NGC 615

Background

NGC 615 was discovered by William Herschel on January 10, 1785. It belongs to the NGC 584 galaxy group, which also includes NGC 584, NGC 596, NGC 600, and NGC 636. The galaxy lies about three degrees northeast from Theta Ceti. Observations have revealed a chemically and dynamically decoupled nucleus, with two decoupled components; the outer one is identified as a disk.

At the center, within 3 arcseconds (0.3 kpc), an inclined circumnuclear disk exists, whose stars have solar metallicity and a median age of 5 billion years. In the bulge (3–6 arcseconds), a counterrotating gaseous component was detected. Further out (8–30 arcseconds) lies the inner decoupled component, where stars and gas rotate together. This component is an oval-shaped inner compact disk containing HII regions; the most prominent pair in Hα imaging lies symmetrically about the nucleus at a radius of 20–23 arcseconds.

The mean stellar age in the inner disk outside the HII regions is also 5 billion years. The presence of a chemically decoupled nucleus and inner disk suggests a secondary burst of star formation, possibly triggered by a close passage of another galaxy some gigayears ago. The outer disk shows a pair of low-contrast outer spiral arms or, more likely, a ring, with an inclination no more than 60°.