The cD galaxy Hydra A: evidence for cooling-flow driven star formation in a fast rotating central disk

Abstract

For the nearby powerful radio galaxy Hydra A, which is thought to lie at the centre of a massive cooling flow, we report the results of optical spectroscopy plus high-resolution Hα and 2.2µ imaging. We detected strong nuclear emission lines with line ratios characteristic of LINERs. The nuclear emission region is resolved and the tilt of the emission lines indicates that they originate in a gaseous disk-like structure with a rotational velocity of ~ 300 kms^-1. We find extended Balmer absorption lines of strengths characteristic of O-B type stars in the spectrum of the central region bluewards of the 4000°A break. The Balmer lines are also tilted indicating that the O-B stars are co-rotating with the gas. This is in contrast with the older stellar population which show no evidence for rotation. Our result indicates a close link between the young stars and the emitting gas. We suggest that the material needed for the formation of these young stars and the gaseous disk may be deposited near the centre by the cooling flow. We examine the physical conditions within the nebula together with possible sources of energy input by confronting the observed emission-line ratios with predictions of models with different excitation mechanisms and conclude that photoionization by a combination of hot stars and X-ray is able to explain the observed line ratios.