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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.