Searching for signatures of self-interacting dark matter in halos from full physics simulations: From 3D structure to projected observables

The gist In this work we explore the imprints of self-interacting dark matter (SIDM) on the structure of simulated galaxy groups and clusters in view of future optical surveys.

Introduction to SIDM Probes

Self-interacting dark matter (SIDM) was originally introduced by Spergel & Steinhardt (2000) as an alternative to CDM at small scales, in order to solve the core-cusp and missing satellites problems (e.g. de Blok 2009; Oman et al. 2015; Bullock & Boylan-Kolchin 2017). More generally, SIDM turned out to change various halo properties from dwarf galaxy (e.g. Vogelsberger et al. 2014) up to galaxy cluster (e.g. Sabarish et al. 2024; Robertson et al. 2019) scales, and these effects are only prevalent in the inner halo since, as the density decreases outward, collisions become rare and SIDM more closely resembles CDM (e.g., Planck Collaboration et al. 2020). The last years have seen an increasing number of studies about possible SIDM probes spanning many different observables (Adhikari et al. 2025), for example: on the field of Milky Way satellites (Correa 2021; Yang et al. 2023; Shah & Adhikari 2024; Roberts et al. 2019; Correa et al. 2025; Nadler et al. 2025; Silverman et al. 1997); ultra-faint (Fischer & Yu 2026; Almeida et al. 1984) or diffuse galaxies (Wang et al. 1984; Zhang et al. 1985) and strong galaxy-galaxy lensing perturbers Despali et al. (2022); Vegetti et al. (2024); Lei et al. (2025); Zhang et al. (2018); core buoyancy (van den Bosch & Dattathri 1986; 1986); DM deficent galaxies Wang et al. (1984); 3D central density of halos (Straight et al. 2025); brightest cluster galaxy dynamics (Harvey et al. 2025) on possible subhalo matter redistribution and stripping (Sirks et al. 1984; Tokayer et al. 1984; Chiang et al. 1986); on galaxy cluster mergers (Kim et al. 1984; Robertson et al. 2007; Fischer et al. 2023); and on baryon and DM misalignment (Sirks et al. 1984).

SIDM Model Framework

The study focuses on two representative classes of SIDM models defined by the angular structure of the differential scattering cross-section per unit mass: rare SIDM (rSIDM), where collisions lead to an efficient exchange of momentum between particles, and frequent SIDM (fSIDM), which requires a significantly higher collision rate due to small deflection angles (e.g. Kahlhoefer et al. 2014). The models considered assume a velocity-dependent cross-section, motivated by the difficulty of reconciling constraints across different astrophysical scales with a velocity-independent interaction (Silverman et al. 2023; Lovell & Zavala 2023; Kaplinghat et al. 1986). The transfer cross-section is expressed as:

σT(vrel)/mDM = σT,0/mDM [1 + vrel 2/vc 2]-2 (Equation 2) where the normalization σT,0 corresponds to the value of a low-velocity plateau of the cross-section, and vc represents the transition velocity after which the cross-section falls as v-4rel (Page 3). The simulations set this normalization to σT,0/mDM = 40 cm2 g−1 and vc = 200 km s−1 (Page 3).

Simulation Setup and Analysis

The research utilized the Dianoga 1x SIDM simulations first presented in Ragagnin, A. et al. (2024), which includes zoom-in regions of the same parent box of comoving length 1 Gpc, assuming a ΛCDM initial power spectrum with cosmological parameters omegam = 0.24, omegab = 0.0456, ns = 0.96, σ8 = 0.8 and h = 0.72 (Bonafede et al. 2011). The DM mass resolution is set to mDM≃ 1.3 × 109 M⊙ for all simulations (Page 3). Six simulated zoom-in regions, each associated with a different galaxy cluster encompassing masses of M200/M⊙ ∈ h 14, h 15i (see Table 1), were re-simulated with a CDM, rSIDM and fSIDM model, both as a DMO run and also including baryons (denoted "FP for full-physics") (Page 3). The mass and density profiles are generated by dividing the particles in 50 logarithmically-spaced radial bins inside the range r/r500c ∈ h-3, 5i for both 3D and projected profiles (Page 4).

Key Findings on Halo Profiles

The main results of the study can be summarized as follows:

1. In lower-mass halos, SIDM effectively produces more cored internal 3D profiles than CDM, with relative differences of ∼ 20% in the DMO case (Page 5). The FP case results in smaller deviations since baryons counteract the effects of SIDM (Page 5).

2. Both DMO and FP SIDM profiles are also characterized by a ∼ 5% mass excess at intermediate radii – especially evident in the density profiles – as a consequence of heat transfer from collisions (Page 5).

3. The most massive SIDM halos are more affected by baryons, leading to cuspier inner profiles than CDM and a much less pronounced mass excess at larger radii (Page 5).

