Self-interacting dark matter in the center of a Local Group dwarf galaxy and its satellites

TL;DR

The paper investigates SIDM with velocity-dependent cross sections in a Local Group dwarf analogue using high-resolution cosmological hydrodynamic zoom-ins. It shows that SIDM forms a central dark matter core of $r_ ext{core} \approx 1~\mathrm{kpc}$ that is robust to baryonic physics, while baryons respond with extended gas distributions and a less centrally concentrated stellar component, reducing the total stellar mass by about $25\%$. Substructure is markedly different, with about half as many luminous satellites and a large increase in stellar-only star clusters; tidal stripping accelerates core collapse in some satellites, producing a broader diversity of rotation curves. These results suggest observable SIDM signatures in low-mass galaxies and highlight the crucial roles of environment and baryonic physics in shaping SIDM phenomenology.

Abstract

We present a detailed comparison of a Local Group dwarf galaxy analogue evolved in two cosmological models: the standard $Λ$CDM and a self-interacting dark matter (SIDM) model with a velocity-dependent cross-section. Both simulations are run with the high-resolution, hydrodynamical LYRA galaxy formation model, allowing us to explore the global and substructure properties of the dwarf in a consistent context. While the overall halo growth, final mass, and subhalo mass functions remain largely unchanged across models, SIDM produces a central dark matter core extending to $\sim$1 kpc, which does not significantly vary with the inclusion of baryons. Baryonic properties, however, differ notably. The SIDM model leads to a 25% reduction in stellar mass and retains more gas within the stellar half-mass radius due to a prolonged quiescent phase in star formation. The stellar distribution is less centrally concentrated, and a population of in-situ star clusters form at late times. Substructure analysis reveals fewer luminous satellites and more stellar-only systems in SIDM, driven in part by tidal stripping that affects the dark matter more than the stars. A subset of satellites undergoes tidal-triggered core collapse after infall, enhancing the diversity of SIDM satellite rotation curves. These differences offer potential observational signatures of SIDM in low-mass galaxies.

Figures (11)

• Figure 1: Three SIDM models with differing velocity dependency. The top panel shows the cross-section as a function of the relative velocity of particles, which is constant for the "SIDM C" (light blue) model and instead has a steep dependence on velocity in "SIDM S" (green) and "SIDM T" (orange). The bottom panel shows the dark matter density profile at $z=0$ of the halo in the dark-matter-only run. In this work, we will study the effects of the "SIDM S" (green) model and its interaction with baryonic physics in more detail.
• Figure 2: Top: Dark matter density profiles. For the DMO simulations, we have reduced the mass by $(1-\Omega_\mathrm{B})$. The SIDM model produces a uniform density region at the center ("core") out to a radius of approximately $1\mathrm{kpc}$. The grey solid line designates 3 times the DM softening length. The dashed gray line encloses 2000 DM particles. Middle: SIDM profile divided by $\Lambda$CDM profile for hydrodynamical and dark matter-only simulations. Core size as shown in legend is defined as the profile ratio reach $90\%$. It is a common SIDM prediction to see a "bump" in the relative profiles where the excess DM that has been pushed out of the core piles up. Bottom: The evolution of the SIDM profile over time. The core is formed very early, before $z=8$, and grows in size as the halo grows with time.
• Figure 3: Ellipticity of the DM measured within $100~\mathrm{pc}$ and within _200 $R_{200}$.
• Figure 4: Top: Neutral hydrogen density projection at $z=0$. Due to the larger core ($\sim 1\mathrm{kpc}$) in the SIDM run, the dense, star forming gas is more extended. Bottom: Stellar surface density in the central 4 kpc. Due to the larger core, the SIDM run has less centrally concentrated SF, instead displaying more, smaller star formation regions, extending out to around $1\mathrm{kpc}$, the approximate size of the DM core.
• Figure 5: Top: Gas density and HI density profiles. Bottom: Stellar surface density profile. Due to the DM core, the SIDM run has less centrally concentrated SF, instead displaying more numerous and smaller star formation regions. The lower stellar surface density extends out to around $50\mathrm{pc}$, only about 5% of the size of the DM core.