NGC 3741: the dark halo profile from the most extended rotation curve

We present new Hi observations of the nearby dwarf galaxy NGC 3741. This galaxy has an extremely extended Hi disc, which allows us to trace the rotation curve out to unprecedented distances in terms of the optical disc: we reach 42 B-band exponential scalelengths or about 7 kpc. The Hi disc is strongly warped, but the warp is very symmetric. The distribution and kinematics are accurately derived by building model data cubes, which closely reproduce the observations. In order to account for the observed features in the data cube, radial motions of the order of 5-13 km s-1 are needed. They are consistent with an inner bar of several hundreds of pc and accretion of material in the outer regions. The observed rotation curve was decomposed into its stellar, gaseous and dark components. The Burkert dark halo (with a central constant density core) provides very good fits. The dark halo density distribution predicted by the A cold dark matter (CDM) theory fails to fit the data, unless NGC 3741 is a 2.5σ exception to the predicted relation between concentration parameter and virial mass and at the same time a high value of the virial mass (though poorly constrained) of 1011 Mȯ. Noticeably, modified Newtonian dynamics (MOND) seems to be consistent with the observed rotation curve. Scaling up the contribution of the gaseous disc also gives a good fit. © 2006 RAS.