Dark matter halo of dwarf galaxies shaped by supernova feedback
21 July 2015
The Cold Dark Matter (CDM) model has proven very successful, however there exists a long standing problem of ubiquitous 'cusps' of dark matter halos, i.e. the dark matter distribution sharply increases to a high value at a central point. Significant improvements in the understanding of detailed physical processes of dark and luminous matter on galactic scales have been achieved by observational data from several galaxy surveys in tandem with advances in cosmological hydrodynamic simulations. These have only been tested in a small number of field galaxies though, owing to the lack of high-quality multi-wavelength data and of standardised analysis tools. In order to provide robust observational constraints to dark matter models, we need to extend the investigation to a larger number of galaxies in a more systematic and consistent manner.
In a new publication, CAASTRO researcher Dr Se-Heon Oh (ICRAR – University of Western Australia) and colleagues present high-resolution (20-300 pc) mass models of 26 dwarf galaxies and discuss the dark matter distributions near their centres, as part of the LITTLE THINGS survey. This is a high-resolution (~6" angular; < 2.6 km/s velocity resolution) HI 21cm survey of nearby (< 11 Mpc) gas-rich dwarf galaxies undertaken with the NRAO Very Large Array (VLA) in the northern sky. In their publication, the team quantified the degree of the central dark matter concentration of the sample galaxies by measuring the logarithmic inner slopes of their dark matter density profiles. The mean value of the inner slopes (-0.32) indicates a mass distribution with a sizeable constant density-core towards the centres of the galaxies. Comparing these observations with latest Lambda CDM simulations of galaxies where realistic baryonic feedback is included, the dark matter cusps of the halos in both samples can effectively be accounted for by supernova (SN) driven gas outflows.
Some LITTLE THINGS sample galaxies, however, such as DDO 210, DDO 101 and Haro 29, have relatively steeper inner density slopes. According to the latest hydrodynamic simulations, SN feedback in low mass dwarf galaxies of less than 106 solar masses is not sufficient to disrupt the central cusps, largely due to low star formation efficiencies in these systems. Dr Oh and his team have submitted a Gemini observing proposal for spectroscopy of DDO 210’s inner dark matter halo density profile where simulations have predicted primordial CDM cusps to have survived. This will be a fundamental test for the presence or absence of a signature of the central cusp in low mass halos, and whatever the outcome, will inform the debate on the formation process of low-mass galaxies in particular, as well as provide a crucial test to the LCDM.