Composite Bulges:(Par.) Zoology of Beasts that Live at Centers of Galaxy Disks (P. Erwin, 28/3/2023)
Channel: Academy of Athens
Date: 17/05/2023

Peter Erwin
Max-Planck-Institute for Extraterrestrial Physics,
Garching bei Muenchen, Germany

Abstract: 
Classically, the centers of z ~ 0 disk galaxies were usually said to be dominated by "bulges": old, spheroidal, kinematically hot stellar systems not unlike low-luminosity elliptical galaxies. In the last two decades, this has been challenged by the "pseudobulge" idea: that some bulges are instead kinematically cool, disklike systems.

I will present evidence that the reality is even more complicated: when disk galaxies have "bulges" in their centers, these are*composite*  systems, consisting of multiple distinct, coexisting stellar structures. These include the boxy/peanut-shaped bulges of bars, nuclear disks, nuclear rings, nuclear bars, compact spheroidal ("classical") bulges, and nuclear star clusters of varying sizes. As part of this, I will discuss recent findings from the Composite Bulge Survey, which uses HST imaging and 2D IFU stellar kinematics for a volume-limited sample of massive S0 and spiral galaxies to investigate the nature and demographics of such systems. One striking (if preliminary) result is that classical bulges are surprisingly rare even in massive, early-type disk galaxies.