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

Abstract

The ultra-diffuse galaxies DF2 and DF4 in the NGC 1052 group share several unusual properties: they both have large sizes¹, rich populations of overluminous and large globular clusters^2-6, and very low velocity dispersions that indicate little or no dark matter^7-10. It has been suggested that these galaxies were formed in the aftermath of high-velocity collisions of gas-rich galaxies^11-13, events that resemble the collision that created the bullet cluster¹⁴ 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 galaxies¹². 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 seven to eleven large, low-luminosity objects. We propose that these all originated in the same event, forming a trail of dark-matter-free galaxies that is roughly more than two megaparsecs long and angled 7° ± 2° from the line of sight. We also tentatively identify the highly dark-matter-dominated remnants of the two progenitor galaxies that are expected¹¹ at the leading edges of the trail.