A trail of dark matter-free galaxies from a bullet dwarf collision

TL;DR

This paper proposes that a group of dark matter-free galaxies, including DF2 and DF4, originated from a single bullet-dwarf collision about eight billion years ago, forming a linear trail of galaxies.

Contribution

It introduces a novel formation scenario linking dark matter-free galaxies to a past collision event, supported by observational data and structural analysis.

Findings

DF2 and DF4's positions and velocities align with a shared collision origin.

A linear substructure of 7-11 galaxies suggests a common formation event.

Identification of potential remnants of progenitor galaxies at the trail's edges.

Abstract

The ultra-diffuse galaxies DF2 and DF4 in the NGC1052 group share several unusual properties: they both have large sizes, rich populations of overluminous and large globular clusters, and very low velocity dispersions indicating little or no dark matter. It has been suggested that these galaxies were formed in the aftermath of high velocity encounters of gas rich galaxies, events that resemble the collision that created the bullet cluster but on much smaller scales. The gas separates from the dark matter in the collision and subsequent star formation leads to the formation of one or more dark matter-free galaxies. Here we show that the present-day line-of-sight distances and radial velocities of DF2 and DF4 are consistent with their joint formation in the aftermath of a single bullet-dwarf collision, around eight billion years ago. Moreover, we find that DF2 and DF4 are part of an apparent linear substructure of 7-11 large, low-luminosity objects. We propose that these all originated in the same event, forming a trail of dark matter-free galaxies that is more than 2 Mpc long and angled 7 +- 2 degrees from the line of sight. We also tentatively identify the highly dark matter-dominated remnants of the two progenitor galaxies that are expected at the leading edges of the trail.