Keck spectroscopy of NGC 1052-DF9: stellar populations in the context of the NGC 1052 group

In this study, we use Keck/Keck Cosmic Web Imager spectroscopy to measure the age, metallicity, and recessional velocity of NGC 1052-DF9 (DF9), a dwarf galaxy in the NGC 1052 group. We compare these properties to those of two other galaxies in the group, NGC 1052-DF2 and NGC 1052-DF4, which have low dark matter content. The three galaxies are proposed constituents of a trail of galaxies recently hypothesized to have formed as part of a 'bullet dwarf' collision. We show that the ages and total metallicities of the three galaxies are within uncertainties of one another which may be expected if they share a related formation pathway. However, the recessional velocity we recover for DF9 (1680 ± 10 km s^-1) is higher than predicted for a linearly projected interpretation of the 'bullet dwarf' trail. DF9 is then either not part of the trail or the correlation of galaxy velocities along the trail is not linear in 2D projection due to their 3D geometry. After examining other proposed formation pathways for the galaxies, none provide a wholly satisfactory explanation for all of their known properties. We conclude further work is required to understand the formation of this interesting group of galaxies.