Galaxy And Mass Assembly (GAMA): deconstructing bimodality - I. Red ones and blue ones
Date
2015
Authors
Taylor, Edward N
Hopkins, Andrew M.
Baldry, Ivan K
Bland-Hawthorn, Jonathan
Brown, Michael J I
Colless, Matthew
Driver, Simon
Norberg, P
Robotham, A. S. G.
Alpaslan, M
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Volume Title
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Blackwell Publishing Ltd
Abstract
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 ‘red’ and ‘blue’ 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 characterize the two populations without having to specify a priori
which galaxies are ‘red’ and ‘blue’. Our results then provide the means to derive objective
operational definitions for the terms ‘red’ and ‘blue’, 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 ‘blue’ galaxies do not depend strongly on mass;
(2) the tight, flat ‘dead sequence’ does not extend much below log M∗ ∼ 10.5; instead, (3)
the stellar colours of ‘red’ galaxies vary rather strongly with mass, such that lower mass
‘red’ galaxies have bluer stellar populations; (4) below log M∗ ∼ 9.3, the ‘red’ 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 existence of an
upturn, of the ‘red’ galaxy mass function below log M∗ ∼ 9.3. Points 1–4 provide meaningful
targets for models of galaxy formation and evolution to aim for.
Key words: galaxies: evolution – galaxies: formation – galaxies: fundamental parameters –
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Monthly Notices of the Royal Astronomical Society
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Journal article
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Open Access
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