A galaxy lacking dark matter (1803.10237v1)

Abstract: Studies of galaxy surveys in the context of the cold dark matter paradigm have shown that the mass of the dark matter halo and the total stellar mass are coupled through a function that varies smoothly with mass. Their average ratio M_{halo}/M_{stars} has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5x10^{10} solar masses) and increases both towards lower masses and towards higher masses. The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy NGC1052-DF2, which has a stellar mass of approximately 2x10^8 solar masses. We infer that its velocity dispersion is less than 10.5 kilometers per second with 90 per cent confidence, and we determine from this that its total mass within a radius of 7.6 kiloparsecs is less than 3.4x10^8 solar masses. This implies that the ratio M_{halo}/M_{stars} is of order unity (and consistent with zero), a factor of at least 400 lower than expected. NGC1052-DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales.

• The paper demonstrates that NGC1052–DF2 exhibits an extremely low dark matter-to-stellar mass ratio, challenging the cold dark matter paradigm.
• The authors employ advanced observational techniques, including Dragonfly imaging, Keck spectroscopy, and HST data, to measure the galaxy’s velocity dispersion.
• The findings prompt a reevaluation of galaxy formation theories and highlight the need for further studies on dark matter-deficient systems.

A Galaxy Lacking Dark Matter

The research presented in the paper "A galaxy lacking dark matter," authored by Pieter van Dokkum and colleagues, addresses a significant challenge to the conventional understanding of galactic formation and composition concerning dark matter. The paper meticulously investigates the ultra-diffuse galaxy NGC1052–DF2, revealing evidence that contradicts standard cosmological models that couple dark matter with baryonic matter on galactic scales.

Key Findings of the Study

The paper focuses on the cold dark matter paradigm, which posits a correlation between the mass of dark matter halos and total stellar mass. Typically, the mass ratio $M_{\rm halo} / M_{\rm stars}$ is expected to exhibit a minimum around galaxies with stellar masses similar to that of the Milky Way, approximately $5 \times 10^{10}$ solar masses. However, this paper details the observation of NGC1052–DF2, a galaxy with a stellar mass estimated to be around $2 \times 10^8$ solar masses, with a notably low velocity dispersion of less than 10.5 km/s. The inferred total mass within a certain radial limit is less than $3.4 \times 10^8$ solar masses, resulting in an expected mass ratio that is surprisingly low and possibly approaching zero. This finding denotes an extraordinary deficiency of dark matter in NGC1052–DF2, challenging the prevailing assumptions about galaxy formation.

Methodology and Observations

NGC1052–DF2 was detected using the Dragonfly Telephoto Array and followed with spectroscopic data from the W.M. Keck Observatory and imaging from the Hubble Space Telescope (HST). The paper identifies ten luminous, globular cluster-like objects associated with the galaxy, possessing a minimal velocity dispersion. The analysis utilizes the Tracer Mass Estimator (TME) method, yielding constraints on the galaxy's dynamical mass that align predominantly with the stellar mass, leaving little room for the presence of a dark matter halo.

Implications and Future Directions

This research has significant theoretical and practical implications. Demonstrating a case where a galaxy exhibits negligible dark matter weakens some arguments supporting alternative gravity models such as Modified Newtonian Dynamics (MOND). It also provides an intriguing counterpoint to assumptions about the omnipresence of dark matter signatures when baryonic matter is present. While results from NGC1052–DF2 show promise in advancing our understanding of galaxy formation, more observations of similar galaxies are needed to ascertain the prevalence and astrophysical origins of such systems.

A potential explanation for NGC1052–DF2 is that it constitutes an old tidal dwarf galaxy formed from galaxy merger remnants or quasar winds. However, the paper acknowledges the need for further exploration to understand the formation mechanism, the peculiar velocity dispersion, and the consequences of having such a notable absence of dark matter.

Conclusion

The apparent deficiency of dark matter in NGC1052–DF2 presents an extraordinary case in astrophysical observations, stimulating ongoing debates regarding galaxy evolution, dark matter distribution, and the overall validity of the cold dark matter approach. Future studies focused on broadening the observational sampling of ultra-diffuse galaxies and deepening the investigation into unconventional dark matter configurations will be critical in refining our models of galaxy formation and understanding the diverse fabric of the universe.