Abstract
In this paper we present the first results from an ongoing serendipitous survey aiming to identify X-ray-selected 'normal' galaxies [i.e. not dominated by active galactic nuclei (AGNs)] by combining archival XMM-Newton data with the Sloan Digital Sky Survey. In the first 4.5 deg 2 of this program, we have identified a total of 11 'normal' galaxy candidates (eight of them with optical spectroscopy) with fluxes f X (0.5-8 keV) 10 -15 -10 -13 erg s -1 cm -2 . These sources are selected to have low X-ray-to-optical flux ratio (log f X /f opt ≤ -2), soft X-ray spectral properties and optical spectra, when available, consistent with the presence of a stellar ionizing continuum. These sources comprise both early- and late-type systems at redshifts z ≤ 0.2 with luminosities L X (0.5-8 keV) 10 39 -10 42 erg s -1 . This data set provides the first tight constraint on the surface density of X-ray-selected 'normal' galaxies at relatively bright fluxes spanning 2 orders of magnitude (10 -15 -10 -13 erg s -1 cm -2 ). The slope of the 'normal' galaxy log N-log S relation in the above flux range is estimated to be -1.4 ± 0.3, consistent with the Euclidean prediction. We also discuss the prospects of 'normal' galaxy studies at X-ray wavelengths using both our continuously expanding survey and future X-ray missions.
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