Cosmological Simulations with Self-Interacting Dark Matter II: Halo Shapes vs. Observations

TL;DR

This work revisits the constraints on velocity-independent SIDM using cosmological simulations, showing that prior cluster-shape bounds were too restrictive due to projection effects and outer-halo triaxiality. By carefully translating 3D halo shapes into observable quantities and examining lensing and X-ray data, the authors demonstrate that cross sections up to around $0.1\ \mathrm{cm^2/g}$ can be consistent with current observations, while 1 cm^2/g is generally disfavored in clusters and elliptical galaxies. A companion paper (Rocha et al. 2012) further analyzes density profiles and substructure, supporting the viability of SIDM0.1 to address several small-scale structure problems. Looking ahead, ensemble cluster lensing and X-ray shape analyses, together with baryonic physics, are identified as promising avenues to tighten SIDM constraints.

Abstract

If dark matter has a large self-interaction scattering cross section, then interactions among dark-matter particles will drive galaxy and cluster halos to become spherical in their centers. Work in the past has used this effect to rule out velocity-independent, elastic cross sections larger than sigma/m ~ 0.02 cm^2/g based on comparisons to the shapes of galaxy cluster lensing potentials and X-ray isophotes. In this paper, we use cosmological simulations to show that these constraints were off by more than an order of magnitude because (a) they did not properly account for the fact that the observed ellipticity gets contributions from the triaxial mass distribution outside the core set by scatterings, (b) the scatter in axis ratios is large and (c) the core region retains more of its triaxial nature than estimated before. Including these effects properly shows that the same observations now allow dark matter self-interaction cross sections at least as large as sigma/m = 0.1 cm^2/g. We show that constraints on self-interacting dark matter from strong-lensing clusters are likely to improve significantly in the near future, but possibly more via central densities and core sizes than halo shapes.

Figures (11)

• Figure 1: Surface density of a halo of mass $M_{\mathrm{vir}} = 1.2\times 10^{14}M_\odot$ projected along the major axis of the moment-of-inertia tensor -- the orientation that dominates the lensing probability. The left column shows the halo for CDM, while the middle and right columns show the same halo simulated using SIDM with $\sigma/m = 0.1$$\hbox{cm}^2/\hbox{g}$ and $1.0$$\hbox{cm}^2/\hbox{g}$, respectively. The bottom row shows the same information, now zoomed in on the central region. The surface density stretches logarithmically from $\approx 10^{-3}\hbox{g}/\hbox{cm}^2$ (blue) to $\approx 10\hbox{g}/\hbox{cm}^2$ (red).
• Figure 2: Surface density profiles for the same halo shown in Fig. \ref{['fig:major4']}, now projected along the intermediate axis. Deviations from axisymmetry are highest along this projection.
• Figure 3: Host halo shapes in shells of radius scaled by the virial radius in three virial-mass bins as indicated. The black solid lines denote the 20th percentile (lowest), median (middle), and 80th percentile (highest) value of $c/a$ at fixed $r/r_{\mathrm{vir}}$ for CDM. The blue dashed lines show the median and 20th/80th percentile ranges for $\sigma/m = 1\hbox{cm}^2/\hbox{g}$, and the green dotted lines show the same for $\sigma/m = 0.1\hbox{cm}^2/\hbox{g}$. There are 440, 65, and 50 halos in each mass bin (lowest mass bin to highest).
• Figure 4: Median subhalo shape vs. radius for galaxy-mass systems compared to host halos of the same mass. Bold lines denote subhalos and lighter lines denote host halos. The radii are normalized by $r_{\rm vir}$ for hosts and $r_{\rm tidal}$ for subhalos. Line colors and styles have the same meanings as in Fig. \ref{['fig:3drrvir']}: dashed blue is SIDM$_{1}$, dotted green is SIDM$_{0.1}$, and solid black is CDM.
• Figure 5: Median axis ratio $c/a$ for host halos as a function of the local scattering rate modulo the cross section: $\rho v_{\mathrm{rms}} \sim \Gamma (\sigma/m)^{-1}$. Smaller values correspond to the outer halo, where the density and scattering rate are low. The quantity is scaled by $10 \hbox{Gyr cm}^2/\hbox{g}$, such that $1$ in these units means that each particle has roughly one interaction per 10 Gyr in SIDM$_{1}$ (blue dashed line) and one interaction per 100 Gyr in SIDM$_{0.1}$ (green dotted line). The black solid line is CDM.