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

The ultra-diffuse galaxies DF2 and DF4 in the NGC 1052 group share unusual properties: 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 such galaxies can form after high-velocity collisions of gas-rich galaxies, akin to a small-scale analogue of the bullet cluster, where gas separates from dark matter and subsequent star formation yields dark-matter-free galaxies. 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 about eight billion years ago. Moreover, DF2 and DF4 lie within an apparent linear substructure of seven to eleven large, low-luminosity objects. We propose that these originated in the same event, forming a >2 Mpc trail of dark-matter-free galaxies angled 7° ± 2° from the line of sight, and tentatively identify highly dark-matter-dominated remnants of the two progenitor galaxies at the leading edges of the trail.

Pieter van Dokkum, Zili Shen, Michael A. Keim, Sebastian Trujillo-Gomez, Shany Danieli, Dhruba Dutta Chowdhury, Roberto Abraham, Charlie Conroy, J. M. Diederik Kruijssen, Daisuke Nagai, Aaron Romanowsky