Hydra A: star formation and dust production in a cooling flow.

Abstract

Optical images and spectroscopy are presented for the radio galaxy Hydra A which contains a substantial flow of cooling X-ray gas. A young stellar population is observed as an excess light around the center showing blue colour and Balmer lines in absorption. The same area also exhibits strong optical emission lines. Our IUE spectra show Lyα and an ultraviolet continuum. The optical luminosity and colour of the excess light, the presence of Balmer absorption lines, and the UV continuum flux are compatible with a burst of recent star formation of an age =~10^7^yr, total mass 10^8^-10^9^M_sun_, and an initial mass function including OB stars. We argue that the young stars and the emission-line gas most likely are products of the cooling flow. This is apparently contradicted by the unexpected presence of a significant amount of dust in Hydra A. The dust is inferred from a substantial far-infrared flux which rivals the X-ray emission in cooling the hot gas. We argue for the hypothesis that the dust is produced in connection with star formation in the cooling flow.