A COMMON SURFACE-DENSITY SCALE FOR THE MILKY WAY AND ANDROMEDA DWARF SATELLITES AS A CONSTRAINT ON DARK MATTER MODELS

In an attempt to place an explicit constraint on dark matter models, we define and estimate the mean surface density of a dark halo within the radius of the maximum circular velocity, which is derivable for various galaxies with any dark matter density profiles. We find that this surface density is generally constant across a wide range of maximum circular velocities of ∼10–400 km s−1, irrespective of the different density distributions in each of the galaxies. This common surface density at high halo-mass scales is found to be naturally reproduced by both cold and warm dark matter (CDM and WDM, respectively) models, even without employing any fitting procedures. However, the common surface density at dwarf-galaxy scales, which we have derived from the Milky Way and Andromeda dwarf satellites, is reproduced only in a massive range of WDM particle masses, whereas CDM provides reasonable agreement with the observed constancy. This is due to the striking difference between mass–concentration relations for CDM and WDM halos at low halo-mass scales. In order to explain the universal surface density of dwarf-galaxy scales in WDM models, we suggest that WDM particles need to be heavier than 3 keV.