What's in a name? Quantifying the interplay between the Definition, Orientation, and Shape of Ultra-diffuse Galaxies Using the Romulus Simulations

TL;DR

This study uses Romulus simulations to analyze how the definition, shape, and viewing angle influence the classification and understanding of ultra-diffuse galaxies across different environments.

Contribution

It demonstrates that UDG classification depends heavily on the chosen definition and viewing orientation, affecting the inferred formation mechanisms.

Findings

Isolated UDGs are more oblate and diskier.

UDG classification varies with definition, affecting population counts.

Viewing angle influences UDG identification more in low-density environments.

Abstract

We explore populations of ultra-diffuse galaxies (UDGs) in isolated, satellite, and cluster environments using the Romulus25 and RomulusC simulations, including how the populations vary with UDG definition and viewing orientation. Using a fiducial definition of UDGs, we find that isolated UDGs have notably larger semi-major (b/a) and smaller semi-minor (c/a) axis ratios than their non-UDG counterparts, i.e., they are more oblate, or diskier. This is in line with previous results that adopted the same UDG definition and showed that isolated UDGs form via early, high-spin mergers. However, the choice of UDG definition can drastically affect what subsets of a dwarf population are classified as UDGs, changing the number of UDGs by up to approximately 45% of the dwarf population. We also find that a galaxy's classification as a UDG is dependent on its viewing orientation, and this dependence decreases as environmental density increases. Overall, we conclude that some definitions for UDGs used in the literature manage to isolate a specific formation mechanism for isolated dwarfs, while less restrictive definitions erase a link to the formation mechanism. Thus, how we define UDG populations must be considered if we want to understand the formation and evolution of UDGs.