Distribution of slow and fast rotators in the Fornax cluster

Nicholas Scott,Alister W Graham,Kevin A Pimbblet,Ryan C W Houghton,Roger L Davies,Michele Cappellari

doi:10.1093/mnras/stu472

Abstract

We present integral field spectroscopy of 10 early-type galaxies in the nearby, low-mass, Fornax cluster, from which we derive spatially resolved stellar kinematics. Based on the morphologies of their stellar velocity maps we classify 2/10 galaxies as slow rotators, with the remaining eight galaxies fast rotators. Supplementing our integral field observations with morphological and kinematic data from the literature, we analyse the ‘kinematic’ type of all 30 galaxies in the Fornax cluster brighter than MK = −21.5 mag (M* ∼ 6 × 109 M⊙). Our sample's slow rotator fraction within one virial radius is |$7^{+4}_{-6}$| per cent. |$13^{+8}_{-6}$| per cent of the early-type galaxies are slow rotators, consistent with the observed fraction in other galaxy aggregates. The fraction of slow rotators in Fornax varies with cluster-centric radius, rising to 16|$^{+11}_{-8}$| per cent of all kinematic types within the central 0.2 virial radii, from 0 per cent in the cluster outskirts. We find that, even in mass-matched samples of slow and fast rotators, slow rotators are found preferentially at higher projected environmental density than fast rotators. This demonstrates that dynamical friction alone cannot be responsible for the differing distributions of slow and fast rotators. For dynamical friction to play a significant role, slow rotators must reside in higher mass sub-haloes than fast rotators and/or form in the centres of groups before being accreted on to the cluster.

Highlights

The population of galaxies that reside in clusters shows significant differences to that in the field and other lower-density environments
On inspection of the velocity maps, the majority of galaxies show regular velocity fields with the rotation aligned along the photometric major axis and with significant rotation velocities. 7/10 galaxies have velocity fields whose morphology is consistent with being fast rotators (FRs)
We derived spatially-resolved maps of the velocity and velocity dispersion for each galaxy, classifying them as either fast or slow rotators based on i) the morphology of their velocity maps and ii) their specific stellar angular momentum, λR

Summary

Introduction

The population of galaxies that reside in clusters shows significant differences to that in the field and other lower-density environments. A number of authors (Stanford et al 1998; van Dokkum et al 2000; Smith et al 2005; Postman et al 2005; Cooper et al 2006; Capak et al 2007; Poggianti et al 2008) have more recently confirmed this picture, showing that in the highest density regions the early-type fraction is already high at z ∼ 1 (though increases still further up to the present day), but in low and intermediate density regions significant morphological evolution is only seen between z = 0.5 and today Despite these long-recognised differences the precise effects of environment on galaxy evolution are poorly understood. It has been suggested that the observed effects of environment are due to a differing mass function between low- and high-density environments (Treu et al 2005), recent studies involving large samples do find a statistically significant variation in e.g. specific star formation rate beyond that which would be expected from a differing mass function alone (Bamford et al 2009; Peng et al 2010)

Results

Discussion

Conclusion