Weighing galaxies to find a more Fundamental Plane

Early-type galaxies are thought to have a very simple structure, and therefore their properties ought to follow well-defined scaling relations. The most prominent of these is the Fundamental Plane. Under the assumption that galaxies are dynamically relaxed systems, the mass of a galaxy should be proportional to its radius and the square of the velocity dispersion of the stars.

However, the observed relation did not quite match the theory – the observed Plane was tilted with respect to the theoretical prediction. One possible explanation for this discrepancy is that the theoretical prediction refers to galaxy mass, whereas most observations have relied on galaxy luminosity as a proxy for mass.  Directly measuring the galaxy mass may remove this; however, measuring galaxy mass is much more challenging than measuring luminosity.

One approach to measuring galaxy mass is to use 3D, integral field spectroscopy. CAASTRO researcher Dr Nicholas Scott and members of the SAMI Pilot Survey obtained spatially resolved spectroscopy of 106 galaxies in three nearby clusters. From these observations they were able to produce maps of the velocities and velocity dispersions within each galaxy. From these maps they can then use a dynamical modeling technique to measure the total mass (stars, gas and dark matter) of each galaxy.

While other studies have used this approach for very nearby galaxies, the SAMI Pilot team was the first to apply this to a large sample of galaxies in clusters. Galaxies in clusters are particularly well suited to this work because we know they are all at the same distance, removing the most significant source of uncertainty when measuring the Fundamental Plane. Using these mass measurements, the researchers were able to construct Fundamental Planes for the galaxies in each of the three clusters. All three galaxy mass planes were consistent with the theoretical prediction, confirming that the difference between galaxy light and mass was responsible for the previously tilted plane.

Publication details:

Nicholas Scott, Lisa Fogarty et al. in MNRAS (2015) “The SAMI Pilot Survey: The Fundamental and Mass Planes in Three Low-Redshift Clusters”