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<sup>2</sup> 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) ~ 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<sup>-15</sup>-10<sup>-13</sup> erg s<sup>-1</sup> cm<sup>-2</sup>). 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.