Slow rotating galaxies join clusters late and migrate to centre
13 July 2015
Previous surveys have shown that early-type galaxies (ETGs), traditionally thought of as dispersion-dominated systems, show strong rotation in ~80% of cases. This led to a new kinematic classification of ETGs into fast rotators (strong rotation signature; FRs) and slow rotators (no sign of rotation; SRs). Due to their drastically different dynamics, these two families must have very different formation histories.
Earlier, it was also found that SRs are almost absent in the field, with the fraction of SRs in the ETG population increasing only in the densest environment. This is now known as the kinematic morphology-density relation and has been seen in other clusters. However, only four clusters had been examined for this effect until CAASTRO researcher Dr Lisa Fogarty (University of Sydney) and her SAMI Pilot Survey team set out to examine the kinematic morphology-density relation of three further clusters.
The SAMI Pilot Survey observed 106 galaxies in the clusters Abell 85, Abell 168 and Abell 2399. The team extracted stellar kinematics for the 80 ETGs in the sample and classified them as FRs or SRs based on their stellar angular momentum.
For two of those clusters, the researchers observed the expected trend of an increasing fraction of SRs in the densest region of the cluster – the cluster centre. But in one of the clusters (Abell 2399), this was not the case. Also contrary to most other studies, their survey found that neither of the brightest cluster galaxies in this cluster is a slow rotator. In the other two clusters, Abell 85 and Abell 2399, the team saw SRs on the outskirts of the cluster, hinting that these SRs are already fully formed as they fall into the cluster environment for the first time.
The formation mechanism for SRs therefore seems to not only occur in clusters but also in many other environments. Dr Fogarty and her team suggest a mechanism by which SRs first form in groups, then these groups grow to form clusters or join existing galaxy clusters. The SRs could then eventually migrate to the densest parts of the cluster, possibly through dynamical friction.