Spatially-Offset AGN Candidates in the CLASS Survey

TL;DR

This study searches for offset active galactic nuclei (AGN) in nearby galaxies using radio and optical data, finding a small fraction with significant offsets and identifying new candidate offset AGN.

Contribution

It provides the first systematic radio-based search for offset AGN in SDSS galaxies and constrains their occurrence rate, highlighting potential candidates for further study.

Findings

Only 0.87% of galaxies have radio offsets >600 mas.

An excess of objects with offsets >150 mas suggests some real offset AGN.

Upper limit of ~17% for offset AGN fraction in the sample.

Abstract

Prompted by a recent claim by Barrows et al. that X-ray AGN are often found significantly offset from the centres of their host galaxies, we have looked for examples of compact radio sources which are offset from the optical centroids of nearby (z < 0.2) galaxies. We have selected a sample of 345 galaxies from the Sloan Digital Sky Survey (SDSS) galaxy catalog which have nearby compact radio sources listed in the Cosmic-Lens All Sky Survey (CLASS) catalog. We find only three matches (0.87 per cent of the sample) with offsets greater than 600 milliarcsec (mas), which is considerably fewer than we would have expected from the Barrows et al. X-ray survey. We fit our histogram of offsets with a Rayleigh distribution with {\sigma} = 60.5 mas, but find that there is an excess of objects with separations greater than approximately 150 mas. Assuming that this excess represents AGN with real offsets, we place an upper limit of approximately 17 per cent on the fraction of offset AGN in our radio-selected sample. We select 38 objects with offsets greater than 150 mas, and find they have some diverse properties: some are well known, such as Mrk 273 and Arp 220, and others have dust lanes which may have affected the optical astrometry, while a few are strong new candidates for offset AGN.