The Dwarf Galaxy Ddo 47 as a Dark Matter Laboratory: Testing Cusps Hiding in Triaxial Halos and the Mond Paradigm

We present an analysis of high resolution HI data of the dwarf galaxy DDO 47, aimed at testing the hypothesis that dark halo triaxiality might induce non-circular motions resulting in rotation curves best fitted by cored halos, even if the dark matter halo is intrinsically cuspy. This hypothesis has been invoked in order to reconcile the predictions of the ΛCDM theory with the rotation curves of disk galaxies. DDO 47 is an ideal case to test this hypothesis because it has a very regular velocity field, its rotation curve is best fitted by a cored halo and a NFW halo is inconsistent with the data. The minor axis kinematics (e.g. Hayashi et al. 2004) that results naturally from the non-circular motions needed to “hide” a cusp is incompatible with the actual observations: along the minor axis, velocities as high as 30% of the rotational velocity should be reached within the stellar exponential scale length, while the observations show that the velocities stay close to zero. We therefore conclude that the dark matter halo around the dwarf galaxy DDO 47 is truly cored and that it cannot be hidden by non-circular motions. We also fit the kinematics of DDO47 with the standard MOND formula and find that MOND is unable to reproduce the observed rotation curve. Subject headings: galaxies: dwarf – galaxies: individual (DDO 47) – galaxies: kinematics and dynamics – dark matter.