A COUNTER-ROTATING DISK IN THE NORMAL SA-GALAXY NGC-4138

As part of a detailed survey of the kinematics and dynamics in a sampl e of non-interacting Sa galaxies, we present H I synthesis maps, optic al images and optical emission and absorption line long slit spectra o f the small-bulged Sa galaxy NGC 4138. Located in a loose group domina ted by early type spirals but with no near companions, NGC 4138 appear s morphologically smooth and undisturbed, except for a dust lane visib le on the southeastern side of the galaxy. As traced by stellar Mg I b absorption lines, the primary stellar disk contains similar to 80% of the stars; the presence of Her absorption suggests that star formatio n in this component ceased similar to 10(8) yr ago. The remaining star s, along with the H I and emission line gas, are rotating counter to t he 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 similar to 22 ''. The velocity dispersion of the primary disk stars is systemati cally 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 r adius of the stellar disk. Overall, the H I surface density is low, an d the derived M(HI)/L(B) is higher than average for an Sa. The well-or dered 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 lengt hs, and appears to decline by more than 100 km s(-1) beyond the optica l edge. However, the presence of gas at large projected distances alon g the minor axis suggest instead that the outer H I disk is strongly w arped. Because of the possibility of a significant warp, mass modellin g is ambiguous, giving ratios of dark to luminous matter ranging from unity (falling rotation curve with no warp) to 5.5 (flat rotation curv e with a strong warp). In either case, the counter-rotation component accounts for similar to 25% of the total luminous mass of the galaxy. The counter-rotating disk may either be the result of a merger of a sp iral with a gas-rich dwarf, or the continual infall of material of opp osite spin vector onto the galaxy. Both models are constrained by the small bulge, the presence of A stars in the primary disk, and the over all smooth morphology of the galaxy. (C) 1996 American Astronomical So ciety.