Institution(s): 1. Kavli Institute for the Physics and Mathematics of the Universe, 2. Tohoku University
We propose a new astrophysical test on the nature of dark matter based on the properties of dark halos associated with dwarf spheroidal galaxies. The method adopts a mean surface density of a dark halo defined within a radius of maximum circular velocity, which is derivable for a wide variety of galaxies with any dark-matter density profiles. We find that even though dark halo density profiles are derived based on the different assumptions for each galaxy sample and determined by several independent methods, this surface density is generally constant across a wide range of maximum circular velocities of about 10 to 400 km/s, irrespective of different density distribution in each of the galaxies.
Furthermore, this constancy allows us to constrain dark matter models, especially particle mass of dark matter, because this surface density can be directly computed from the theoretical predictions based on N-body simulations. We find that at higher halo-mass scales, this constancy for real galaxies can be naturally reproduced by both cold and warm dark matter (CDM and WDM) models, even though we do not perform any fitting to the data. However, at dwarf-galaxy mass scales, for which we have estimated from the Milky Way and Andromeda dwarf satellites, the mean surface density derived from WDM largely deviates from the observed constancy, whereas CDM is in reasonable agreement with observations. This difference is due to the different mass-concentration relations between CDM and WDM halos at low halo-mass scales, namely, WDM halos show a much weaker concentration than CDM halos at these low-mass scales.
In order to explain this universal mean surface density of dwarf-galaxy scales in WDM models, we suggest that WDM particles need to be heavier than 3 keV.