2023MNRAS.523.1310B

Stringent observational constraints on the subgalactic matter power spectrum would allow one to distinguish between the concordance ΛCDM and the various alternative dark-matter models that predict significantly different properties of mass structure in galactic haloes. Galaxy-galaxy strong gravitational lensing provides a unique opportunity to probe the subgalactic mass structure in lens galaxies beyond the Local Group. Here, we demonstrate the first application of a novel methodology to observationally constrain the subgalactic matter power spectrum in the inner regions of massive elliptical lens galaxies on 1-10 kpc scales from the power spectrum of surface-brightness anomalies in highly magnified galaxy-scale Einstein rings and gravitational arcs. The pilot application of our approach to Hubble Space Telescope (HST/WFC3/F390W) observations of the SLACS lens system SDSS J0252+0039 allows us to place the following observational constraints (at the 99 per cent confidence level) on the dimensionless convergence power spectrum $\Delta ^{2}_{\delta \kappa }$ and the standard deviation in the aperture mass σ_AM: $\Delta ^{2}_{\delta \kappa }\lt 1$ (σ_AM < 0.8 × 10⁸ M_⊙) on 0.5-kpc scale, $\Delta ^{2}_{\delta \kappa }\lt 0.1$ (σ_AM < 1 × 10⁸ M_⊙) on 1-kpc scale and $\Delta ^{2}_{\delta \kappa }\lt 0.01$ (σ_AM < 3 × 10⁸ M_⊙) on 3-kpc scale. These first upper-limit constraints still considerably exceed the estimated effect of CDM subhaloes. However, future analysis of a larger sample of galaxy-galaxy strong lens systems can substantially narrow down these limits and possibly rule out dark-matter models that predict a significantly higher level of density fluctuations on the critical subgalactic scales.