Identification of AGN in the XMM-Newton X-ray survey of the SMC

TL;DR

This study combines X-ray and infrared data from XMM-Newton, ALLWISE, and VISTA to identify and characterize active galactic nuclei behind the Small Magellanic Cloud, revealing a diverse range of AGN types and new candidates for further study.

Contribution

The paper presents a new catalog of AGN behind the SMC using combined X-ray and infrared data, including newly identified candidates validated by initial spectroscopy.

Findings

Detected AGN from nearby Seyfert galaxies to distant quasars.

X-ray hardness ratios and infrared colors are consistent with AGN properties.

Identified new candidate AGN, including obscured sources, for future follow-up.

Abstract

Context: Finding Active Galactic Nuclei (AGN) behind the Magellanic Clouds (MCs) is difficult because of the high stellar density in these fields. Although the first AGN behind the Small Magellanic Cloud (SMC) were reported in the 1980s, it is only recently that the number of AGN known behind the SMC has increased by several orders of magnitude. Aims: The mid-infrared colour selection technique has been proven to be an efficient means of identifying AGN, especially obscured sources. The X-ray regime is complementary in this regard and we use XMM-Newton observations to support the identification of AGN behind the SMC. Methods: We present a catalogue of AGN behind the SMC by correlating an updated X-ray point source catalogue from our XMM-Newton survey of the SMC with already known AGN from the literature as well as a list of candidates obtained from the ALLWISE mid-infrared colour selection criterion. We studied the properties of the sample with respect to their redshifts, luminosities and X-ray spectral characteristics. We also identified the near-infrared counterpart of the sources from the VISTA observations. Results: The redshift and luminosity distributions of the sample (where known) indicate that we detect sources from nearby Seyfert galaxies to distant and obscured quasars. The X-ray hardness ratios are compatible with those typically expected for AGN. The VISTA colours and variability are also consistent in this regard. A positive correlation was observed between the integrated X-ray flux (0.2--12 keV) and the ALLWISE and VISTA magnitudes. We further present a sample of new candidate AGN and candidates for obscured AGN. All of these make an interesting subset for further follow-up studies. An initial spectroscopic follow-up of 6 out of the 81 new candidates showed all six sources are active galaxies, albeit two with narrow emission lines.