X-ray bright optically inactive galaxies inXMM-Newton/Sloan Digital Sky Survey fields: more diluted than absorbed?

Abstract

We explore the properties of X-ray bright optically inactive galaxies (XBONGs) detected in the 0.5-8 keV spectral band in 20 public XMM-Newton fields overlapping with the Sloan Digital Sky Survey (SDSS). We constrain our sample to optically extended systems with log f X /f opt > -2 that have spectroscopic identifications available from the SDSS (r < 19.2 mag). The resulting sample contains 12 objects with L X (0.5-8 keV) = 5 x 10 41 -2 x 10 44 erg s -1 in the redshift range 0.06 < z < 0.45. The X-ray emission in four cases is extended, suggesting the presence of hot gas associated with a cluster or group of galaxies. The X-ray spectral fits show that two additional sources are best fit with a thermal component emission (kT ∼ 1 keV). Three sources are most likely associated with active galactic nuclei (AGNs): their X-ray spectrum is described by a steep photon index r ∼ 1.9 typical of unobscured AGNs, while they are very luminous in X-rays [L X (0.5-8 keV) 10 43 -10 44 erg s -1 ]. Finally, three more sources could be associated with either normal galaxies or unobscured low-luminosity AGNs (L X < 10 42 erg s -1 ). We find no evidence for significant X-ray absorbing columns in any of our XBONGs. The above suggest that XBONGs, selected in the total 0.5-8 keV band, comprise a mixed bag of objects primarily including normal elliptical galaxies and type 1 AGNs whose optical nuclear spectrum is probably diluted by the strong stellar continuum. Nevertheless, as our sample is not statistically complete we cannot exclude the possibility that a fraction of optically fainter XBONGs may be associated with heavily obscured AGNs.