The Spur and the Gap in GD-1: Dynamical evidence for a dark substructure in the Milky Way halo (1811.03631v2)

Abstract: We present a model for the interaction of the GD-1 stellar stream with a massive perturber that naturally explains many of the observed stream features, including a gap and an off-stream spur of stars. The model involves an impulse by a fast encounter, after which the stream grows a loop of stars at different orbital energies. At specific viewing angles, this loop appears offset from the stream track. A quantitative comparison of the spur and gap features prefers models where the perturber is in the mass range of $10^6\,\rm M_\odot$ to $10^8\,\rm M_\odot$. Orbit integrations back in time show that the stream encounter could not have been caused by any known globular cluster or dwarf galaxy with a determined orbit, and mass, size and impact-parameter arguments show that it could not have been caused by a molecular cloud in the Milky Way disk. The most plausible explanation for the gap-and-spur structure is an encounter with a dark-matter substructure, like those predicted to populate galactic halos in LCDM cosmology. However, the expected densities of LCDM subhalos in this mass range and in this part of the Milky Way are $2-3\,\sigma$ lower than the inferred density of the GD-1 perturber. This observation opens up the possibility that detailed observations of streams could measure the mass spectrum of dark-matter substructures and even identify individual substructures and their orbits in the Galactic halo.

• The paper demonstrates that the GD-1 gap and spur arise from a gravitational impulse by a dark matter subhalo with a mass between 10^6 and 10^8 solar masses.
• The study uses detailed dynamical modeling to rule out known luminous objects as the perturber, reinforcing the dark substructure hypothesis.
• The findings challenge conventional cold dark matter models and underscore the need for high-precision observations of stellar streams in the Milky Way halo.

An Analysis of Dark Matter Influence on the GD-1 Stellar Stream

The paper presented by Bonaca et al. explores the dynamical interactions between the GD-1 stellar stream and an enigmatic, massive perturber hypothesized to reside in the Milky Way halo. The research primarily focuses on elucidating the observed features within the GD-1 stream, particularly a gap and an offset spur of stars. The proposed model suggests that these features result from the stream's interaction with a massive substructure, potentially a dark matter subhalo.

Modeling the GD-1 Perturbation

The researchers developed a model simulating the interaction between the GD-1 stream and a massive perturbing agent. This model effectively attributes the gap and spur observed in GD-1 to a gravitational impulse instigated by a close encounter with a high-density substructure. Crucially, the parameters of this model infer that the perturber's mass lies between $10^6\,\rm M_\odot$ to $10^8\,\rm M_\odot$. The paper applies these parameters to demonstrate that neither known Milky Way globular clusters nor dwarf galaxies fit the role of the perturber, both in terms of mass and orbital trajectories.

Key Findings and Numerical Implications

The numerical analysis within the paper shows that the GD-1 perturbation could hardly be attributed to any luminous object with a known orbit within the galaxy, reinforcing the hypothesis of a dark matter substructure as the cause. The perturber's effect, manifesting as a gap and spur in the stellar stream, suggests a density slightly exceeding the expectations for dark subhalos formed by cold dark matter models. The researchers propose that despite the challenges in direct observation, such perturbations could allow measurements of dark matter mass distributions and potentially identify distinct substructures within the galactic halo.

Implications and Future Directions

The implications of these findings are notable in both a theoretical and practical context. The inferred properties of the GD-1 perturber point to discrepancies in current dark matter halo theories, particularly regarding their density profiles. These discrepancies pave the way for potential breakthroughs in understanding the small-scale structure of dark matter. As the data suggests interactions that are unusually dense for dark matter subhalos, it pushes the boundaries of existing models and prompts a reevaluation of dark matter characteristics, perhaps even questioning cold dark matter paradigms.

In a future-oriented perspective, the paper anticipates methods to enhance detection and characterization of such substructures. Detailed kinematic studies of the GD-1’s perturbed features, enabled by high-precision astrometric data and spectroscopy, could discern these interactions further, testing the limits of current gravitational models. Moreover, the pursuit of identifying individual substructures may lead to more sophisticated observational strategies, transforming how we interpret gravitational interactions within the Milky Way.

Conclusion

Bonaca et al. have made a compelling case for the presence of a massive dark substructure in the Milky Way halo impacting the GD-1 stellar stream. Their findings emphasize the critical need for high-resolution observations and detailed modeling to illuminate the extensive and nuanced role of dark matter in galactic dynamics. By bridging gaps in current astrophysical understanding, this research contributes significantly to the broader field of cosmology and the ongoing effort to decode the universe's dark matter component.