Majority of galaxies in Fornax Cluster are fast rotating disks

Since the time of Edwin Hubble, astronomers have been classifying galaxies into different types based on their appearance, and this approach has taught us much about how galaxies formed and have evolved. However, this approach has significant limitations because we cannot observe the true, three-dimensional shape of a galaxy, only a two-dimensional projection from an arbitrary viewing angle. For example, we cannot reliably tell the difference between a spheroidal galaxy and a disk galaxy seen face-on. This degeneracy can be broken by studying the motions of the stars within a galaxy – the stellar kinematics – and accurately classifying galaxies as either slow rotators (spheroidal systems with mainly disordered stellar kinematics) or fast rotators (disk-like systems with ordered rotation).

CAASTRO researcher Dr Nicholas Scott (University of Sydney), along with CAASTRO Partner Investigator Prof Roger Davies (University of Oxford) and a team of international astronomers, used the Wide Field Spectrograph on the Australian National University 2.3m telescope to study the stellar kinematics of galaxies in the nearby Fornax cluster. The team found that the majority of galaxies in the cluster are disk-like fast rotators or spiral galaxies, with slow rotators making up only 7% of the total population.

Combining their result with complementary studies of galaxies in other clusters, they found that slow rotators mainly reside in the dense cores of galaxy clusters. By controlling for other galaxy parameters, such as mass, they concluded that migration due to dynamical friction cannot be solely responsible for the over-representation of slow rotators in the centres of clusters but that a more complex mechanism is required. The researchers suggest that slow rotators either preferentially form in the centre of clusters – so they do not need to migrate – or they merge into the cluster as a central group galaxy – so the relevant mass for dynamical friction is the group mass, not the galaxy stellar mass.

by N. Scott

Publication details:

Nicholas Scott, Roger L Davies, Ryan C W Houghton, Michele Cappellari, Alister W Graham, Kevin A Pimbblet in MNRAS (2014) “Distribution of Slow and Fast Rotators in the Fornax Cluster”