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The causes of the red sequence, the blue cloud, the green valley, and the green mountain

Eales, Stephen A.; Baes, Maarten; Bourne, Nathan; Bremer, Malcolm; Brown, Michael J.I.; Clark, Christopher; Clements, David; de Vis, Pieter; Driver, Simon; Dunne, Loretta; Dye, Simon; Furlanetto, Cristina; Holwerda, Benne; Ivison, R J; Kelvin, L S; Lara-Lopez, Maritza; Leeuw, Lerothodi; Loveday, Jon; Maddox, Steve; Micha?owski, Micha? J.; Phillipps, Steven; Robotham, Aaron; Smith, Dan; Smith, Matthew; Valiante, Elisabetta; van der Werf, Paul; Wright, Angus


Stephen A. Eales

Maarten Baes

Nathan Bourne

Malcolm Bremer

Michael J.I. Brown

Christopher Clark

David Clements

Pieter de Vis

Simon Driver

Loretta Dunne

Associate Professor

Cristina Furlanetto

Benne Holwerda

R J Ivison

L S Kelvin

Maritza Lara-Lopez

Lerothodi Leeuw

Jon Loveday

Steve Maddox

Micha? J. Micha?owski

Steven Phillipps

Aaron Robotham

Dan Smith

Matthew Smith

Elisabetta Valiante

Paul van der Werf

Angus Wright


The galaxies found in optical surveys fall in two distinct regions of a diagram of optical colour versus absolute magnitude: the red sequence and the blue cloud, with the green valley in between. We show that the galaxies found in a submillimetre survey have almost the opposite distribution in this diagram, forming a ‘green mountain’. We show that these distinctive distributions follow naturally from a single, continuous, curved Galaxy Sequence in a diagram of specific star formation rate versus stellar mass, without there being the need for a separate star-forming galaxy main sequence and region of passive galaxies. The cause of the red sequence and the blue cloud is the geometric mapping between stellar mass/specific star formation rate and absolute magnitude/colour, which distorts a continuous Galaxy Sequence in the diagram of intrinsic properties into a bimodal distribution in the diagram of observed properties. The cause of the green mountain is Malmquist bias in the submillimetre waveband, with submillimetre surveys tending to select galaxies on the curve of the Galaxy Sequence, which have the highest ratios of submillimetre-to-optical luminosity. This effect, working in reverse, causes galaxies on the curve of the Galaxy Sequence to be underrepresented in optical samples, deepening the green valley. The green valley is therefore not evidence (1) for there being two distinct populations of galaxies, (2) for galaxies in this region evolving more quickly than galaxies in the blue cloud and the red sequence, and (3) for rapid- quenching processes in the galaxy population.


Eales, S. A., Baes, M., Bourne, N., Bremer, M., Brown, M. J., Clark, C., …Wright, A. (2018). The causes of the red sequence, the blue cloud, the green valley, and the green mountain. Monthly Notices of the Royal Astronomical Society, 481(1), 1183-1194.

Journal Article Type Article
Acceptance Date Aug 10, 2018
Online Publication Date Aug 23, 2018
Publication Date Nov 30, 2018
Deposit Date Nov 12, 2018
Publicly Available Date Nov 12, 2018
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Oxford University Press
Peer Reviewed Peer Reviewed
Volume 481
Issue 1
Pages 1183-1194
Keywords galaxies: evolution – galaxies: general
Public URL
Publisher URL
Additional Information This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2018 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.


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