The Stellar and Gaseous Kinematics in NGC 253

Abstract

This paper presents observations made at intermediate spectral and spatial resolutions along the major and minor axes of the starburst galaxy NGC 253. The spectral ranges analyzed are in the region of the stellar Mg I b (~5175 Å) line, in the near-IR Ca II triplet (~8550 Å) absorption features, and in the region of the Hα emission line. We compare the shape of the stellar features with those of reference stars and determine the line-of-sight velocity distribution of the stellar component by using a two-dimensional Gaussian decomposition algorithm, and we show for the first time the rotation curve of the stellar component in NGC 253. Comparing the recession velocity curves of the gas and stars, we show that the stellar component has a decoupled kinematics with respect to the gas, displaying a shallower velocity gradient and larger velocity dispersion than the gas in the inner regions. The minor-axis kinematics, together with the kinematics across the central 40'' along the major axis, suggest the presence of a rotating body with a kinematically misaligned axis with respect to the main disk of the galaxy. The asymmetries in the LOS velocity distribution along the minor axis, together with the steep velocity gradient of the gaseous component, suggest a merger scenario as the source of these kinematic signatures. The enclosed mass in the central regions is computed to be (2.4 ± 0.5) × 107 M☉ for a radius of r = 07 (10 pc). A double gaseous component in the central 6'' is detected from the [S III] λ9069 Å data along the minor axis; this seems to be the signature of a superbubble, due to a supernova rate of 0.05 yr-1.