Galaxy Zoo: Disentangling the Environmental Dependence of Morphology and Colour

TL;DR

This study uses clustering statistics from Galaxy Zoo data to analyze how galaxy morphology and color depend on environment, revealing that color is a stronger environmental indicator than morphology and highlighting different evolutionary paths for galaxy types.

Contribution

It provides the first detailed clustering analysis of visually classified galaxy morphologies, quantifies the morphology-environment relation, and explores the implications for galaxy evolution and the red spiral population.

Findings

Color-environment correlation remains strong at fixed morphology.

Red spiral galaxies are often in moderately dense environments.

Morphology and color are linked but can evolve independently.

Abstract

We analyze the environmental dependence of galaxy morphology and colour with two-point clustering statistics, using data from the Galaxy Zoo, the largest sample of visually classified morphologies yet compiled, extracted from the Sloan Digital Sky Survey. We present two-point correlation functions of spiral and early-type galaxies, and we quantify the correlation between morphology and environment with marked correlation functions. These yield clear and precise environmental trends across a wide range of scales, analogous to similar measurements with galaxy colours, indicating that the Galaxy Zoo classifications themselves are very precise. We measure morphology marked correlation functions at fixed colour and find that they are relatively weak, with the only residual correlation being that of red galaxies at small scales, indicating a morphology gradient within haloes for red galaxies. At fixed morphology, we find that the environmental dependence of colour remains strong, and these correlations remain for fixed morphology \textit{and} luminosity. An implication of this is that much of the morphology--density relation is due to the relation between colour and density. Our results also have implications for galaxy evolution: the morphological transformation of galaxies is usually accompanied by a colour transformation, but not necessarily vice versa. A spiral galaxy may move onto the red sequence of the colour-magnitude diagram without quickly becoming an early-type. We analyze the significant population of red spiral galaxies, and present evidence that they tend to be located in moderately dense environments and are often satellite galaxies in the outskirts of haloes. Finally, we combine our results to argue that central and satellite galaxies tend to follow different evolutionary paths.