Self-Interacting Dark Matter with Mass Segregation: A Unified Explanation of Dwarf Cores and Small-Scale Lenses

TL;DR

This work presents a two-component SIDM model with mass segregation, where a heavier and a lighter dark matter species exchange energy through inter- and intra-species scatterings, producing denser inner regions in dwarfs while maintaining cluster-scale constraints. By developing a conditioned universality and a parametric SIDM2c model calibrated to gravothermal evolution (including baryonic contraction via a form factor), the authors show that mass segregation can both form cores in dwarf halos and enhance strong-lensing signals through larger Einstein radii. Their results demonstrate that two-component SIDM with inter-species interactions can simultaneously address dwarf-core, core-collapse, and GGSL excess challenges, with significant cross-section enhancements in lensing even within cluster bounds. The study provides a practical, testable framework with predictions for GGSL cross sections, Einstein radii distributions, and halo-density evolutions, offering a unified pathway to understanding small-scale structure across cosmic scales.

Abstract

In two-component self-interacting dark matter (SIDM) models with inter-species interactions, mass segregation arises naturally from collisional relaxation, enhancing central densities and gravothermal evolution. We demonstrate that models with velocity-dependent interactions, both within and between species, can connect several small-scale observations while remaining consistent with cluster-scale constraints. This combination enables core formation in dwarf halos, where the presence of baryons increases the inner densities and enhances the predicted strong lensing signatures. Using cosmological and controlled simulations alongside an accurate parametric model, we show that this framework explains the structure of dark perturbers observed in strong lensing systems, and significantly increases the efficiency of small-scale lenses by a factor of $\sim 5$, consistent with the galaxy-galaxy strong lensing excess reported in clusters. Importantly, mass segregation can enhance the Einstein radii of SIDM halos relative to their CDM counterparts, overcoming a key challenge in one-component SIDM scenarios. Our results establish mass segregation in two-component SIDM as a self-consistent and testable model capable of simultaneously addressing multiple small-scale challenges in structure formation.

Figures (12)

• Figure 1: Effective cross section per mass $\sigma_{\rm eff}/m$ as a function of $V_{\rm max}$ for various SIDM models. The self-interaction among the heavier component ($\chi_H-\chi_H$) and the inter-species interaction ($\chi_H-\chi_L$) in the SIDM2v model are represented by red and magenta solid curves. This model is featured by its capability of reproducing SIDM effects analogous to one dark matter component with $\sigma/m = 0.3 ~{\rm cm^2/g}$ (SIDM03) in massive dwarf halos of mass around $M_h = 10^{11}~\rm M_{\odot}$, as favored by the findings of Ref. Zhang:2025bju. The two orange stars feature the two kinds of strengths in the two-component SIDM2c model, which takes constant effective cross sections as in SIDM2v in a $M_h = 10^{11}~\rm M_\odot$ halo with median concentration. For $V_{\rm max} \gtrsim 200~\mathrm{km/s}$, the inter-species interaction dominates over the intra-species interactions and remains significant up to the cluster scale at $V_{\rm max} \approx 1600~\mathrm{km/s}$. In clusters, the coexistence of both types of scattering yields density profiles consistent with observations. Particularly, the intra-species interaction remains well below $0.1~\rm cm^2/g$, as indicated by the light-red arrow.
• Figure 2: Rescaled halo density profiles at 60% of the core collapse time $\tau\equiv t/t_c=0.6$, compared to a single-component (SIDM1c) model at the same evolution stage. The profiles of three halos of mass $2\times10^{10}~M_{\odot}$ (solid) and three of mass $10^{11}~M_{\odot}$ (dashed) with median, $+1\sigma$, and $-1\sigma$ concentrations have almost identical shapes, illustrating the high quality of universality supporting the construction of the parametric model.
• Figure 3: Dwarf halo evolution and core formation in one- and two-component SIDM. Left: Density profiles for the median halo with a collisionless stellar component at $t=0$, $3$, $6$, and $9~$Gyr, with deeper colors corresponding to later times. Baryons (dotted tomato lines) induce higher DM densities (solid) compared to DM-only cases (dashed). Unlike DM-only evolution, central density increases even as the cores grow in size. The parametric model predictions are shown in dash-dotted lines. Right: Evolution of core size ($r_c$) for the median halo with (solid and dashed) and without (dotted) baryons. The core growth in SIDM2c remains comparable to that in SIDM03 in the presence of baryons.
• Figure 4: Distribution of the gravothermal phase $\tau$ from parametric model predictions for the SIDM2c model. The sample includes halos of masses higher than $10^{10}~\rm M_{\odot}$ and within $5~\mathrm{Mpc}/h$ of the host cluster, separated into cluster subhalos (Group A, light red) within the cluster's virial radius and isolated halos (Group B, light blue). Means and standard deviations for both groups are shown in the inset. Most halos in both groups have cored inner profiles ($\tau < 0.6$). Cluster subhalos, however, show a greater frequency of core collapsed systems ($\tau > 1$), as a consequence of tidal stripping.
• Figure 5: Projected logarithmic density slope $\gamma_{\rm 2D}$ (averaged over $0.75$--$1.25\,\mathrm{kpc}$) vs. projected mass within 1 kpc for benchmark halos: M11 ($10^{11}\,M_\odot$, red), M08 ($8 \times 10^{10}\,M_\odot$, green), and M05 ($5 \times 10^{10}\,M_\odot$, blue), all with $+2.5\sigma$ concentration. Cyan region denotes the $95\%$ confidence contour enclosed for the SDSSJ0946+1006 lensing perturber from Ref. minor:2020hic. Arrows trace the evolution of each halo under SIDM2c, which naturally drives systems into the favored region within a Hubble time. For comparison, we also show subhalos of the cluster host halo in the SIDM2c model. Their density profiles are obtained using the SIDM2c parametric model, whose functional form limits $\gamma_{\mathrm{2D}} \lesssim 1.5$.