The time evolution of the (mis)alignment between angular momentum and principal axis in simulated Massive ETGs

The stellar halos of massive early type galaxies (ETGs) have been studied in a variety of different contexts. A good deal of that work has leveraged the capabilities of large-scale cosmological simulations to closely analyze simulated galactic halos in a controlled environment, focusing on characterizing the distribution of morphological and kinematic properties of these galaxies. In this project, we analyze a large sample (~4000 galaxies) of massive (M_*>10¹¹ M_☉at z=0) ETGs in TNG300, the largest volume initialization of the IllustrisTNG (TNG) simulation suite. First, we characterize their distribution in the parameter space defined by morphological and kinematic properties. In particular, we focus on the relationship between the intrinsic misalignment angle between the angular momentum and morphological axes and the broader structural properties of these systems, such as the degree of rotational support. We find that a majority of the galaxies with these high masses are prolate, but, within this designation, display a diverse range of behaviors, both dynamically and structurally. This diversity in mind, we find that a significant fraction of the galaxies, particularly the ones we might classify as prolate rotators, occupy such a classification for only a short phase of their evolution. That is, the current stage of these galaxies’ evolution is not well characterized by their instantaneous properties. Rather, we find that analyzing the variations in angular momentum and principal axis directions over time provides a new perspective on understanding the instantaneous properties we observe. This approach gives us a way to reliably classify these galaxies from their point of major merger to the current time and is fairly resistant to the complex evolutionary effects that perturb the aforementioned instantaneous galaxy states. As a result, we are also working on connecting this new understanding to the merger histories of these systems, in order to try and understand how they first arose.