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  1. ABSTRACT The presence of an invisible substructure has previously been detected in the gravitational lens galaxy SDSSJ0946+1006 through its perturbation of the lensed images. Using flexible models for the main halo and the subhalo perturbation, we demonstrate that the subhalo has an extraordinarily high central density and steep density slope. We robustly infer the subhalo’s projected mass within 1 kpc to be ∼2–3.7 × 109 M⊙ at >95 per cent CL for all our lens models, while the average log-slope of the subhalo’s projected density profile over the radial range 0.75–1.25 kpc is constrained to be steeper than isothermal (γ2D ≲ −1). By modeling the subhalo light, we infer a conservative upper bound on its luminosity LV < 1.2 × 108L⊙ at 95 per cent CL that shows that the perturber is dark matter dominated. We analyse lensing galaxy analogues in the Illustris TNG100-1 simulation over many lines of sight, and find hundreds of subhalos that achieve a mass within 1 kpc ≳ 2 × 109M⊙. However, less than 1 per cent of the mock observations yield a log-slope steep enough to be consistent with our lensing models, and they all have stellar masses exceeding that allowed by observations by an order of magnitude or more. We conclude that the presence of such a darkmore »highly concentrated subhalo is unexpected in a Lambda cold dark matter universe. While it remains to be determined whether this tension can be reduced by adding more complexity to the primary lens model, it is not significantly alleviated if the perturber is assumed to be a LOS structure, rather than a subhalo.« less
  2. ABSTRACT We demonstrate that the perturbations of strongly lensed images by low-mass dark matter subhaloes are significantly impacted by the concentration of the perturbing subhalo. For subhalo concentrations expected in Lambda cold dark matter (ΛCDM), significant constraints on the concentration can be obtained at Hubble Space Telescope (HST) resolution for subhaloes with masses larger than about $10^{10}\, {\rm M}_\odot$. Constraints are also possible for lower mass subhaloes, if their concentrations are higher than the expected scatter in CDM. We also find that the concentration of lower mass perturbers down to $\sim 10^8\, {\rm M}_\odot$ can be well constrained with a resolution of ∼0.01 arcsec, which is achievable with long-baseline interferometry. Subhalo concentration also plays a critical role in the detectability of a perturbation, such that only high-concentration perturbers with mass $\lesssim 10^9\, {\rm M}_\odot$ are likely to be detected at HST resolution. If scatter in the ΛCDM mass–concentration relation is not accounted for during lens modelling, the inferred subhalo mass can be biased by up to a factor of 3 (6) for subhaloes of mass $10^9 \, {\rm M}_\odot \,(10^{10} \, {\rm M}_\odot$); this bias can be eliminated if one varies both mass and concentration during lens fitting. Alternatively, onemore »may robustly infer the projected mass within the subhalo’s perturbation radius, defined by its distance to the critical curve of the lens being perturbed. With a sufficient number of detections, these strategies will make it possible to constrain the halo mass–concentration relation at low masses in addition to the mass function, offering a probe of dark matter physics as well as the small-scale primordial power spectrum.« less

    We analyse strongly lensed images in eight galaxy clusters to measure their dark matter density profiles in the radial region between 10 kpc and 150 kpc, and use this to constrain the self-interaction cross-section of dark matter (DM) particles. We infer the mass profiles of the central DM haloes, bright central galaxies, key member galaxies, and DM subhaloes for the member galaxies for all eight clusters using the qlens code. The inferred DM halo surface densities are fit to a self-interacting dark matter model, which allows us to constrain the self-interaction cross-section over mass σ/m. When our full method is applied to mock data generated from two clusters in the Illustris-TNG simulation, we find results consistent with no dark matter self-interactions as expected. For the eight observed clusters with average relative velocities of $1458_{-81}^{+80}$ km s−1, we infer $\sigma /m = 0.082_{-0.021}^{+0.027} \rm cm^2\, g^{ -1}$ and $\sigma /m \lt 0.13~ \rm cm^2\, g^{ -1}$ at the 95 per cent confidence level.