galaxies have been selected/defined. Accordingly, we model our data as two naturally overlapping populations, each with their own mass function and colour-mass relation, which enables us characterise the two populations without having to specify a priori which galaxies are &#39;red&#39; and &#39;blue&#39;. Our results then provide the means to derive objective operational definitions for the terms &#39;red&#39; and &#39;blue&#39;, which are based on the phenomenology of the colour-mass diagrams. Informed by this descriptive modelling, we show that: 1.) after accounting for dust, the stellar colours of &#39;blue&#39; galaxies do not depend strongly on mass; 2.) the tight, flat &#39;dead sequence&#39; does not extend much below log M* ~ 10.5; instead, 3.) the stellar colours of &#39;red&#39; galaxies vary rather strongly with mass, such that lower mass &#39;red&#39; galaxies have bluer stellar populations; 4.) below log M* ~ 9.3, the &#39;red&#39; population dissolves into obscurity, and it becomes problematic to talk about two distinct populations; as a consequence, 5.) it is hard to meaningfully constrain the shape, including the possibility of an upturn, of the &#39;red&#39; galaxy mass function below log M* ~ 9. Points 1-4 provide meaningful targets for models of galaxy formation and evolution to aim for.