Diversifying halo structures in two-component self-interacting dark matter models via mass segregation

Abstract

Self-interacting dark matter (SIDM), through gravothermal evolution driven by elastic self-scatterings, offers a compelling explanation for the observed diversity of inner halo densities. In this work, we investigate SIDM dynamics in a two-component dark matter model with mass ratios of order unity, motivated by an asymmetric dark matter framework that naturally evades constraints from relic abundance and mediator decay, while enabling strong, velocity-dependent self-interactions. We show that cross-component scatterings significantly enhance mass segregation, driving the formation of dense, core collapsed-like halos. This effect couples naturally to SIDM-induced diversity, introducing a new mechanism for generating structural variations beyond those arising from gravothermal evolution alone. Our results reveal a novel mechanism for reconciling SIDM with small-scale observational tensions by enabling shifts in central densities while preserving the flexibility to generate diverse halo structures. We further highlight that halo structural diversity may serve as a diagnostic of dark sector composition, opening a new observational window into the particle nature of SIDM.

Figures (4)

• Figure 1: Projected dark matter densities of MW analogs in the two-component SIDM2c simulation. Densities of the heavier (blue-to-white) and lighter (red-to-white) species are shown, with brighter regions indicating higher densities. The inset shows viscosity cross sections Tulin:2013teo for the $\chi_H-\chi_H$ (red), $\chi_L-\chi_L$ (orange), and $\chi_H-\chi_L$ (magenta) interactions in the SIDM2c simulation. For comparison, the $\chi_0-\chi_0$ (blue) interaction in the one-component case is shown as a dashed curve. All channels assume the same mediator mass and coupling. Model parameters for these interactions can be derived by fixing $\sigma_0/m = 147.1~\rm cm^2/g$ and $w = 24.33~\rm km/s$ in the $\chi_0\text{--}\chi_0$ case; see Supplemental Material for details.
• Figure 2: Halo density profiles of MW subhalos of $M_{\rm vir}>5\times 10^8~\rm M_{\odot}/h$ in simulated models. The main panels show CDM1c (green) and SIDM1c (orange) on the left panel, SIDM2c (red) and SIDMx (magenta) on the middle and right panels, respectively. The CDM2c (blue) is shown in all panels for comparison. The smaller sub-panels display the fractional number density of the lighter species, $n_L/(n_H+n_L)$. While the CDM2c case closely resembles the CDM1c scenario, with only slightly shallower inner regions, SIDM significantly redistributes the lighter species to larger radii, leading to denser inner regions dominated by the heavier species. In the SIDM2c case, self-interactions among the heavier species introduce both core and cusp profiles.
• Figure 3: Extrapolated inner halo density, $\rho_{\rm in}=\rho(r=150{\ \rm pc})$, vs fractional number density, $f_L(<0.2 R_{\rm vir})$, for MW subhalos of $M_{\rm vir}>5\times 10^8~\rm M_{\odot}/h$ in two-component simulations SIDM2c (red), SIDMx (magenta), and CDM2c (blue). For comparison, the $\rho_{\rm in}$ of observed MW satellites, derived from Ref. kaplinghat:2019svz under the assumptions of NFW (gray) and isothermal (brown) profiles, are shown as tick marks along the y-axis, with their Gaussian fits overlaid to highlight the probability distributions. The size of the points is proportional to the virial radius of the corresponding halos.
• Figure 4: The parametric model fitting results (solid black) for the density profiles of MW subhalos with $M_{\rm vir}>5 \times 10^8~\rm M_{\odot}/h$ in CDM2c (left), SIDM2c (middle), and SIDMx (right) simulations. The relative differences between the simulated (Sim) and fitted (Fit) curves are measured as $2\rm (Fit-Sim)/(Fit+Sim)$ and shown in the sub-panels. Compared with the profiles in the main article, more bins are used here to improve the fit. The fitted curves are used to obtain the $\rho_{\rm in}=\rho(r=150{\ \rm pc})$, as referenced in the main text.