Green valley galaxies in the cosmic web: internal versus environmental quenching

Abstract

We analyze the SDSS data to classify the galaxies based on their colour using a fuzzy set-theoretic method and quantify their environments using the local dimension. We find that the fraction of the green galaxies does not depend on the environment and 10%–20% of the galaxies at each environment are in the green valley depending on the stellar mass range chosen. Approximately 10% of the green galaxies at each environment host an AGN. Combining data from the Galaxy Zoo, we find that ∼ 95% of the green galaxies are spirals and ∼ 5% are ellipticals at each environment. Only ∼ 8% of green galaxies exhibit signs of interactions and mergers, ∼ 1% have dominant bulge, and ∼ 6% host a bar. We show that the stellar mass distributions for the red and green galaxies are quite similar at each environment. Our analysis suggests that the majority of the green galaxies must curtail their star formation using physical mechanism(s) other than interactions, mergers, and those driven by bulge, bar and AGN activity. We speculate that these are the massive galaxies that have grown only via smooth accretion and suppressed the star formation primarily through mass driven quenching. Using a Kolmogorov-Smirnov test, we do not find any statistically significant difference between the properties of green galaxies in different environments. We conclude that the environmental factors play a minor role and the internal processes play the dominant role in quenching star formation in the green valley galaxies.