A deep ROSAT survey .10. X-ray-luminous narrow-emission-line galaxies

X-ray-luminous narrow-emission-line galaxies (NELGs) have been previously identified and proposed as an important class of extragalactic X-ray source, with a potentially significant contribution to the total extragalactic X-ray flux at energies below similar to 10 keV. In order to investigate and clarify this possibility, we have used a sample of NELGs found in five deep ROSAT fields, and similar samples belonging to the Cambridge-Cambridge ROSAT Serendipity Survey and to the Einstein Observatory Extended Medium Sensitivity Survey sample. We have been able to study their X-ray properties, derive their number-flux relationship, investigate their cosmological evolution and derive a preliminary X-ray-luminosity function (XLF) for this class of objects. We have compared the above-mentioned properties to those exhibited by soft-X-ray-selected broad-line active galactic nuclei (BLAGN) and/or normal galaxies. The principal results of this investigation are as follows: (a) for a given optical luminosity, the typical X-ray luminosity of NELGs is about one or two orders of magnitude higher than that of normal galaxies; (b) the ratio of the surface density of NELGs compared with BLAGN increases from similar to 0.04 at f(x) greater than or similar to 6 x 10(-13) erg cm(-2) s(-1) to similar to 0.1 at f(x) greater than or similar to 10(-14) erg cm(-2) s(-1), suggesting that the surface density of NL galaxies might be very close to that of BLAGN at f(x) similar to 10(-15) erg cm(-2) s(-1); (c) we find that these objects are described by a cosmological evolution rate similar to that of soft-X-ray-selected BLAGN; and (d) the de-evolved (z = 0) XLF of NELGs in the luminosity range 10(41) to 5 x 10(43) erg s(-1) is steeper than the BLAGN (z = 0) XLF in the same luminosity interval. The spatial density of NELGs is significantly lower than that of X-ray-selected BLAGN at L(x) (z = 0) similar to 5 x 10(43) erg s(-1), but this difference decreases at lower luminosities such that at L(x) (z = 0) less than or equal to 10(42) erg s(-1) the spatial density of NELGs is very close to that of BLAGN. The implications of these results for the contribution of this class of objects to the cosmic X-ray background are discussed.