Quantifying the detectability of Milky Way satellites with image simulations: Case study with KiDS

¹ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), German Centre for Cosmological Lensing, 44780 Bochum, Germany
² Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada
³ INAF – Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
⁴ Leiden Observatory, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands
⁵ TU Dortmund University, Department of Physics, 44227 Dortmund, Germany

^⋆ Corresponding author: shiyang@astro.ruhr-uni-bochum.de

Received: 19 February 2025
Accepted: 18 April 2025

Abstract

Ultra-faint dwarf galaxies, which can be detected as resolved satellite systems of the Milky Way, are critical to our understanding of galaxy formation, evolution, and the nature of dark matter, as they are the oldest, smallest, most metal-poor, and most dark matter-dominated stellar systems known thus far. Quantifying the sensitivity of surveys is essential to investigating their capability and limitations in searching for ultra-faint satellites. In this paper, we present the first study of the image-level observational selection function for Kilo-Degree Survey (KiDS) based on the Synthetic UniveRses For Surveys (SURFS) based on KiDS Legacy-like simulations. We generated mock satellites and simulated images that included resolved stellar populations of the mock satellites and the background galaxies, capturing realistic observational effects such as source blending, photometric uncertainties, and star-galaxy separation. We applied the matched-filter method to recover the injected satellites. We derived the observational selection function of the survey in terms of the luminosity, half-light radius, and heliocentric distance of the satellites. Compared to the catalogue-level simulation typically used in previous studies, the image-level simulation provides a more realistic assessment of survey sensitivity, accounting for observational limitations that are neglected in catalogue-level simulations. The image-level simulation shows a detection loss for compact sources with a distance d ≳ 100 kpc. We argue that this is because compact sources are more likely to be identified as single sources, rather than being resolved during the source extraction process.