The Post-Merger Elliptical NGC 1700: Stellar Kinematic Fields to Four Effective Radii

Abstract

We have measured the stellar motions in the elliptical galaxy NGC 1700 along four position angles, to very large radii, using absorption features in spectra obtained with the Multiple Mirror Telescope. Our data extend the coverage of the stellar velocity field by a factor of 5 (2.5 times further in radius and twice as many PAs) beyond previous work. We have attained 10 km/s accuracy in the mean velocity out to nearly 2 effective radii (r_e), and errors are < 15% of the maximum rotation speed out to nearly 3 r_e. The lack of detectable minor-axis rotation and the nearly identical kinematics on the +/- 45 degree PAs suggest that NGC 1700 is nearly oblate for r < 2.5 r_e. Beyond this radius, twisting of the morphological and kinematic axes indicate increasing triaxiality, an intrinsic twist, or both. The velocity distribution in the low-amplitude counterrotating core is weakly skewed in the direction of rotation, arguing against a central stellar disk. The small skewness and the depression of the central velocity dispersion are consistent with the accretion of a low-mass stellar companion in a retrograde orbit. Photometric fine structure at large radii (Schweizer \& Seitzer 1992) is also indicative of a merger; a velocity reversal 50 arcsec northeast suggests a major event. However, radially increasing prograde rotation in the main body of the galaxy implies that this was not the same event responsible for the counterrotating core. The strong rotation at large R and the nearly oblate shape are consistent with N-body simulations of group mergers (Weil 1995); that all disturbances inside 2.5 r_e have phase-mixed out suggests that NGC 1700 owes its present form to a merger of 3 or more stellar systems 2 to 4 h^-1 Gyr ago.