Strong Lensing Perturbers from the SIDM Concerto Suite

TL;DR

This work analyzes the SIDM Concerto zoom-in simulations to study how gravothermal evolution—spanning core expansion to core collapse—modifies the inner structure of halos and their lensing signatures. By computing the projected enclosed mass $M_{2D}$ and the projected density slope $\\gamma_{2D}$ at $R=1$ kpc for subhalos and field halos across multiple environments, the authors compare SIDM predictions against CDM and observed strong-lensing perturbers (J0946, B1938, SDP.81, SPT2147-50). They demonstrate that core-collapsed SIDM halos can naturally attain high central densities and steep inner slopes, providing plausible analogs to the lensing perturbers and offering a pathway to constrain velocity-dependent SIDM cross sections via lensing data. The study also highlights the role of tidal forces and merger history in accelerating or delaying gravothermal evolution, and it advocates forward-modeling lensing signals with high-resolution SIDM halos to exploit future surveys for robust SIDM tests.

Abstract

Motivated by recent detections of low-mass perturbers in strong gravitational lensing systems, we investigate analogs of these objects in the Concerto suite, a set of cosmological N-body zoom-in simulations of self-interacting dark matter (SIDM) with high-amplitude, velocity-dependent cross sections. We investigate characteristic halo properties relevant to gravitational imaging measurements, focusing on the projected enclosed mass and the central density slope. In SIDM, these quantities evolve continuously through gravothermal processes, spanning core-expansion and core-collapse phases, in sharp contrast to cold dark matter, where they remain nearly static after halo formation. This SIDM evolution further depends on tidal environment and merger history, which can be probed through strong lensing. We also identify simulated SIDM halos whose properties are consistent with the properties of low-mass perturbers inferred from recent observations, and we demonstrate that the core-collapse mechanism offers a compelling explanation for their observed high densities. Our results highlight the potential of strong gravitational lensing as a powerful probe of dark matter self-interactions.

Figures (13)

• Figure 1: Effective dark matter self-interaction cross sections as a function of the halo maximum circular velocity in GroupSIDM-70 (orange) and GroupSIDM-147 (red) models. The shaded region shows the $V_{\rm max}$ range derived from the observed strong-lensing perturber models we consider in this work.
• Figure 2: Distributions of effective concentration (Eq. \ref{['eq:ceff']}) as a function of peak mass for subhalos (top) and field halos (bottom) in the Group (circles), MW (pentagons), and LMC (triangles) simulations, shown for CDM (left) and GroupSIDM-147 (right). Halos with more than $2000$ particles are shown as large filled markers, and those with $1000\textup{--}2000$ particles as small open markers.
• Figure 3: Profiles of the 3D density (left), surface density (middle) and projected enclosed mass (right) for two representative SIDM subhalos (red) and their CDM counterparts (gray) at $z=0$ in Group CDM and GroupSIDM-147 simulations. The solid and dashed curves denote subhalos with masses $2\times10^{9}~{\rm M_\odot}$ and $3.5\times10^{9}~{\rm M_\odot}$ at $z=0$, respectively. In the left panel, the vertical dashed line marks the resolution limit $2.8\epsilon = 0.67~\mathrm{kpc}$.
• Figure 4: Probability distributions of the projected density slope $\gamma_{\rm 2D}$ of subhalos at redshifts $z=0.89$ (blue), $0.31$ (green), $0.23$ (pink), and $0$ (brown) in the Group simulations for CDM (left), GroupSIDM-70 (middle), and GroupSIDM-147 (right). We include all subhalos resolved with more than $1000$ particles, corresponding to masses above $4\times10^{8}~{\rm M_\odot}$.
• Figure 5: Evolution of the projected enclosed mass $M_{\rm 2D}$ and density slope $\gamma_{\rm 2D}$ and for example subhalos (left), non-growing field halos (middle), and growing field halos (right). The subhalos and growing field halos evolve from $z=0.94$ to $z=0$ (solid). The non-growing halos evolve from $z=4.52$ to $z=0.94$ (dashed) and then to $z=0$ (solid). Gray, orange, and red lines correspond to CDM, GroupSIDM-70, and GroupSIDM-147 halos, respectively. Black arrows indicate the direction of evolution.