The green valley is a red herring : Galaxy Zoo reveals two evolutionary pathways towards quenching of star formation in early- and late-type galaxies

Schawinski, Kevin and Urry, C. Megan and Simmons, Brooke D. and Fortson, Lucy and Kaviraj, Sugata and Keel, William C. and Lintott, Chris J. and Masters, Karen L. and Nichol, Robert C. and Sarzi, Marc and Skibba, Ramin and Treister, Ezequiel and Willett, Kyle W. and Wong, O. Ivy and Yi, Sukyoung K. (2014) The green valley is a red herring : Galaxy Zoo reveals two evolutionary pathways towards quenching of star formation in early- and late-type galaxies. Monthly Notices of the Royal Astronomical Society, 440 (1). pp. 889-907. ISSN 0035-8711

Abstract

We use SDSS+GALEX+Galaxy Zoo data to study the quenching of star formation in low-redshift galaxies. We show that the green valley between the blue cloud of star-forming galaxies and the red sequence of quiescent galaxies in the colour–mass diagram is not a single transitional state through which most blue galaxies evolve into red galaxies. Rather, an analysis that takes morphology into account makes clear that only a small population of blue early-type galaxies move rapidly across the green valley after the morphologies are transformed from disc to spheroid and star formation is quenched rapidly. In contrast, the majority of blue star-forming galaxies have significant discs, and they retain their late-type morphologies as their star formation rates decline very slowly. We summarize a range of observations that lead to these conclusions, including UV–optical colours and halo masses, which both show a striking dependence on morphological type. We interpret these results in terms of the evolution of cosmic gas supply and gas reservoirs. We conclude that late-type galaxies are consistent with a scenario where the cosmic supply of gas is shut off, perhaps at a critical halo mass, followed by a slow exhaustion of the remaining gas over several Gyr, driven by secular and/or environmental processes. In contrast, early-type galaxies require a scenario where the gas supply and gas reservoir are destroyed virtually instantaneously, with rapid quenching accompanied by a morphological transformation from disc to spheroid. This gas reservoir destruction could be the consequence of a major merger, which in most cases transforms galaxies from disc to elliptical morphology, and mergers could play a role in inducing black hole accretion and possibly active galactic nuclei feedback.