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

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 $\sigma /m\approx 2{\mathrm{cm}}^2/\mathrm{g}$ on galaxy scales to $\sigma /m\approx 0.1{\mathrm{cm}}^2/\mathrm{g}$ 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.

Figure 1

Self-interaction cross section measured from astrophysical data, given as the velocity-weighted cross section per unit mass as a function of mean collision velocity. Data include dwarfs (red), LSB galaxies (blue), and clusters (green), as well as halos from SIDM $N$-body simulations with $\sigma /m=1{\mathrm{cm}}^2/\mathrm{g}$ (gray). Diagonal lines are contours of constant $\sigma /m$ and the dashed curve is the velocity-dependent cross section from our best-fit dark photon model (Sec. 5).

Figure 2

Top: SIDM density profile fit to cluster A2537 (orange) compared to the NFW profile (cyan) and comparison to stellar kinematics data (inset). Bottom: SIDM fit to the rotation curve of galaxy IC2574 (orange) with contributions from the SIDM halo (solid), the gas disk (dashed), and the stellar disk (dotted).

Figure 3

Parameter space for the dark photon model of self-interactions (with ${\alpha }^{\prime }={\alpha }_{\mathrm{EM}}$), preferred by dwarfs (red), LSB spiral galaxies (blue), and clusters (green), each at 95% C.L. The combined 95% (99%) region is shown by the solid (dashed) contours. The estimated Bullet Cluster excluded region lies below the dot-dashed curve and the ensemble merging cluster excluded region below the long-dashed curve.