A Counter-Rotating Disk in the Normal SA Galaxy NGC 4138

Abstract

As part of a detailed survey of the kinematics and dynamics in a sample of non-interacting Sa galaxies, we present H I synthesis maps, optical images and optical emission and absorption line long slit spectra of the small-bulged Sa galaxy NGC 4138. Located in a loose group dominated by early type spirals but with no near companions, NGC 4138 appears morphologically smooth and undisturbed, except for a dust lane visible on the southeastern side of the galaxy. As traced by stellar Mg I b absorption lines, the primary stellar disk contains ~80% of the stars; the presence of Hα absorption suggests that star formation in this component ceased ~10^8^ yr ago. The remaining stars, along with the H I and emission line gas, are rotating counter to the primary stellar disk with comparable observed velocity. The counter-rotating stars are present throughout the disk of the galaxy but peak at the same location as local maxima in the I-band luminosity and the H I distribution and a ring of H II regions at a radius of ~22". The velocity dispersion of the primary disk stars is systematically larger than that of the counter-rotating stars, suggesting mild heating of the older component. The H I gas extends to 2.5 times the radius of the stellar disk. Overall, the H I surface density is low, and the derived M_H I_/L_B_ is higher than average for an Sa. The well-ordered appearance of the H I velocity field implies quiescent rotation in a disk. The H I rotation curve can be traced to 16 disk scale lengths, and appears to decline by more than 100 km s^-1^ beyond the optical edge. However, the presence of gas at large projected distances along the minor axis suggests instead that the outer H I disk is strongly warped. Because of the possibility of a significant warp, mass modelling is ambiguous, giving ratios of dark to luminous matter ranging from unity (falling rotation curve with no warp) to 5.5 (flat rotation curve with a strong warp). In either case, the counter- rotation component accounts for ~25% of the total luminous mass of the galaxy. The counter-rotating disk may either be the result of a merger of a spiral with a gas-rich dwarf, or the continual infall of material of opposite spin vector onto the galaxy. Both models are constrained by the small bulge, the presence of A stars in the primary disk, and the overall smooth morphology of the galaxy.