Forensics of Subhalo-Stream Encounters: The Three Phases of Gap Growth (1412.6035v2)

Abstract: There is hope to discover dark matter subhalos free of stars (predicted by the current theory of structure formation) by observing gaps they produce in tidal streams. In fact, this is the most promising technique for dark substructure detection and characterization as such gaps grow with time, magnifying small perturbations into clear signatures observable by ongoing and planned Galaxy surveys. To facilitate such future inference, we develop a comprehensive framework for studies of the growth of the stream density perturbations. Starting with simple assumptions and restricting to streams on circular orbits, we derive analytic formulae that describe the evolution of all gap properties (size, density contrast etc) at all times. We uncover complex, previously unnoticed behavior, with the stream initially forming a density enhancement near the subhalo impact point. Shortly after, a gap forms due to the relative change in period induced by the subhalo's passage. There is an intermediate regime where the gap grows linearly in time. At late times, the particles in the stream overtake each other, forming caustics, and the gap grows like $\sqrt{t}$. In addition to the secular growth, we find that the gap oscillates as it grows due to epicyclic motion. We compare this analytic model to N-body simulations and find an impressive level of agreement. Importantly, when analyzing the observation of a single gap we find a large degeneracy between the subhalo mass, the impact geometry and kinematics, the host potential and the time since flyby.

• The paper demonstrates that subhalo impacts produce gaps in stellar streams through three distinct phases: compression, expansion, and caustic formation.
• It employs analytic expressions validated by N-body simulations to capture the precise temporal evolution of gap properties.
• The study reveals that gap growth rates depend on the galactic potential, providing a novel method to infer dark matter substructure characteristics.

Forensics of Subhalo-Stream Encounters: The Three Phases of Gap Growth

The paper "Forensics of Subhalo-Stream Encounters: The Three Phases of Gap Growth" by Denis Erkal and Vasily Belokurov provides a meticulous investigation into the perturbations caused by dark matter subhalos on galactic stellar streams. These gaps are of significant interest as they offer a promising avenue to infer the properties of otherwise invisible dark matter substructure.

Summary of Findings

The authors present a comprehensive framework to analyze how subhalo-induced gaps in streams evolve over time. Their analysis is primarily focused on streams following circular orbits around galaxies, which simplifies the problem to one solvable analytically. The process is broken down into three distinct phases: compression, expansion, and caustic formation. Initially, the stream experiences a compression phase where particles are gravitationally pulled together at the impact point, forming a temporary density enhancement. Subsequently, in the expansion phase, the alteration in particle orbital periods due to the perturbation induces a gap that grows linearly with time. Finally, in the caustic phase, the growth rate transitions from linear to $\sqrt{t}$ as caustics (density peaks) emerge at the gap's edges.

Technical Approach and Numerical Results

The authors develop analytic formulae that describe the full temporal evolution of gap properties such as size and density contrast. A prominent feature of the analysis is the agreement of these formulae with outcomes from N-body simulations, which serve as a robust validation of the theoretical model. The inclusion of factors such as the host potential and the gravitational influence of the subhalo allows a detailed examination of the degeneracies involved in interpreting observational data.

The numerical results underscore the high level of precision achievable by the analytic model. For instance, the gap grows at a rate influenced by the characteristics of the host galaxy's potential, illustrated by different growth rates within models of Keplerian and NFW potentials. This dependence connects observable features of stream gaps to theoretical models of galaxy mass distribution.

Implications for Cosmology and Future Research

This paper enhances the understanding of subhalo-stream interactions by providing clear, quantitative expressions for gap dynamics, thereby illustrating the potential of these features as tools for mapping dark matter's influence in galactic halos. However, the significant degeneracies uncovered, particularly in extracting subhalo properties from a single gap, pose a challenge, emphasizing the necessity for statistical approaches when deriving constraints from observational data.

Future research may expand on this foundational work by extending the model to incorporate non-circular orbits and streams with realistic velocity dispersions. Additionally, adapting the framework to a broader spectrum of subhalo mass profiles would enhance its applicability.

The paper contributes significantly to the theoretical groundwork needed for the interpretation of forthcoming galactic stream data from surveys. As galactic dynamics simulations grow increasingly sophisticated, the insights offered by this paper will be instrumental in bridging the gap between theoretical predictions and observational analyses, providing guidance for the constraints on the subhalo mass spectrum.