Galaxy Zoo: the interplay of quenching mechanisms in the group environment★

R J Smethurst,R E Hart,S P Bamford,S J Kruk,R C Nichol,B D Simmons,K L Masters,C J Lintott

doi:10.1093/mnras/stx973

Abstract

Does the environment of a galaxy directly influence the quenching history of a galaxy? Here we investigate the detailed morphological structures and star formation histories of a sample of SDSS group galaxies with both classifications from Galaxy Zoo 2 and NUV detections in GALEX. We use the optical and NUV colours to infer the quenching time and rate describing a simple exponentially declining SFH for each galaxy, along with a control sample of field galaxies. We find that the time since quenching and the rate of quenching do not correlate with the relative velocity of a satellite but are correlated with the group potential. This quenching occurs within an average quenching timescale of $\sim2.5~\rm{Gyr}$ from star forming to complete quiescence, during an average infall time (from $\sim 10R_{200}$ to $0.01R_{200}$) of $\sim 2.6~\rm{Gyr}$. Our results suggest that the environment does play a direct role in galaxy quenching through quenching mechanisms which are correlated with the group potential, such as harassment, interactions or starvation. Environmental quenching mechanisms which are correlated with satellite velocity, such as ram pressure stripping, are not the main cause of quenching in the group environment. We find that no single mechanism dominates over another, except in the most extreme environments or masses. Instead an interplay of mergers, mass & morphological quenching and environment driven quenching mechanisms dependent on the group potential drive galaxy evolution in groups.

Highlights

Over half of all galaxies are found clustered together in groups (Zwicky 1938; Abell 1958; Huchra & Geller 1982; Eke et al 2004), sharing one large dark matter halo
The extensive morphological classifications provided by Galaxy Zoo 21 (GZ2) allow for the investigation of how more detailed morphological structure is affected by the group environment
We quantify the trends observed with R/R200 seen across Figs 9–11 by performing a linear regression fit to the middle bin in each figure panel, with the uncertainty on the median value in each R/R200 bin represented by the error on the median

Summary

Introduction

Over half of all galaxies are found clustered together in groups (Zwicky 1938; Abell 1958; Huchra & Geller 1982; Eke et al 2004), sharing one large dark matter halo (groups with ∼100 or more galaxies are referred to as clusters; Bower & Balogh 2004). Some galaxies are found isolated from others in less dense environments (often referred to as the field), either because they are fossil groups (where all members have eventually merged; Ponman et al 1994; Jones, Ponman & Forbes 2000; Jones et al 2003) or because they have been isolated for their entire lifetimes This environmental density is found to be correlated with morphology (Dressler 1980; Smail et al 1997; Poggianti et al 1999; Postman et al 2005; Bamford et al 2009), colour (Butcher & Oemler 1978; Pimbblet et al 2002), quenched galaxy fraction (Kauffmann et al 2003; Baldry et al 2006; Peng et al 2012; Darvish et al 2016) and star formation rate (SFR; Gomez et al 2003). Instead, the correlation of increased quenched galaxy fractions with environment density is due to a superposition of other possible quenching mechanisms each of which depends on more local factors

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Conclusion