Radio and optical orientations of galaxies

We investigate the correlations between optical and radio isophotal position angles for 14 302 Sloan Digital Sky Survey galaxies with r magnitudes brighter than 18 and which have been associated with extended Faint Images of the Radio Sky at Twenty centimetres (FIRST) radio sources. We identify two separate populations of galaxies using the colour, concentration and their principal components. Surprisingly, strong statistical alignments are found: late-type galaxies are overwhelmingly biased towards a position angle differences of 0° and early-type galaxies to 90°. The late-type alignment can be easily understood in terms of the standard picture in which the radio emission is intimately related to areas of recent star formation. In early-type galaxies, the radio emission is expected to be driven by accretion on to a nuclear black hole. We argue that the observed correlation of the radio axis with the minor axis of the large-scale stellar distribution gives a fundamental insight into the structure of elliptical galaxies, for example, whether or not the nuclear kinematics are decoupled form the rest of the galaxy. Our results imply that the galaxies are oblate spheroids with their radio emission aligned with the minor axis. Remarkably, the strength of the correlation of the radio major axis with the optical minor axis depends on radio loudness. Those objects with a low ratio of FIRST radio flux density to total stellar light show a strong minor axis correlation while the stronger radio sources do not. This may reflect different formation histories for the different objects, and we suggest we may be seeing the different behaviour of slow rotating and fast rotating ellipticals. A simple analysis to estimate the effects of measurement errors indicates that the intrinsic degree of anti-alignment in the roundest early-type galaxies may be as small as ±15° and a similar value is obtained for the degree of alignment in the late-type population.