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arXiv:1001.

1409v1 [astro-ph.CO]

9 Jan

2010 **FULL TITLE** ASP Conference Series, Vol. **VOLUME**, **YEAR OF PUBLICATION** **NAMES OF EDITORS** A dichotomy in radio jet orientations in elliptical galaxies I.W.A. Browne and R.A. Battye Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester, M13 9PL Abstract. We have investigated the correlations between optical and radio isophotal position angles for

14302 SDSS galaxies with r magnitudes brighter than 18. All the galaxies are identi?ed with extended FIRST radio sources. For passive early-type galaxies, which we distinguish from the others by using the colour, concentration and their principal components, we ?nd a strong statistical alignment of the radio axes with the optical minor axes. Since the radio emission is driven by accretion on to a nuclear black hole we argue that the observed correlation gives new insight into the structure of elliptical galaxies, for example, whether or not the nuclear kinematics are decoupled from the rest of the galaxy. Our results imply that a signi?cant fraction of the galaxies are oblate spheroids, perhaps rotationally supported, with their radio emission aligned with the stellar 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 ?ux density to total stellar light show a strong minor axis correlation while the stronger radio sources do not. This split may re?ect di?erent formation histories and we suggest this may be a new manifestation of the better known dichotomy between slow rotating and fast rotating ellipticals. 1. Introduction Searching for statistical alignments between radio and optical axes is motivated by the desire to ?nd a connection between radio emission mechanism and the geometry of the host galaxy. Though there has been a long history of searching for such alignments, generally the results have been inconclusive (for example, Mackay (1971);

Palimaka et al. (1979);

Valtonen (1983);

Birkinshaw &

Davies (1985);

Sansom et al. (1987)). In most investigations there appears to be a slight preponderance of objects where the radio elongation is more aligned with the optical minor axis than the major axis. The clearest result was obtained by Condon et al. (1991) who found that extended radio jets in

125 UGC galaxies were preferentially aligned with the optical minor axes of their hosts, with the e?ect being strongest for elliptical galaxies. Recent work indicates that elliptical galaxies have complex kinematics. These include decoupled cores in a signi?cant fraction of galaxies (for example, Halliday et al. (2001);

Loubser et al. (2008);

Krajnovi` c et al. (2008)), and the separation of such galaxies into two distinct classes, cored on non-cored (Kormendy et al. (2009)) and/or fast and slow ro- tators (Emsellem et al. (2000)). Therefore studying the alignment between the stellar population and the radio emission assumes added signi?cance. All the historic results have been based on relatively small numbers of ob- jects (? 100) but with the advent of deep radio and optical surveys like FIRST (Becker et al. (1995)) and the SDSS (York et al. (2000)), respectively, it is pos-

1 2 Browne and Battye sible to construct samples with orders of magnitude larger numbers. But going deeper in radio ?ux density means that the galaxies identi?ed with the radio sources are no longer dominated by the ellipticals, which were the targets of early studies, but are now a mixture of ellipticals and disk-dominated star- forming galaxies. Since the source of the emission is fundamentally di?erent in the two types of galaxy one might well expect their alignment properties also to be di?erent. Hence we need to distinguish reliably between early- and late-type galaxies and we do this by using SDSS photometry. In this paper we will focus almost exclusively on radio sources hosted by elliptical galaxies. 2. Sample selection We use optical identi?cations of FIRST radio sources listed in the SDSS DR6 database (Adelman-McCarthy et al. (2008)) which were selected using the pro- cedure de?ned in Ivezi? c et al. (2002). There are