The disk and dark halo mass of the barred galaxy NGC 4123. II. Fluid-dynamical models

We report a dynamical determination of the separate contributions of disk and dark halo masses to the rotation curve of a spiral galaxy. We use fluid-dynamical models of gas flow in the barred galaxy NGC 4123 to constrain the dynamical properties of the galaxy: disk M/L, bar pattern speed, and the central density and scale radius of the dark halo. We derive a realistic barred potential directly from the light distribution. For each model we assume a value of the stellar M/L and a bar pattern speed Ω_p and add a dark halo to fit the rotation curve. We then compute the gas flow velocities with a two-dimensional gasdynamical code and compare the model flow patterns to a two-dimensional velocity field derived from Fabry-Perot observations. The strong shocks and noncircular motions in the observed gas flow require a high stellar M/L and a fast-rotating bar. Models with I-band disk M/L of 2.0-2.5 h₇₅, or 80%-100% of the maximum disk value, are highly favored. The corotation radius of the bar must be ≤1.5 times the bar semimajor axis. These results contradict some recent claimed "universal" galaxy disk/halo relations, since NGC 4123 is of modest size (rotation curve maximum 145 km s^-1 and V_flat = 130 km s^-1) yet quite disk-dominated. The dark halo of NGC 4123 is less concentrated than favored by current models of dark halos based on cosmological simulations. Since some 30% of bright disk galaxies are strongly barred and have dust lanes indicating shock morphology similar to that of NGC 4123, it is likely that they also have high stellar M/L and low-density halos. We suggest that luminous matter dominates inside the optical radius R₂₅ of high surface brightness disk galaxies.