| Abstract | The mass distribution of the gas-rich dwarf irregular galaxy DDO 154 is studied. The large extent of the H I component allows us to determine unambiguously the rotation curve out to 4R_HO_ (7R_25_, 15α^-1^). In terms of optical scale lengths, this is one of the longest rotation curves ever measured. At those large galactocentric distances, the flat part of the rotation curve is reached which permits much better constraints on the parameters of the mass model and especially of the dark halo component. The best-fitting model yields a core radius r_c_ = 3.0 kpc and a central density ρ_0_ = 0.016 M_sun_ pc^-3^ for the isothermal halo. For the stellar disk, we find (M/L_B_)_*_ = 1.0. For r > 1 kpc, the rotation curve is completely dominated by the dark component. At the last point of the rotation curve (7.6 kpc), more than 90% of the mass is provided by the dark component. At that radius, (M/L_B_)_total_ = 80. The results for DDO 154 show that there exist galaxies where the luminous matter (stars and gas) is only a minor component of the total galaxy mass. This also suggests that it is possible that many of the smallest galaxies could be optically invisible. |
