A dichotomy in radio jet orientations in elliptical galaxies

TL;DR

This study reveals a strong alignment between radio and optical axes in elliptical galaxies, with the alignment strength varying by radio loudness, offering insights into galaxy structure and formation history.

Contribution

It uncovers a dichotomy in radio jet orientations in elliptical galaxies, linking radio emission alignment to galaxy rotation and formation processes.

Findings

Radio axes align with optical minor axes in passive early-type galaxies.

Alignment strength varies with radio loudness, indicating different galaxy formation histories.

Results suggest many ellipticals are oblate spheroids with rotation-supported structures.

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 identified 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 find 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 significant 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 flux density to total stellar light show a strong minor axis correlation while the stronger radio sources do not. This split may reflect different formation histories and we suggest this may be a new manifestation of the better known dichotomy between slow rotating and fast rotating ellipticals.