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.
Highlights
Deep and wide area X-ray surveys (e.g. Alexander et al 2003; Lehmer et al 2005; Kim et al 2007; Elvis et al 2009; Watson et al 2009; Xue et al 2010) have resolved up to ≈90 per cent of the X-ray background into discrete sources over the 0.5–8 keV band (e.g. Lumb et al 2002; Moretti et al 2003; Bauer et al 2004; Worsley et al 2005; Hickox & Markevitch 2006)
The basic question posed at the beginning of this work was: what can power more than 1042 erg s−1 of X-ray luminosity from a galaxy with no unambiguous signatures of active galactic nucleus (AGN) activity? We have examined this by selecting three galaxies at a redshift of ∼0.1 from SDSS for further study at X-ray energies, based on the requisite high apparent X-ray luminosity and an optical spectrum betraying only signs of star formation processes
The work presented in the previous section details the results of several simple tests with the power to diagnose whether the X-ray emission originates in star formation processes or an AGN
Summary
Deep and wide area X-ray surveys (e.g. Alexander et al 2003; Lehmer et al 2005; Kim et al 2007; Elvis et al 2009; Watson et al 2009; Xue et al 2010) have resolved up to ≈90 per cent of the X-ray background into discrete sources over the 0.5–8 keV band (e.g. Lumb et al 2002; Moretti et al 2003; Bauer et al 2004; Worsley et al 2005; Hickox & Markevitch 2006). The majority of the objects detected in these surveys are luminous X-ray sources at z ≈ 0.2–5 with LX ≥ 1042 erg s−1 (e.g. Hornschemeier et al 2001; Barger 2003; Brandt & Hasinger 2005; Treister, Urry & Lira 2005; Silverman et al 2010). Moran, Filippenko & Chornock 2002; Maiolino et al 2003; Goulding & Alexander 2009a,b) or due to star formation activity These objects are distinct from the population of X-ray dim, ‘normal’, star-forming galaxies being revealed in deep high-Z surveys (e.g. Lehmer et al 2008)
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