The Distribution of Dark Matter in Galaxies: a Constant-Density Halo around DDO 47

Abstract

In this paper we present strong evidence for a core in the density distribution of the dark halo around a (dwarf) galaxy. DDO 47 has a rotation curve that increases linearly from the first data point, at 300 pc, up to to the last one, at 5 kpc. This profile cannot be reproduced unless we consider a dark halo with a flat density core. The optical region of this galaxy is dominated by a homogeneous halo of costant density of ~ 10^{-24} g/cm^3 and a size of ~3 kpc, about twice that of the stellar disk. This result is a formidable case of the inability of standard Lambda Cold Dark Matter scenario to account for the dark mass distribution around galaxies, and points toward the existence of an intriguing halo scale-length. With respect to the recent claim that the "status of CDM on galactic scales" is satisfactorily in agreement with observations (Primack, 2002), this paper adds up to the results of Blais-Ouellette et al. (2002), Trott & Webster (2002), Binney & Evans (2002), de Blok & Bosma (2002), Bottema (2002) in providing evidence that, "succesfull CDM" is instead obliged to implement the "soft core" feature as a general property of the distribution of galactic dark matter.