MARVELously Dark: the gravothermal evolution of dwarf halos in velocity-dependent SIDM

Self-interacting dark matter (SIDM) with a sufficiently large cross section has been shown to naturally produce constant dark matter (DM) cores, as well as core-collapse, at the centers of dwarf halos on cosmic timescales, potentially reducing tensions with observation. Here, we present halos from a new dark matter only (DMO) cosmological (SIDM) simulation: Ms.Marvel DMO with a velocity-dependent self-interaction cross section with $σ/m_\text{max} = 50$ cm$^2$/g at $v_\text{max} = 35$ km/s. We compare these to the CDM suite of Storm simulations including both DMO and dark matter + hydrodynamics runs, in order to test core-formation (and core-collapse) across different dark matter models. We show that Ms.Marvel DMO can reproduce core slopes consistent with observations of isolated dwarf galaxies and more massive ($\text{M}{vir} \gtrsim 10^{10} M{\odot}$) CDM dwarf halos that include stellar feedback from the matched CDM run (Storm CDM+baryons). We identify nine Ms.Marvel SIDM DMO halos in the core-collapse phase of gravothermal evolution with halo masses below $2\times 10^9 M_{\odot}$. We find that using core slope to measure the core-collapse timescales of Ms.Marvel DMO halos agrees well with predicted collapse times estimated with the parametric model for SIDM halos introduced by Yang et al.(2023). Additionally, compared to central density, core slope is less sensitive to both the radius of measurement and halo merger history. These results indicate that the slope of the inner DM density profile more cleanly differentiates core-collapsed versus core-forming halos than central density amplitude.

💡 Research Summary

This paper presents a new dark‑matter‑only (DMO) cosmological simulation, Ms.Marvel DMO, that implements a velocity‑dependent self‑interacting dark matter (SIDM) model. The model adopts a Yukawa‑type interaction with a transfer cross‑section that peaks at σmax/mχ = 50 cm² g⁻¹ for a characteristic circular velocity vmax = 35 km s⁻¹. In the classical dwarf regime this yields effective cross‑sections of order 10–100 cm² g⁻¹, sufficient to drive strong heat conduction in halo interiors.

The authors compare 228 isolated halos from Ms.Marvel DMO with 235 CDM DMO halos and 20 CDM+hydrodynamics (“Storm”) galaxies that share the same initial conditions. All halos contain at least 10⁴ dark‑matter particles, giving a mass resolution of 8.07 × 10³ M⊙ and a force softening of 65 pc. Halo properties are identified with the Amiga Halo Finder, and the inner density profile is characterized by two diagnostics: (i) the slope α = d log ρ/d log r measured between 0.5 % and 1 % of the virial radius, and (ii) the central density ρ₀ at the same radius.

Key findings are:

1. Core formation in massive dwarfs – For halos with Mvir ≳ 10¹⁰ M⊙, the SIDM simulation produces shallow inner slopes (α ≈ −0.3 to −0.5) that match the cores generated by stellar feedback in the Storm CDM+baryons run and are consistent with observed isolated dwarf galaxies (e.g., LITTLE THINGS).

2. Gravothermal collapse in low‑mass dwarfs – Nine halos with Mvir ≲ 2 × 10⁹ M⊙ exhibit steep inner slopes (α ≈ −0.9 to −1.2), indicating they have entered the gravothermal collapse phase. Their central densities are 2–5 times higher than comparable CDM halos, reproducing the high‑density cores inferred for ultra‑faint dwarfs.

3. Slope versus central density as a collapse indicator – The evolution of α with cosmic time is far less sensitive to the exact measurement radius or to recent merger events than ρ₀. Consequently, α provides a more robust estimator of the collapse timescale.

4. Validation of analytic collapse models – The authors compute expected collapse times for each halo using the parametric model of Yang et al. (2023). The collapse times inferred from the measured α agree with the analytic predictions to within ~10–30 %, confirming the model’s applicability to a statistical sample of isolated dwarf halos with diverse merger histories.

5. Observational relevance – By comparing the simulated inner slopes to rotation‑curve data from surveys such as SPARC and LITTLE THINGS, the SIDM model reproduces the observed diversity of dwarf rotation curves, including both cored and cuspy systems. This demonstrates that SIDM can simultaneously address the “cusp‑core” and “too‑big‑to‑fail” problems without invoking finely tuned baryonic feedback.

The study acknowledges resolution limits: while halos are resolved with ≥10⁴ particles, prior work suggests ≥10⁵ particles may be required to fully capture the late stages of core collapse. Appendix C shows that the spatial resolution (≈0.33 kpc) is adequate for the density slopes considered, but higher‑resolution runs are needed to confirm the detailed inner structure of the smallest halos.

In conclusion, the velocity‑dependent SIDM model implemented in Ms.Marvel DMO naturally yields both constant‑density cores in more massive dwarfs and rapid gravothermal collapse in low‑mass dwarfs. The inner density slope emerges as a powerful, radius‑independent diagnostic that cleanly separates collapsed from non‑collapsed systems, outperforming central density amplitude. These results strengthen the case for SIDM as a viable alternative to CDM on dwarf‑galaxy scales and provide concrete predictions for upcoming observations with Rubin LSST, ELT, and future strong‑lensing surveys.