The kinematics and morphology of NGC 520

Abstract

The peculiar galaxy NGC 520 (Arp 157) is often interpreted as an interacting pair of galaxies. The identification of two bulges and overall morphology of two galaxies has long been a puzzle which we attempt to solve in this paper. New long-slit optical spectroscopy and optical and near- infrared images of NGC 520 are presented. Velocity dispersion and radial velocity profiles are obtained from the spectra with the Fourier quotient and cross-correlation techniques. At the northwestern optical emission peak, a sharp rise in the velocity dispersions implies a concentration of mass. The northwest optical peak is also apparent as a peak in the K band image whose near-IR colors are consistent with the presence of a typical bulge stellar population. These data suggest that the northwest peak may be the bulge of one of two galaxies in the system. A larger bulge is clearly evident in the K band image in the middle of an optical dust lane. The optical colors and Hα emission imply that both bulges and the disk of the northwest galaxy have experienced recent star formation. While possibly solving one puzzle, the new data have confirmed another. The radial velocity profile in the central 10" at the larger bulge is consistent with a counterrotating core, in agreement with the CO radial velocity profile in the same region (as reported by Sanders and colleagues in 1988). The elongated shape of the primary core is seen in the K band, the radio continuum (as reported by Condon and colleagues in 1982), and the CO line emission. The nature and origin of this core could be explained as an accreted gas-rich irregular galaxy which is forming a counterrotating stellar disk.