Galaxy Zoo: evidence for diverse star formation histories through the Green Valley

R. J. Smethurst, C. J. Lintott, B. D. Simmons, K. Schawinski, P. J. Marshall, S. Bamford, L. Fortson, S. Kaviraj, K. L. Masters, Thomas Melvin, R. C. Nichol, R. A. Skibba, K. W. Willett

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Does galaxy evolution proceed through the green valley via multiple pathways or as a single population? Motivated by recent results highlighting radically different evolutionary pathways between early- and late-type galaxies, we present results from a simple Bayesian approach to this problem wherein we model the star formation history (SFH) of a galaxy with two parameters, [t, τ ] and compare the predicted and observed optical and near-ultraviolet colours. We use a novel method to investigate the morphological differences between the most probable SFHs for both disc-like and smooth-like populations of galaxies, by using a sample of 126 316 galaxies (0.01 < z <0.25) with probabilistic estimates of morphology from Galaxy Zoo. We find a clear difference between the quenching time-scales preferred by smooth- and disc-like galaxies, with three possible routes through the green valley dominated by smooth- (rapid time-scales, attributed to major mergers), intermediate- (intermediate time-scales, attributed to minor mergers and galaxy interactions) and disc-like (slow time-scales, attributed to secular evolution) galaxies.We hypothesize that morphological changes occur in systems which have undergone quenching with an exponential time-scale τ < 1.5Gyr, in order for the evolution of galaxies in the green valley to match the ratio of smooth to disc galaxies observed in the red sequence. These rapid time-scales are instrumental in the formation of the red sequence at earlier times; however, we find that galaxies currently passing through the green valley typically do so at intermediate time-scales
Original languageEnglish
Pages (from-to)435-453
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Early online date17 Apr 2015
Publication statusPublished - 11 Jun 2015


  • astro-ph.GA
  • RCUK
  • STFC
  • ST/J500665/1


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