Dark Matter Halos as Particle Colliders: A Unified Solution to Small-Scale Structure Puzzles from Dwarfs to Clusters

Abstract: Astrophysical observations spanning dwarf galaxies to galaxy clusters indicate that dark matter (DM) halos are less dense in their central regions compared to expectations from collisionless DM N-body simulations. Using detailed fits to DM halos of galaxies and clusters, we show that self-interacting DM (SIDM) may provide a consistent solution to the DM deficit problem across all scales, even though individual systems exhibit a wide diversity in halo properties. Since the characteristic velocity of DM particles varies across these systems, we are able to measure the self-interaction cross section as a function of kinetic energy and thereby deduce the SIDM particle physics model parameters. Our results prefer a mildly velocity-dependent cross section, from $\sigma/m \simeq 2\; {\rm cm^2/g}$ on galaxy scales to $\sigma/m \simeq 0.1\; {\rm cm^2/g}$ on cluster scales, consistent with the upper limits from merging clusters. Our results dramatically improve the constraints on SIDM models and may allow the masses of both DM and dark mediator particles to be measured even if the dark sector is completely hidden from the Standard Model, which we illustrate for the dark photon model.

• The paper introduces a self-interacting dark matter framework with a velocity-dependent cross-section that unifies density profiles from dwarf galaxies to clusters.
• It develops a semi-analytic halo model incorporating hydrostatic equilibrium and a dark photon mechanism to constrain dark matter and mediator masses.
• The results challenge collisionless dark matter models by quantifying cross-section variations, offering new insights into dark matter microphysics.

Overview of "Dark Matter Halos as Particle Colliders: A Unified Solution to Small-Scale Structure Puzzles from Dwarfs to Clusters"

The paper by Kaplinghat, Tulin, and Yu presents a thorough examination of self-interacting dark matter (SIDM) as a coherent explanation for discrepancies observed in dark matter (DM) distributions from the scale of dwarf galaxies to galaxy clusters. The researchers address the issue that DM halos show deficits in density at central regions when compared to predictions from collisionless DM N-body simulations.

Key Contributions

1. Self-Interacting Dark Matter Framework: The authors propose that SIDM, with a velocity-dependent cross-section, can reconcile the observed density profiles across multiple scales. By examining a range of astrophysical systems, the study provides a comprehensive framework for measuring the self-interaction cross-section as a function of kinetic energy.
2. Astrophysical Measurements: The paper reports a self-interaction cross-section that varies from approximately $\sigma/m \approx 2 \, \text{cm}^2/\text{g}$ at galaxy scales down to $\sigma/m \approx 0.1 \, \text{cm}^2/\text{g}$ at cluster scales. Such variation suggests a mildly velocity-dependent cross-section, constrained by observational data such as dwarf galaxies, low surface brightness (LSB) spiral galaxies, and relaxed galaxy clusters.
3. Analytical Halo Model: The paper introduces a semi-analytic model to describe SIDM halos, extending beyond phenomenological profiles to integrate observational data more accurately. This model determines the DM density profile by imposing hydrostatic equilibrium in the inner regions and conforms to a Navarro-Frenk-White-like profile externally.
4. Dark Photon Model: The study employs a dark photon model to illustrate the nature of self-interactions. It calculates the repercussions on DM and dark mediator particle masses, constrained through observed velocity-dependent cross-sections. Their results indicate a preference for a dark matter mass around 15 GeV and a dark photon mass of approximately 17 MeV.

Numerical Results and Constraints

The paper's results show a notable constraint on SIDM models using astrophysical data. They successfully measure the velocity-weighted cross-section per unit mass across diverse systems. In particular, the study’s application to astronomical data from halo structures delineates a clear outline of SIDM parameters that are aligned with simulator outputs. Thus, offering a progressive step towards understanding DM microphysics.

Practical and Theoretical Implications

• Astrophysical Alignment: The findings encourage significant reconsideration of CDM models, particularly in explaining small-scale structure anomalies. SIDM not only provides a suitable framework across diverse scales but also accommodates varying halo properties unaccounted for by CDM assumptions.
• Velocity Dependence and DM Physics: The velocity-dependent cross-section elucidates potential implications for the underlying particle dynamics. This insight advances the exploration of DM beyond gravitational interactions, further stimulating development in DM particle physics integrated with a consistent astrophysical paradigm.
• Future Directions: Further exploration of velocity dependencies in SIDM could potentially detail the nature of DM interactions more explicitly. Additionally, astronomical observations and large-scale simulations could enhance constraints on DM halo models, emphasizing the role of baryonic and non-baryonic interactions in shaping cosmic structures.

Conclusion

By leveraging astrophysical data as a substitute for particle colliders, the researchers exceptionalize their study of DM microphysics and propose a unified solution to DM density discrepancies observed from dwarf galaxies to clusters. This paper crucially advances the concept of cosmological structures being instrumental in unveiling the granular nature of DM, with significant implications for future theoretical and observational pursuits.