Structure and Kinematics of NGC 615 and Its Nuclear Star Formation History

The results of a complex photometric and spectral investigation of the regular Sb galaxy NGC 615 are presented. The observations were made with the 6 m, 1 m, and 0.6 m telescopes of the Special Astrophysical Observatory RAS (Nizhnij Arkhyz, Russia) and the 1.5 m ESO telescope (La Silla, Chile). The analysis of the radial brightness profiles reveals the existence of at least two decoupled exponential components; the parameters of the outer component are typical for the global disks of Sb galaxies, and the inner component seen in the radius range 9"-30" (1.0-3.5 kpc) is distinguished by compactness and high surface brightness. A combination of two-dimensional velocity fields for stars and ionized gas in the central part of the galaxy and of the long-slit velocity profiles along the major axis up to the optical border of NGC 615 provides some interesting findings. In the very center, R<3" (0.3 kpc), stars and gas rotate together, and the dynamical and photometric major axes are turned with respect to the global line of nodes; we conclude that an inclined circumnuclear disk is detected. In the bulge, R=3"-6" (0.35-0.75 kpc), the stellar velocity dispersion is more than 100 km s^-1, the stars rotate twice as slowly as the ionized gas, and a counterrotating gaseous component is observed. In the inner exponential component the stars rotate together with the ionized gas, and the stellar velocity dispersion drops to about 50 km s^-1. As the photometric major axis of this component is turned with respect to the line of nodes, we call it ``an oval inner disk.'' The mean age of the stellar populations in the nucleus and in the inner disk beyond its H II regions is 5 Gyr, while the bulge is much older. Both the chemically decoupled nucleus of NGC 615 and the inner disk probably formed the bulk of their more luminous stars in a secondary burst of star formation, perhaps provoked by the close passage of another galaxy some gigayears ago. Based on observations collected with the 6 m, 1 m, and 0.6 m telescopes of the Special Astrophysical Observatory (SAO) of the Russian Academy of Sciences (RAS) and with the 1.52 m ESO telescope at La Silla.