The abundance of dwarf galaxies around low-mass giants in the Local Volume

Abstract: The abundance of satellite dwarf galaxies has long been considered a crucial test for the current model of cosmology leading to the well-known missing satellite problem. Recent advances in both simulations and observations have allowed to study dwarf galaxies around host galaxies in more detail. We have surveyed a 72 deg2 area of the nearby Sculptor group using the Dark Energy Camera - also encompassing the two low-mass Local Volume galaxies NGC24 and NGC45 residing behind the group - to search for hitherto undetected dwarf galaxies. Apart from the previously known dwarf galaxies we have found only two new candidates down to a 3 sigma surface brightness detection limit of 27.4 r mag arcsec-2. Both systems are in projection close to NGC24. However, one of these candidates could be an ultra-diffuse galaxy associated to a background galaxy. We compared the number of known dwarf galaxy candidates around NGC24, NGC45, and five other well-studied low-mass giant galaxies (NGC 1156, NGC2403, NGC5023, M33, and the LMC) with predictions from cosmological simulations and found that for the stellar-to-halo mass models considered, the observed satellite numbers tend to be on the lower end of the expected range. This could either mean that there is an over-prediction of luminous subhalos in LambdaCDM or - and more likely - that we are missing some of the satellite members due to observational biases.