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 clusters2–6, and very low velocity dispersions that indicate little or no dark matter7–10. It has been suggested that these galaxies were formed in the aftermath of high-velocity collisions of gas-rich galaxies11–13, 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.

Characteristics of Effective Dark Matter in Nonlocal Gravity

“…Therefore, NLG predicts less effective dark matter for dwarf galaxies as compared with the standard picture. This prediction seems to be consistent with observational data regarding dwarf galaxies that lack dark matter (van Dokkum et al 2018;Guo et al 2019;Mancera Piña et al 2019;Hammer et al 2020;Shen et al 2021). In the next section, we apply NLG to three UDGs, namely, AGC 114905, 242019, and 219533 and compute their effective dark matter content.…”

“…This situation with NLG is in sharp contrast with the standard CDM paradigm, where the rotation curves can be fit very well; however, the resulting predicted total amount of dark matter for each UDG is too small by about an order of magnitude. This shortcoming, on the other hand, could be the result of the rare collision events recently proposed by van Dokkum et al (2022).…”

“…In the process of galaxy formation within the potential well provided by dark matter, the total mass of the stars in the galaxy (M stars ) is expected to become correlated with the corresponding halo mass (M halo ). In this framework, the ratio of the halo mass to the mass of the stars, M halo /M stars , when plotted versus M stars , has a minimum value of around 30 for the Milky Way type of galaxies and increases toward lower and higher galactic masses (van Dokkum et al 2018;Guo et al 2019;Mancera Piña et al 2019;Hammer et al 2020;Shen et al 2021). For the UDGs under consideration here the ratio M halo /M stars is about unity; therefore, they pose a significant challenge to the standard dark matter paradigm.…”

“…This directly means that UDGs contain less dark matter and do not respect the baryonic Tully-Fisher relation (McGaugh et al 2000). In recent years, some nearby dark-matter-free UDG candidates have attracted much attention; for example, see van Dokkum et al (2018), Guo et al (2019), Mancera Piña et al (2019), Hammer et al (2020), and Shen et al (2021.…”

DHOST gravity in Ultra-diffuse galaxies -- Part I: the case of NGC1052-DF2

“…Indeed, hydrodynamical simulations [40,45,46] have shown that it is even possible to create a galaxy lacking DM in the standard cosmological scenario, and in [47][48][49] it has been suggested that NGC1052-DF2 may have formed as a consequence of high velocity collisions of gas-rich galaxies. In a most recent work [50], it was added that it might represent a sample of a possibly larger family, as a total of eleven low surface galaxies with the same properties have been found.…”