4. While projecting the mass and density profiles along the line of sight alleviates the effects of SIDM at all halo masses – leading to cuspier profiles than the 3D case – the weak lensing shear (surface density excess) profiles still reach deviations of up to ∼ 20% above resolution limit for M200 ∼ 1013 M⊙ halos, along all lines of sight considered (Page 7).

Observational Viability and Constraints

The study suggests that weak lensing inference of the surface density excess profiles could prove particularly useful for detecting SIDM effects in M200 ∼ 1013 M⊙ clusters, although further modeling of observational systematics is required for a more realistic detection analysis (Page 7). The median NFW concentration remains only slightly affected by SIDM at all mass scales, with deviations at the ≲ 5% level in the projected FP case (Page 9).

Conclusion and Future Work

The study concludes that while SIDM imprints on halo mass and density profiles are washed out from both baryons and 2D projection – thus reducing the observational viability of such probes – we also show that there are certain observationally promising radial scales that may potentially act as robust probes of such SIDM effects through weak lensing (such as in the internal regions of group-sized halos, see Fig. 6) and galaxy-traced mass reconstructions (such as in the cores of very massive clusters, see Fig. 8) (Page 10). Future work will focus on studying the velocity dispersion profiles of halos to assess if the halo velocity anisotropy could potentially represent a viable probe for the detection of SIDM (Page 10).

Concentration-Mass Relation

The median NFW concentration remains only slightly affected by SIDM at all mass scales, with deviations at the ≲ 5% level in the projected FP case (Page 9). The comparison of median concentrations between all models is shown in Fig. 9 (Page 11).

Comparison with Observations

The comparison between our theoretical profiles and the reconstructed mass distribution of real galaxy clusters shows that, while none of our individual halos match perfectly with the observed profiles, the observational uncertainty in the cluster core might potentially be small enough to allow for distinction of SIDM models (Page 8). The cores of very massive clusters might offer the opportunity for precise constraints on SIDM models (Page 9).

Summary

In this work we conducted an analysis of DM halo profiles of groups and clusters of galaxies in order to investigate the complex interplay between baryons and DM in the SIDM framework, aiming to inform future weak lensing and galaxy kinematics observational analyses on which halo properties are most sensitive probes of SIDM (Page 10). We performed stacking and fitting to robustly explore the inner regions of halos (r ≲ r500c) where SIDM is expected to play a prominent role (Page 10). The main results of our study can be summarized as follows:

– In lower-mass halos, SIDM effectively produces more cored internal 3D profiles than CDM, with relative differences of ∼ 20% in the DMO case (Page 5).

1. Improved AI system can perform robust parameter estimation for dark matter self-interactions by using weak lensing shear mass reconstructions as a primary probe, which may potentially serve as a robust probe of SIDM in M200 1013 Msun halos.

2. The AI can distinguish between DM models by analyzing projected surface density excess profiles, noting that these are characterized by a greater SIDM suppression at small radii compared to 3D profiles, which allows for a clearer deviation from CDM.

3. The system can assess the impact of baryonic physics on halo structure, specifically noting that the addition of baryons results in a significant increase in the halo central mass and density as a result of the baryonic gas cooling and clustering in the center of the DM potential well.

4. The AI can perform high-fidelity internal DM slope reconstructions using kinematic data by combining it with gravitational lensing, as suggested by "mass reconstruction techniques based on the kinematics of member galaxies, and their combination with gravitational lensing (Mamon et al. 2013; Pizzuti et al. 2021) can provide high-fidelty internal DM slope reconstructions."

5. The system can evaluate the viability of current observational probes by noting that the cores of very massive clusters might offer the opportunity for precise constraints on SIDM models because the observational uncertainty in the cluster core might potentially be small enough to allow for distinction of SIDM models.

In this work we explore the imprints of self-interacting dark matter (SIDM) on the structure of simulated galaxy groups and clusters in view of future optical surveys. We find that both baryonic and projection effects lead to a significant alleviation of the SIDM core-forming behavior on the mass and density profiles, resulting in very small (5%) deviations from standard collisionless dark matter. The opposite trend emerges for our most massive halos, where SIDM tends to significantly enhance the central density instead; we compare those halos with the observed dark matter profiles of two galaxy clusters, MACS J1206 and Abell S1063, finding that SIDM may leave potentially detectable imprints on cluster cores when considering the small observational uncertainty of current kinematic mass measurements. Finally, we find that weak lensing observations provide a promising tool for testing SIDM on cluster scales: in an idealized scenario where systematics are under control, weak lensing shear profiles exhibit up to about 20% level deviations for M 200 10 13 M halos. Conversely, the projected concentration-mass relation is only slightly modified by our SIDM models. We also discuss the limitiations of our analysis and the impact of the simulation resolution, as well as possible extension of this work. Overall, our results provide a baseline for future SIDM studies: although line-of-sight projection and baryonic effects significantly reduce SIDM imprints on halo mass profiles, weak lensing and kinematic analyses in galaxy clusters may potentially allow for robust detection and constraining of dark matter self-interactions in massive structures from megaparsec down to kiloparsec scales.