Optical and H i Studies of the ``Gas-rich'' Dwarf Irregular Galaxy DDO 154

A study of the luminous and dark components of the dwarf irregular galaxy DDO 154 is presented. From the surface photometry, a scale length α^-1^ = 0.5 kpc and a central surface brightness B(0)_c_ = 23.17 are derived for the stellar disk with mean colors typical of Im galaxies. The photometry of the brightest blue stars, combined with the likely association with the CVn I cloud, suggest a distance {DELTA} <= 4.0 Mpc. DDO 154 is one of the most gas-rich galaxy known with (M_HI_/L_B_) = 5.4. The H I disk reaches nearly 5 Holmberg diameters (D_H0_). From the very regular velocity field, the rotation curve is derived unambiguously out to 15 optical scale lengths. There is a clear flattening of the rotation curve for r > 4' and an indication that it may be declining in the very outer parts. The shape of the rotation curve, and the fact that for r > 2 kpc it is completely dominated by the dark component, allow one to constrain almost uniquely the parameters of the mass model. The final model has (M/L_B_)_*_ = 1.0 (M_sun_/L_sun_) for the stellar disk, a core radius r_c_ = 3.0 kpc, and a central density ρ_0_ = 0.015 M_sun_ pc^-3^ for the dark isothermal halo. At the last observed point (r = 7.6 kpc), more than 90% of the mass is dark.