Evidence for Ultra-Diffuse Galaxy `Formation' Through Galaxy Interactions

TL;DR

This paper presents evidence that some ultra-diffuse galaxies (UDGs) may form through galaxy interactions, either by expanding normal dwarfs or collapsing from tidal material, based on the discovery of two UDGs with tidal features.

Contribution

It provides observational evidence linking UDG formation to galaxy interactions, suggesting a possible formation pathway for these diffuse galaxies.

Findings

Discovery of two UDGs with tidal features and associated stellar streams.

Evidence supporting galaxy interactions as a formation mechanism for some UDGs.

Implication that UDGs can be tidal dwarf galaxies or expanded normal dwarfs.

Abstract

We report the discovery of two ultra-diffuse galaxies (UDGs) which show clear evidence for association with tidal material and interaction with a larger galaxy halo, found during a search of the Wide portion of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). The two new UDGs, NGC2708-Dw1 and NGC5631-Dw1, are faint ($M_g$=$-$13.7 and $-$11.8 mag), extended ($r_h$=2.60 and 2.15 kpc) and have low central surface brightness ($\mu(g,0)$=24.9 and 27.3 mag arcsec$^{-2}$), while the stellar stream associated with each has a surface brightness $\mu(g)$$\gtrsim$28.2 mag arcsec$^{-2}$. These observations provide evidence that the origin of some UDGs may connect to galaxy interactions, either by transforming normal dwarf galaxies by expanding them, or because UDGs can collapse out of tidal material (i.e. they are tidal dwarf galaxies). Further work is needed to understand the fraction of the UDG population `formed' through galaxy interactions, and wide field searches for diffuse dwarf galaxies will provide further clues to the origin of these enigmatic stellar systems.