A deep ROSAT Survey X: X-ray Luminous Narrow Emission Line Galaxies

X-ray luminous narrow emision-line galaxies (NELG) have been previously identified and proposed as an important class of extragalactic X-ray sources, with a potentially significant contribution to the total extragalactic X-ray flux at energies below $\sim$ 10 keV. In order to investigate and clarify this possibility, we have used a sample of NELG found in 5 deep ROSAT fields and similar samples belonging to the Cambridge-Cambridge ROSAT Serendipity Survey and to the {\it Einstein} Observatory Extended Medium Sensitivity Survey sample. 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 about 0.04 at fx >= 6 x 10^{-13} erg cm^{-2} s^{-1} to about 0.1 at fx >= 10^{-14} erg cm^{-2} s^{-1}, suggesting that the surface density of NL galaxies might be very close to that of BLAGN at fx ~ 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; d) the de-evolved (z=0) XLF of NELGs in the luminosity range 10^{41} - 5 x 10^{43} erg s^{-1} is steeper than the BLAGN (z=0) XLF in the same luminosity interval. Their spatial density is significantly lower than the spatial density of X-ray selected BLAGN at Lx(z=0) ~ 5 x 10^{43} erg s^{-1}, but this difference decreases at lower luminosities such that at Lx(z=0) < 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.