Star formation in the radio galaxy NGC 4410A

The NGC 4410 group of galaxies provides us with a rare opportunity to study a nearby (97(75)(-1) Mpc) example of a radio galaxy (NGC 4410A) embedded in an extended X-ray source, with evidence for star formation that can be readily spatially distinguished from regions dominated by the active galactic nucleus and shocks. We present broadband and narrowband optical images, along with optical and IUE ultraviolet spectroscopy, for the radio galaxy NGC 4410A and its companion, NGC 4410B. Our Halpha + [N II] images reveal six luminous H II regions (L-Halpha similar to 10(40) ergs s(-1)) distributed in an arc near NGC 4410A. Partially completing the ring is a prominent stellar loop containing diffuse ionized gas. This filamentary gas, in contrast to the H II regions, shows spectroscopic signatures of shock ionization. The star formation in this system may have been triggered by a collision or interaction between the two galaxies, perhaps by an expanding density wave, as in classical models of ring galaxies. Alternatively, the star formation may have been induced by the impact of a radio jet on the interstellar matter. Extended Lyalpha is detected in the ultraviolet IUE spectrum. The ultraviolet continuum, which is presumably radiated by the nucleus of NGC 4410A, is not extended. NGC 4410A appears to be interacting with its neighbors in the NGC 4410 group and could be an example of a spiral galaxy transforming into an elliptical.