Research Byte

Published in the RSAA Lunations
Vol1 Issue5 1–30 June 2020

Galaxies come in a range of types, with (bulge-dominated) ellipticals and (disk-dominated) late-type spirals at the extremes. Most galaxies exhibit features that are intermediate between these two extremes, with both bulge and disk components. We have been motivated to decompose the bulge and disk components to describe the rotation velocities (ordered motions) and velocity dispersions (random motions) in galaxies, as there is a close correlation between the mix of these types of motions and galaxy morphological types. We studied 826 galaxies using 3D spectroscopy data from the SAMI Galaxy Survey (https://sami-survey.org), using a novel technique to separate the random-motion-supported (bulge) and ordered-motion-supported (disk) components. Our sample is the largest to date with spectroscopic bulge-disk decomposition and the first such sample to include all morphological types. 

In our recently-published paper (June 2020), we quantitatively show that galaxy bulges and disks have distinct properties: bulges are pressure-dominated systems and disks are supported by rotation. The combination of these two distinct components provides a consistent description of the major rotation features of galaxies over a wide range of morphological types. Furthermore, we found a tight correlation between the stellar mass and rotation velocities for the disk components of all morphological types, the so-called Tully-Fisher relation, which previously had only applied to spirals. Similarly, we found a tight correlation between the stellar mass and the velocity dispersions of both the bulge and disk components for all morphological types, extending the Faber-Jackson relation, which previously had only applied to early-type galaxies. Both of these scaling relations are thus not exclusive relations for specific galaxy types, but general relations between mass and the rotation (disk) or dispersion (bulge) components of all galaxy types. Our findings suggest that the relative contributions of the two components explain, at least to first order, the complex rotation behaviour of galaxies.

The free version of the paper is available on arXiv: https://arxiv.org/abs/2005.06474

Dr Sree Oh