New CDM Crisis Revealed by Multi-Scale Cluster Lensing

TL;DR

Massive galaxy clusters provide a stringent test of dark matter microphysics by probing subhalo structure across scales from tens of kiloparsecs to a few kiloparsecs. The authors compare lensing-derived subhalo properties of three clusters with CDM predictions from the TNG-Cluster simulations, focusing on the subhalo mass function, radial distribution, inner density profiles, and tidal truncation radii. They find agreement with CDM for the SHMF and outer extents but strong tensions in inner density profiles and inner subhalo distributions, implying GGSL requires steep inner slopes ($γ \gtrsim 2.5$) consistent with SIDM core collapse; while outer radii constraints disfavour strongly collisional SIDM, suggesting a hybrid or new DM theory. The results motivate models with inactive self-interactions in halos’ outskirts but active interactions in dense cores, and call for larger cluster samples and higher-resolution simulations to robustly test these ideas.

Abstract

The properties of substructure in galaxy clusters, exquisitely probed by gravitational lensing, offer a stringent test of dark matter models. Combining strong and weak lensing data for massive clusters, we map their total mass--dominated by dark matter--over the dynamic range needed to confront small-scale predictions for collisionless cold dark matter (CDM). Using state-of-the-art lens models, we extract four key subhalo properties: the mass function, projected radial distribution, internal density profile, and tidal truncation radius. We find that the subhalo mass function and truncation radii are consistent with CDM expectations. In contrast, the inner density profiles and radial distribution of subhalos are strongly discrepant with CDM. The incidence of galaxy-galaxy strong lensing (GGSL) from subhalo cores exceeds CDM predictions by nearly an order of magnitude, requiring inner density slopes as steep as $γ\gtrsim 2.5$ within $r \lesssim 0.01\,R_{200}$ consistent with core-collapsed self-interacting dark matter (SIDM), while the same subhalos behave as collisionless in their outskirts. Additionally, the observed radial distribution of subhalos hosting bright cluster member galaxies, explicitly modeled in the lens reconstructions, remains incompatible with CDM. Together, these small-scale stress tests reveal an intriguing paradox and challenge the dark matter microphysics of purely collisionless CDM and motivate hybrid scenarios, such as a dual-component model with both CDM and SIDM, or entirely new classes of dark matter theories.

Figures (4)

• Figure 1: The mass function of subhalos associated with spectroscopically confirmed bright cluster member galaxies in observed lensing clusters (Lenstool, color-shaded) compared with CDM predictions from simulated analogs (TNG-Cluster, solid lines) under the same selection criteria; see Section \ref{['sec:sim_tngc']} for details.
• Figure 2: Top panel: The projected radial distribution of subhalos associated with spectroscopically confirmed member galaxies in observed clusters (color-shaded) and in simulated analogs (solid lines; averaged over five best-matched analogs and over three random projections). Bottom panels: The projected spatial distribution of subhalos around cluster centers in observations (left column) and simulated analogs (right column; from the best mass-matched analog along a random projection), annotated by the respective $V$-band absolute magnitude of associated galaxies. As compared to observations, simulated analogs clearly show a dearth of, and fail to reproduce the spatial clustering, of inner substructures within $R/R_{200}\lesssim 0.2$.
• Figure 3: Tidal radii from lensing-based observational inferences (gray line; shading indicates conservative $5\sigma$ uncertainties), overlaid with individual CDM (blue horizon dashes) and strongly collisional SIDM (red; conservative upper bounds) predictions over a sample of 418 spectroscopically confirmed member galaxies across eight massive clusters (Abell 2218, 383, 963, 209, 2390, and MACS J0416, J1206, J1149). We also plot the respective median (circles), central 68% (triangles), and central 95% ranges (horizontal bars). Lensing inferences are fully statistically consistent with CDM and discrepant with SIDM.
• Figure 4: Schematic illustrating the implications for CDM and SIDM models. Left panel: Collisionless CDM matches subhalo tidal extents but severely under-predicts GGSL. Right panel: Core-collapsed SIDM yields steep inner profiles that match GGSL, but this strongly collisional regime also results in more pronounced mass stripping that significantly reduces tidal extents even after accounting for the slight expansion of the outskirts due to outward energy transfer during core collapsing. The inset axes show the run of $\log\rho$ versus $\log r$ (not to scale).