Ultra Diffuse Galaxies are a Subset of Cluster Dwarf Elliptical/Spheroidal Galaxies

TL;DR

This paper demonstrates that Ultra Diffuse Galaxies are not a new galaxy type but are a subset of known cluster dwarf elliptical/spheroidal galaxies formed mainly through cluster processes like galaxy harassment.

Contribution

It clarifies the nature of UDGs by linking them to established low-surface brightness galaxies and emphasizes their formation via cluster interactions.

Findings

UDGs are part of the known low-surface brightness galaxy population.

Most UDGs are likely formed through cluster processes such as galaxy harassment.

UDGs are not a distinct new galaxy class but a subset of dwarf ellipticals.

Abstract

Since 2015 there has been a great deal of interest in a supposed new class of galaxy called Ultra Diffuse Galaxies (UDGs). These are large systems with sizes $> 1.5$ kpc and have surface brightness values which are $\mu > 25$ mag arcsec$^{-2}$. Because of their low-surface brightness they are proposed to be `failed' Milky Way type galaxies given their similar size, but much lower stellar masses. As such, these systems are considered by some as a new type of galaxy, yet we show that they are a subset of a well-established and well studied population of low-surface brightness galaxies found mostly in dense areas of the universe - clusters of galaxies. We argue based on previous literature that the most likely method for forming these galaxies is through cluster processes such as `Galaxy Harassment', where through multiple high speed encounters an infalling galaxy is gradually removed of its mass, until it resembles a dwarf elliptical. Future studies of UDGs should consider the above and their more general connection to previously studied populations.