Kinematics of two dwarf galaxies in the NGC 6946 group

Abstract

We present high velocity resolution (1.6 km s -1 ) Giant Meterwave Radio Telescope (GMRT) HI 21 cm synthesis images of the dwarf galaxies KK98 250 and KK98 251, as well as optical broad band images of KK98 250. We find that, despite being faint (M B < -14), both galaxies have regular velocity fields. In both cases the velocity fields are consistent with rigid body rotation. We derive rotation curves for the galaxies and fit mass models to them. For both galaxies, we find acceptable fits using isothermal halos while NFW halos provide a poor fit. Finally, we compile from literature a sample of galaxies with HI synthesis observations and I band magnitudes. From this sample we find, in agreement with earlier studies (which used single dish HI data), that dwarf galaxies tend to lie below the I band Tully-Fisher relation defined by brighter galaxies.