Survival of the most compact: the life and death of satellite halos in self-interacting dark matter

Abstract

Self-interacting dark matter (SIDM) models feature short-range interactions between dark matter (DM) particles that lead to larger diversity in the inner parts of galactic rotation curves and potentially unique gravitational lensing signatures. Satellite galaxies and dark subhalos provide a valuable testing ground for such models. We develop a simulation framework to explore subhalo evolution and its gravothermal collapse for velocity- and angle-dependent self-interacting cross section in these SIDM models. Our results are essential for testing these models. We perform N-body simulations, treating the host halo analytically and modelling the scattering-induced subhalo-halo interaction process using virtual host particles, a central innovation of our work. We use the Eddington inversion method to accurately model the local velocity distribution in the halo. Our approach is significantly less computationally expensive than simulations with a fully resolved host, while incorporating tidal stripping and tidal heating. We test both isotropic and forward-dominated self-scattering, which represent limiting cases for the angular dependence of the self-interaction cross section. Environmental effects, especially the scattering-induced subhalo-halo interaction, have a strong impact on the subhalo evolution and drive a complex structural evolution. As a result, SIDM subhalos have a larger range of central densities and density profile slopes compared to collisionless DM. Our cost-efficient simulation framework enables modelling of SIDM subhalos in realistic environments. Our results highlight the necessity of accurately modelling the scattering-induced subhalo-halo interaction to predict SIDM subhalo density profiles. For the SIDM models we investigate, the enhanced diversity in the mass profiles of subhalos would leave an observable imprint on strong lensing systems and satellite galaxies.

Figures (23)

• Figure 1: Left panel: Illustration of the evolution and the mechanisms acting on an isolated SIDM halo, which becomes bound to a host system over time. Right panel: Density map in the orbital plane of a subhalo on an elliptical orbit between $r_\mathrm{apocenter}=2 \, r_\mathrm{s, \, host}$ and $r_\mathrm{pericenter}= r_\mathrm{s, \, host}$. The radii $r_\mathrm{s, \, host}$ and $r_\mathrm{s, \ sub}$ are the scale radius of the initial NFW host halo and subhalo. The top row shows the evolution of a CDM subhalo, the middle row shows an SIDM subhalo without the SSHI process, and the bottom row shows the evolution with the SSHI process. Our simulations of both SIDM subhalos use an isotropic velocity-dependent cross section.
• Figure 2: Left panel: The adopted velocity-dependent cross sections are shown together with the typical relative velocities $\langle v_\mathrm{rel}\rangle$ within the host (black) and subhalo (grey). Right panel: Evolution of the core size $r_\mathrm{core}$ of an isolated halo with three different effective cross sections. The radius $r_\mathrm{core}$ is given in units of the scale radius of the initial NFW profile. For each effective cross section, we simulate a constant isotropic cross section (solid), a velocity-dependent isotropic cross section (dotted), and a velocity-dependent forward-dominated cross section (dashed).
• Figure 3: Evolution of the maximum circular velocity $v_\mathrm{max}$ and its corresponding radius $r_\mathrm{max}$. The results for various orbits and cross sections are displayed using the symbols as given in Table \ref{['tab:orbits']}. The colour indicates the DM core size, $r_\mathrm{core}$. The radii $r_\mathrm{max}$ and $r_\mathrm{core}$ are given in units of the scale radius of the initial NFW subhalo.
• Figure 4: Bound mass $M_\mathrm{bound}$ (top panel) and core size $r_\mathrm{core}$ (bottom panel) as a function of time. The radius $r_\mathrm{core}$ is given in units of the scale radius of the initial NFW subhalo. The three different orbits are distinguished by colour, and the corresponding pericentres are indicated by the dashed vertical lines. Each orbit is shown for CDM (solid) and for SIDM with isotropic scattering for two different cross sections (dotted and dashed).
• Figure 5: The left panel shows the distance to the centre of the host halo. The radius $r$ is given in units of the scale radius of the initial NFW host halo. The right panel shows the gradient of the velocity dispersion $\mathrm{d} v_\mathrm{dispr}/\mathrm{d}r$ within the initial $r_\mathrm{s}$ of the subhalos for the higher cross section $\sigma_\mathrm{eff}/m_\chi = 50 \ \mathrm{cm}^2 \ \mathrm{g}^{-1}$. The dashed vertical lines indicate the corresponding pericentres.