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Does galaxy evolution proceed through the green valley via multiple pathwaysor as a single population? Motivated by recent results highlighting radicallydifferent evolutionary pathways between early- and late-type galaxies, wepresent results from a simple Bayesian approach to this problem wherein wemodel the star formation history (SFH) of a galaxy with two parameters, [t,\tau] and compare the predicted and observed optical and near-ultravioletcolours. We use a novel method to investigate the morphological differencesbetween the most probable SFHs for both disc-like and smooth-like populationsof galaxies, by using a sample of 126,316 galaxies (0.01 < z < 0.25) withprobabilistic estimates of morphology from Galaxy Zoo. We find a cleardifference between the quenching timescales preferred by smooth- and disc-likegalaxies, with three possible routes through the green valley dominated bysmooth- (rapid timescales, attributed to major mergers), intermediate-(intermediate timescales, attributed to minor mergers and galaxy interactions)and disc-like (slow timescales, attributed to secular evolution) galaxies. Wehypothesise that morphological changes occur in systems which have undergonequenching with an exponential timescale \tau < 1.5 Gyr, in order for theevolution of galaxies in the green valley to match the ratio of smooth to discgalaxies observed in the red sequence. These rapid timescales are instrumentalin the formation of the red sequence at earlier times; however we find thatgalaxies currently passing through the green valley typically do so atintermediate timescales.

Galaxy Zoo: Evidence for Diverse Star Formation Histories through the Green Valley