Dwarf galaxy rotation curves and the core problem of dark matter haloes

The standard cold dark matter (CDM) model has recently been challenged by t he claim that dwarf galaxies have dark matter haloes with constant-density cores, whereas CDM predicts haloes with steeply cusped density distribution s. Consequently, numerous alternative dark matter candidates have recently been proposed. In this paper we scrutinize the observational evidence for t he incongruity between dwarf galaxies and the CDM model. To this end, we an alyse the rotation curves of 20 late-type dwarf galaxies studied by Swaters . Taking the effects of beam smearing and adiabatic contraction into accoun t, we fit mass models to these rotation curves with dark matter haloes with different cusp slopes, ranging from constant-density cores to r(-2) cusps. Even though the effects of beam smearing are small for these data, the unc ertainties in the stellar mass-to-light ratio and the limited spatial sampl ing of the halo's density distribution hamper a unique mass decomposition. Consequently, the rotation curves in our sample cannot be used to discrimin ate between dark haloes with constant-density cores and r(-1) cusps. We sho w that the dwarf galaxies analysed here are consistent with CDM haloes in a Lambda CDM cosmology, and that there is thus no need to abandon the idea t hat dark matter is cold and collisionless. However, the data are also consi stent with any alternative dark matter model that produces dark matter halo es with central cusps less steep than r(-1.5). In fact, we argue that based on existing H I rotation curves alone, at best weak limits can be obtained on cosmological parameters and/or the nature of the dark matter. In order to make progress, rotation curves with higher spatial resolution and indepe ndent measurements of the mass-to-light ratio of the disc are required.