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

By Kevin Schawinski, C. Urry, B. Simmons, L. Fortson, S. Kaviraj, William Keel, Chris J. Lintott, Karen L. Masters, Robert Nichol, M. Sarzi and 6 more. 2014

522 citations recorded by the source corpus.

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.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/14694eeda36d6b56c9a315b9295e6cca9cd7006d

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In Astronomy and Astrophysics

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Citations in the corpus

• The star formation main sequence and stellar mass assembly of galaxies in the Illustris simulation (2014, background): “…population of galaxies that form their stars late and early, respectively, is consistent with the red and blue cloud, which are connected through the green valley, established from observations of galaxies (e.g. Strateva et al. 2001; Faber et al. 2007; Bowler et al. 2014; Schawinski et al. 2014).”
• Inferring the star formation histories of massive quiescent galaxies with bagpipes: evidence for multiple quenching mechanisms (2017, background): “Barro et al. (2013) and Schawinski et al. (2014) present evidence for a scheme in which there are both fast and slow tracks towards quiescence, with different mechanisms driving quenching.”
• The star formation activity of IllustrisTNG galaxies: main sequence, UVJ diagram, quenched fractions, and systematics (2018, background): “Morphological studies of galaxies have been proposed to explain the bending (Abramson et al. 2014; Schawinski et al. 2014; Schreiber et al. 2016), albeit the most likely scenario is related to secular internal processes, like AGN feedback, that drive the quenching of massive galaxies.”
• Evolution of density profiles in high-z galaxies: compaction and quenching inside-out (2015, background): “…in the local universe (Kauffmann et al. 2003; Franx et al. 2008; Robaina et al. 2012; Mendel et al. 2013; Cibinel et al. 2013a; Fang et al. 2013; Schawinski et al. 2014; Omand et al. 2014; Bluck et al. 2014; Woo et al. 2015) and at high-z (Wuyts et al. 2011, 2012; Bell et al. 2012; Cheung et…”
• Constraining the properties of AGN host galaxies with spectral energy distribution modelling (2015, background): “…et al. 2001; Baldry et al. 2004; Bell et al. 2004; Baldry et al. 2006) has been interpreted as evidence for star-formation quenching that may be driven by either internal (e.g. feedback) or external (e.g. environment) processes (e.g. Bell et al. 2004; Faber et al. 2007; Schawinski et al. 2014).”
• The Final Season Reimagined: 30 Tidal Disruption Events from the ZTF-I Survey (2022, background): “The top panel of Figure 3 also shows the limits of the green valley, the transition region between blue, star-forming galaxies and red, quiescent galaxies, originally defined by Schawinski et al. (2014).”
• STELLAR MASSES AND STAR FORMATION RATES FOR 1 M GALAXIES FROM SDSS+WISE (2015, background): “However, it could also signal a decline in SF activity as suggested by Schawinski et al. (2014).”
• Cosmic X-ray surveys of distant active galaxies (2015, background): “…in addition to galaxies transiting from the blue cloud to the red sequence due to the (potentially rapid) shut down of star formation in the host galaxy (e.g., Faber et al 2007; Martin et al 2007; Hasinger 2008; Schawinski et al 2009, 2014; Cimatti et al 2013; Goulding et al 2014; Pan et al 2014).”
• A TURNOVER IN THE GALAXY MAIN SEQUENCE OF STAR FORMATION AT M* ∼ 1010 M☉ FOR REDSHIFTS z < 1.3 (2015, background): “Schawinski et al. (2014) find that disk galaxies and elliptical galaxies likely quench their star formation rates through different processes with very different timescales.”
• The Illustris simulation: the evolving population of black holes across cosmic time (2014, background): “In fact, 108M black holes have the highest Eddington ratios on average and reside within galaxies with colours g−r ∼ 0.5 and with a mix of morphologies, resembling the so-called “green valley” objects (Schawinski et al. 2014).”
• MOSFIRE Spectroscopy of Quiescent Galaxies at 1.5 < z < 2.5. II. Star Formation Histories and Galaxy Quenching (2018, result): “Other studies, at both low and high redshift, have reached similar conclusions (Schawinski et al. 2014; Belli et al. 2015; Moutard et al. 2016; Wild et al. 2016; Wu et al. 2018a).”
• First results from the IllustrisTNG simulations: the galaxy colour bimodality (2017, methodology): “Focusing on morphology, Schawinski et al. (2014) used Galaxy Zoo classifications and found two distinct transition pathways, with different physical timescales, correlated with early versus late-type morphology.”