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 Ωp and add a dark halo to fit the rotation curve. We then compute the gas flow velocities with a 2-D gas dynamical code, and compare the model flow patterns to a 2-D velocity field derived from FabryPerot observations. The strong shocks and non-circular 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 – 2.5h75, or 80 – 100% of the maximum disk value, are highly favored. The corotation radius of the bar must be ≤ 1.5 times the bar semi-major 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, and Vflat = 130 km s ) yet is 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. Subject headings: galaxies: kinematics and dynamics — galaxies: halos — galaxies: structure — hydrodynamics — dark matter