Blue lobes in the Hydra A cluster central galaxy

We present new U- and I-band images of the centrally dominant galaxy in the Hydra A cluster, obtained with the 2. 5 m Isaac Newton Telescope at La Palma. The galaxy is centered in a poor, X-ray luminous cluster whose gaseous intracluster medium is apparently cooling at a rate of mᵈotCF_~300 Mₛun_ yr^-1^. The galaxy's structure is that of a normal giant elliptical galaxy, apart from the central ~8 x 6 arcsec (~12 x 9 kpc) region which contains an unusually blue, lobelike structure that is spatially coincident with a luminous emission-line nebula in rotation about the nucleus. Based on the near spatial coincidence of the central continuum structure and the emission-line nebula, we suggest that the blue continuum is due to a warm stellar population in a central disk. In order to isolate and study the structure of the disk, we have subtracted a smooth galactic background model from the U-band image. The disk's surface brightness profiles along its major and minor axes decline roughly exponentially with radius. The disk's axial ratio is consistent with a nearly edge-on thick disk or a thin disk that is inclined with respect to the line of sight. The bluest regions, located a few arcsec on either side of the nucleus (giving the lobelike appearance), may be due to locally enhanced star formation or a seeing-blurred ring of young stars embedded in the disk observed nearly edge-on. If star formation is occurring with the local initial mass function, the central color, surface brightness, and dynamical mass would be consistent with models for star formation at a rate of <~ 1 Mₛun_ yr^-1^ which has persisted for the past ~10⁹ʸr, a short burst (10⁷^ yr) of star formation at a rate of ~30 Mₛun_ yr^-1^ which occurred <~10⁸^ yr ago, or an instantaneous burst of star formation which occurred ~5 x 10⁷^ yr ago. While the young population contributes ~30%-40% of the central U-band luminosity, its mass would be <~1% to <~10% (10⁸^ Mₛun_-2 x 10⁹^ Mₛun_) of the galaxy's central dynamical mass. We consider a number of possible origins for the disk material. The misaligned disk and stellar isophotal axes and the apparent rarity of such large disks in elliptical galaxies does not support fueling by mass loss from stars in the galaxy. Fueling by the cluster cooling flow or infalling material from an interloping galaxy are more likely origins. The jets of the powerful double-lobed radio source 3C 218 emerge from the nucleus parallel to the rotation axis of the emission-line nebula and nearly along the minor axis of the continuum structure. This configuration resembles, in a number of aspects, standard models for double-lobed radio sources powered by accretion of disk material onto a compact object (e. g. , a black hole). Hydra A's large radio luminosity compared to other FR I and cooling-flow radio sources may be the result of the abundant fuel supply provided by the circumnuclear disk. Although the cause of the radio-aligned lobelike structures in two other cooling flow cluster galaxies (Abell 1975 and Abell 2597) remains undetermined, the disk-like nature of the blue optical structure in Hydra A appears to be quite different from the optical lobes in those objects.