The Needles in the Haystack Survey: Searching for X-ray-selected normal galaxies

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) approximate to 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)less than or similar to-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 zless than or similar to 0.2 with luminosities L-X(0.5-8 keV) approximate to 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.