Spatially Offset Active Galactic Nuclei I: Selection and Spectroscopic Properties

TL;DR

This study presents a sample of 18 spatially offset active galactic nuclei (AGN) from SDSS and Chandra data, using their offsets to explore AGN properties in galaxy mergers and their complex gas kinematics.

Contribution

The paper introduces a new sample of spatially offset AGN identified through combined optical and X-ray data, enabling unbiased studies of AGN in merging galaxies.

Findings

Offset AGN range from 0.8 to 19.4 kpc in projected distance.

Spectroscopic offset AGN selection may be up to 89% incomplete.

Velocity offsets suggest complex gas kinematics in merging systems.

Abstract

We present a sample of 18 optically-selected and X-ray detected spatially offset active galactic nuclei (AGN) from the Sloan Digital Sky Survey (SDSS). In 9 systems, the X-ray AGN is spatially offset from the galactic stellar core that is located within the 3'' diameter SDSS spectroscopic fiber. In 11 systems, the X-ray AGN is spatially offset from a stellar core that is located outside the fiber, with an overlap of 2. To build the sample, we cross-matched Type II AGN selected from the SDSS galaxy catalogue with archival Chandra imaging and employed our custom astrometric and registration procedure. The projected angular (physical) offsets span a range of 0."6 (0.8 kpc) to 17."4 (19.4 kpc), with a median value of 2."7 (4.6 kpc). The offset nature of an AGN is an unambiguous signature of a galaxy merger, and these systems can be used to study the properties of AGN in galaxy mergers without the biases introduced by morphological merger selection techniques. In this paper (Paper I), we use our sample to assess the kinematics of AGN photoionized gas in galaxy mergers. We find that spectroscopic offset AGN selection may be up to 89% incomplete due to small projected velocity offsets. We also find that the magnitude of the velocity offsets are generally larger than expected if our spatial selection introduces a bias toward face-on orbits, suggesting the presence of complex kinematics in the emission line gas of AGN in galaxy mergers.