2023ApJ...954..197I

The lensing power spectra for gravitational potential, astrometric shift, and convergence perturbations are powerful probes to investigate dark matter structures on small scales. We report the first lower and upper bounds of these lensing power spectra on angular scale ∼1″ toward the anomalous quadruply lensed quasar MG J0414+0534 at a redshift z = 2.639. To obtain the spectra, we conducted observations of MG J0414+0534 using the Atacama Large Millimeter/submillimeter Array with high angular resolution (0.″02-0.″05). We developed a new partially nonparametric method in which Fourier coefficients of potential perturbation are adjusted to minimize the difference between linear combinations of weighted mean de-lensed images. Using positions of radio-jet components, extended dust emission on scales >1 kpc, and mid-infrared flux ratios, the range of measured convergence, astrometric shift, and potential powers at an angular scale of ∼1.″1 (corresponding to an angular wavenumber of l = 1.2 × 10⁶ or ∼9 kpc in the primary lens plane) within 1σ are Δ_κ = 0.021-0.028, Δ_α = 7-9 mas, and Δ_ψ = 1.2-1.6 mas², respectively. Our result is consistent with the predicted abundance of halos in the line of sight and subhalos in cold dark matter models. Our partially nonparametric lens models suggest the presence of a clump in the vicinity of object Y, a possible dusty dwarf galaxy, and some small clumps in the vicinity of other lensed quadruple images. Although much fainter than the previous report, we detected weak continuum emission possibly from object Y with a peak flux of ∼100 μJy beam^–1 at the ∼4σ level.