A Third Galaxy Missing Dark Matter along a Trail of Galaxies in the NGC 1052 Field

TL;DR

This study investigates the dark matter content of galaxy DF9 in the NGC 1052 trail, finding it also lacks dark matter, supporting the idea of a unique galaxy formation process involving dark matter separation.

Contribution

It provides the first measurement of DF9's dark matter content, confirming its lack of dark matter and supporting the bullet dwarf collision scenario.

Findings

DF9's stellar velocity dispersion matches expectations from stellar mass alone.

DF9's dispersion is much lower than predicted by the stellar mass--halo mass relation.

Results support a common formation channel for trail galaxies involving dark matter separation.

Abstract

While most dwarf galaxies are strongly dark matter dominated, two remarkable objects in the NGC 1052 field, DF2 and DF4, appear to lack dark matter. DF2 and DF4 were recently found to be part of a trail of low luminosity galaxies that follow a linear relation between their position on the trail and their radial velocity. If the other galaxies on this trail formed together with DF2 and DF4, e.g., from gas that was separated from dark matter through a 'bullet dwarf' collision, they may lack dark matter as well. Here we constrain the dark matter content of DF9, the galaxy on the trail that most closely resembles DF2 and DF4. Using Keck/KCWI absorption line spectroscopy we find that DF9's stellar velocity dispersion is $6.4^{+4.0}_{-4.3}$ km s$^{-1}$. This is consistent with the $8.3^{+0.9}_{-1.4}$ km s$^{-1}$ dispersion that is expected from DF9's $1.4\times 10^8$ M$_\odot$ stellar mass alone, and we conclude that -- like DF2 and DF4 -- dark matter is not required to explain the kinematics of DF9. The dispersion is far below the $27\pm3$ km s$^{-1}$ expected if DF9 fell on the stellar mass--halo mass relation. Our results are further evidence that the trail of low mass galaxies in the NGC 1052 field formed together in a unique galaxy formation channel, and are consistent with the prediction of the bullet dwarf scenario that other trail galaxies should show the same lack of dark matter as DF2 and DF4.