The Disk and Dark Halo Mass of the Barred Galaxy NGC 4123. I. Observations

The non-circular streaming motions in barred galaxies are sensitive to themass of the bar and can be used to lift the degeneracy between disk and darkmatter halo encountered when fitting axisymmetric rotation curves of diskgalaxies. In this paper, we present photometric and kinematic observations ofNGC 4123, a barred galaxy of modest size (V_rot = 130 km/sec, L = 0.7 L_*),which reveal strong non-circular motions. The bar has straight dust lanes andan inner Lindblad resonance. The disk of NGC 4123 has no sign of truncation outto 10 scale lengths, and star-forming regions are found well outside R_25. AFabry-Perot H-alpha velocity field shows velocity jumps of >100 km/sec at thelocation 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 massmodels yield good fits to the rotation curve outside the bar regionfor diskI-band M/L of 2.25 or less, and dark halos with either isothermal or power-lawprofiles can fit the data well. In a companion paper, we model the full 2-Dvelocity field, including non-circular motions, to determine the stellar M/Land the mass of the dark halo.Comment: accepted by ApJ, 16 pages, 9 figures (1 color), uses emulateapj.sty, onecolfloat.st