The distribution of dark matter in the dwarf galaxy DDO 170

DDO 170 is a dwarf galaxy of absolute magnitude M_B_ = -15.15 with a rotation curve measured to ~2 Holmberg radii. Using H I images and new photometry, we build three component models of the mass distribution: a luminous disk with a constant, M/L, a gas disk whose mass is determined by H I flux, and a dark halo. This galaxy is remarkable in a number of ways: (1) the mass- to-light ratio is 56, (2) the ratio of gas mass to blue luminosity is 3.7 in solar units, (3) it is the smallest galaxy where the basic halo parameters (core radius r_c_ and asymptotic velocity v_max_) have been determined by fitting detailed mass models, (4) it has a companion with a projected separation of 62 kpc and a line-of-sight velocity difference of 85 km s^-1^. Over one-third of the "visible" mass of this galaxy is in the form of gas. In the best-fit model of the rotation curve, the halo parameters are r_c_ = 2.4 kpc and v_max_ = 73 km s^-1^. The "minimum phase density" model has r_c_ = 4.1 kpc, v_max_ = 79 km s^-1^. The central density of the halo, its density at the Holmberg radius and the ratio of light-to-dark mass at the Holmberg radius are all comparable to values found for bright galaxies. An extrapolation of the L-v relation for bright disk galaxies (L is proportional to v^4.4^_max_) correctly predicts the rotation velocity at the Holmberg radius. The velocity separation of it and its companion supports the L is proportional to {DELTA}v^8^ scaling found for the velocity differences in binary pairs of galaxies. The constraints are robust to gross changes and are the best current limits on the phase density of dark matter. The minimum mass to bind the companion is 5.2 X 10^10^ M_sun_, while the standard statistical mass estimator yields 5.2 X 10^11^ M_sun_. While the luminosity of DDO 170 is ~1% of L_*_ (the characteristic luminosity defined by the luminosity function), its total mass is nearly the same as that of an L_*_ galaxy.