One of the tightest scaling relations in extragalactic astronomy is that between galaxies' circular velocity (i.e., the speed at which matter in a galaxy orbits around its centre) and luminosity.
This is known as the Tully-Fisher relation. The circular velocities necessary to construct this relation are typically measured from HI data. For this reason, the relation is mostly
populated by gas-rich late-type galaxies, while placing the usually gas-poor early-type (E/S0) galaxies on it has historically been challenging. Using data collected with the Westerbork telescope,
den Hejier et al. (2015) (including CASS' P. Serra) have now been able to do exactly that. They found that
early-type galaxies lie systematically below the relation for late-type galaxies, and that this is mostly due to differences in the stellar populations and, therefore, mass-to-light ratios.
A key result of this study is that once these differences are taken into account, and the relation is build using galaxies' baryonic mass instead of luminosity,
late-type galaxies (black circles in the figure) and early-type galaxies (red circles) lie on the same relation. This result strengthens previous claims that the baryonic Tull-Fisher is a
more fundamental scaling relation than the classical (luminosity) Tully-Fisher.