Quiescent Ultra-diffuse galaxies in the field originating from backsplash orbits

TL;DR

This study uses cosmological simulations to demonstrate that isolated quiescent ultra-diffuse galaxies in the field are likely backsplash galaxies that were previously satellites, having lost gas and dark matter due to past interactions.

Contribution

It reveals that isolated quenched UDGs can originate from satellite galaxies that experienced tidal stripping and gas removal during past interactions, explaining their quiescent nature in the field.

Findings

Quiescent UDGs are often backsplash galaxies with a history of satellite interactions.

Stripped outer halos of these UDGs lack dark matter and gas.

Isolated quenched UDGs can be found in filaments and voids.

Abstract

Ultra-diffuse galaxies (UDGs) are the lowest-surface brightness galaxies known, with typical stellar masses of dwarf galaxies but sizes similar to larger galaxies like the Milky Way. The reason for their extended sizes is debated, with suggested internal processes like angular momentum, feedback or mergers versus external mechanisms or a combination of both. Observationally, we know that UDGs are red and quiescent in groups and clusters while their counterparts in the field are blue and star-forming. This dichotomy suggests environmental effects as main culprit. However, this scenario is challenged by recent observations of isolated quiescent UDGs in the field. Here we use $\Lambda$CDM cosmological hydrodynamical simulation to show that isolated quenched UDGs are formed as backsplash galaxies that were once satellites of another galactic, group or cluster halo but are today a few Mpc away from them. These interactions, albeit brief, remove the gas and tidally strip the outskirts of the dark matter haloes of the now quenched seemingly-isolated UDGs, which are born as star-forming field UDGs occupying dwarf-mass dark matter haloes. Quiescent UDGs may therefore be found in non-negligible numbers in filaments and voids, bearing the mark of past interactions as stripped outer haloes devoid of dark matter and gas compared to dwarfs with similar stellar content.