On the nature of high X-ray luminosities in SDSS galaxies

TL;DR

This study investigates high X-ray luminosity SDSS galaxies lacking optical AGN signatures, finding that their X-ray emission is likely powered by obscured or diluted AGN activity rather than star formation.

Contribution

It provides evidence that high X-ray luminosity in some SDSS galaxies is due to AGN activity hidden by star formation signatures, based on variability and spectral analysis.

Findings

X-ray emission is compact and variable, indicating AGN activity.

Spectral analysis shows no signs of star formation in X-ray data.

Optical signatures are diluted by host galaxy star formation.

Abstract

Surveys have revealed a class of object displaying both high X-ray luminosities (Lx > 10^42 erg/s), and a lack of a discernible active galactic nucleus (AGN) in the optical band. If these sources are powered by star formation activity alone, they would be the most extreme X-ray luminosity star forming galaxies known. We have investigated the mechanism driving the X-ray luminosities of such galaxies by studying the X-ray emission of three moderate redshift (z ~ 0.1) examples of this class, selected from a cross-correlation of the SDSS-DR5 and 2XMMp-DR0 catalogues. X-ray spatial and long-term variability diagnostics of these sources suggest that they are compact X-ray emitters. This result is supported by the detection of rapid short term variability in an observation of one of the sources. The X-ray spectra of all three sources are best fitted with a simple absorbed power-law model, thus betraying no significant signs of star formation. These results indicate that the X-ray emission is powered by AGN activity. But why do these sources not display optical AGN signatures? We show that the most likely explanation is that the optical AGN emission lines are being diluted by star formation signatures from within their host galaxies.