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Strong gravitational lensing offers a compelling test of the cold dark matterparadigm, as it allows for subhaloes with masses of $\sim10^{9}$ M$_\odot$ andbelow to be detected. We test commonly-used techniques for detecting subhaloessuperposed in images of strongly lensed galaxies. For the lens we take asimulated galaxy in a $\sim10^{13}$ M$_\odot$ halo grown in a high-resolutioncosmological hydrodynamical simulation, which we view from two differentdirections. Though the resolution is high, we note the simulated galaxy stillhas an artificial core which adds additional complexity to the baryon dominatedregion. To remove particle noise, we represent the projected galaxy massdistribution by a series of Gaussian profiles which precisely capture thefeatures of the projected galaxy. We first model the lens mass as a (broken)power-law density profile and then search for small haloes. Of the twoprojections, one has a regular elliptical shape, while the other has distinctdeviations from an elliptical shape. For the former, the broken power-law modelgives no false positives and correctly recovers the mass of the superposedsmall halo, but for the latter we find false positives and the inferred halomass is overestimated by $\sim4-5$ times. We then use a more complex model inwhich the lens mass is decomposed into stellar and dark matter components. Inthis case, we show that we can capture the simulated galaxy's complex projectedstructures and correctly infer the input small halo.

Testing strong lensing subhalo detection with a cosmological simulation