The disk and dark halo mass of the barred galaxy NGC 4123. I. Observations

The noncircular streaming motions in barred galaxies are sensitive to the mass of the bar and can be used to lift the degeneracy between disk and dark matter halo encountered when fitting axisymmetric rotation curves of disk galaxies. In this paper we present photometric and kinematic observations of NGC 4123, a barred galaxy of modest size (F_rot = 130 km s^-1, L = 0.7L_*, that reveal strong noncircular motions. The bar has straight dust lanes and an inner Lindblad resonance. The disk of NGC 4123 has no sign of truncation out to 10 scale lengths, and star-forming regions are found well outside R₂₅. A Fabry-Perot Hα velocity field shows velocity jumps of greater than 100 km s^-1 at the location of the dust lanes within the bar, indicating shocks in the gas flow. VLA observations yield the velocity field of the H i disk Axisymmetric mass models yield good fits to the rotation curve outside the bar region for disk I-band M/L of 2.25 or less, and dark halos with either isothermal or power-law profiles can fit the data well. In a companion paper we model the full two-dimensional velocity field, including noncircular motions, to determine the stellar M/L and the mass of the dark halo.