| Abstract
| - 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 deg2 of this program, we have identified a total of 11 ‘normal’ galaxy candidates (eight of them with optical spectroscopy) with fluxes fX(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 fX/fopt≲−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 LX(0.5-8 keV) ≈ 1039-1042 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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