Testing strong lensing subhalo detection with a cosmological simulation

Strong gravitational lensing offers a compelling test of the cold dark matter paradigm, as it allows for subhaloes with masses of $\sim10^{9}$ M$_\odot$ and below to be detected. We test commonly-used techniques for detecting dark matter subhaloes superposed in images of strongly lensed galaxies. For the lens we take a simulated galaxy in a $\sim10^{13}$ M$_\odot$ halo grown in a high-resolution cosmological hydrodynamics simulation, which we view from two different directions. To remove particle noise, we represent the projected galaxy mass distribution by a series of analytic Gaussian profiles which precisely capture the features of the projected galaxy. We first model the lens mass as a broken power-law density profile (the parameterization of which includes a core ensuring the results are unaffected by a numerical central core in the simulated galaxy) and then search for small haloes. Of the two projections, one has a regular elliptical shape, while the other has distinct deviations from an elliptical shape. For the former, the broken power-law model gives no false positives and correctly recovers the mass of the superposed small halo, but for the latter we do find false positives and the inferred halo mass is overestimated by a factor of $\sim4-5$. We then replace the broken power-law model by a more complex model in which the lens mass is decomposed into separate stellar and dark matter components. In this case, we show that we can precisely capture the complex projected structures of the simulated galaxy and correctly infer the input parameters of the superposed small halo. By improving the lens galaxy mass model, we argue this enables more robust subhalo inference that can detect lower mass dark matter subhaloes, strengthening the technique's ability to test alternative models of dark matter which do not form dark matter subhaloes below a cut-off mass.Comment: Submitted to MNRAS. Comments welcome