Sleeping Giants? - X-ray search for low-luminosity AGN candidates in nearby optically bright galaxies

TL;DR

This study investigates the X-ray properties of nearby bright galaxies to identify low-luminosity AGN that optical methods might miss, using XMM-Newton data and spectral analysis.

Contribution

It provides a systematic X-ray analysis of a large, optically classified galaxy sample to uncover hidden low-luminosity AGN candidates.

Findings

Approximately 43% of HII nuclei are potential LLAGN.

Around 40% of transition-LINER nuclei are potential LLAGN.

Most spectra fit thermal plasma and power-law models, indicating diverse nuclear activity.

Abstract

In this Master's project, the X-ray nuclear properties of a sample of bright nearby galaxies are explored. This is done by matching their comprehensive optical spectroscopic classification to the latest available XMM-Newton catalogue - 3XMM-DR4. The good coverage (approx. 38 per cent) ensures that a statistically representative sample is investigated. All nuclear and morphological subsets found within the original sample of 486 galaxies are encompassed, but early-type galaxies and galaxies with optical features characteristic for active galactic nuclei (AGN) are favoured. The results from the investigation of the properties of our cross-matched sample are overall consistent with the presence of a large fraction of X-ray - detected low-luminosity AGN (LLAGN). The X-ray - detected galaxies within our HII and transition-LINER subsets are of particular interest, as they could harbour LLAGN missed by optical spectroscopic selection. The properties of these nuclei are explored by X-ray spectral fitting of available XMM-Newton observations. The majority of the examined spectra are found to be consistent with a thermal plasma model and an underlying power-law continuum, and the relative contributions of a potential LLAGN and of thermal emission to the source luminosity are presented. In the case that these spectra are not highly contaminated, as confirmed for some of the nuclei by examining available higher-resolution Chandra images, we classify approx. 43 per cent of the HII nuclei and 40 per cent of the transition-LINER nuclei as optically undetected LLAGN-candidates. This classification is found to be reasonably successful, given the project limitations, when compared to X-ray and multi-wavelength results found in literature for many of the nuclei.