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X-ray luminous narrow emision-line galaxies (NELG) have been previouslyidentified and proposed as an important class of extragalactic X-ray sources,with a potentially significant contribution to the total extragalactic X-rayflux at energies below $\sim$ 10 keV. In order to investigate and clarify thispossibility, we have used a sample of NELG found in 5 deep ROSAT fields andsimilar samples belonging to the Cambridge-Cambridge ROSAT Serendipity Surveyand to the {\it Einstein} Observatory Extended Medium Sensitivity Surveysample. The principal results of this investigation are as follows: a) for agiven optical luminosity, the typical X-ray luminosity of NELGs is about one ortwo orders of magnitude higher than that of normal galaxies; b) the ratio ofthe surface density of NELGs compared with BLAGN increases from about 0.04 atfx >= 6 x 10^{-13} erg cm^{-2} s^{-1} to about 0.1 at fx >= 10^{-14} ergcm^{-2} s^{-1}, suggesting that the surface density of NL galaxies might bevery close to that of BLAGN at fx ~ 10^{-15} erg cm^{-2} s^{-1}; c) we findthat these objects are described by a cosmological evolution rate similar tothat of soft X-ray selected BLAGN; d) the de-evolved (z=0) XLF of NELGs in theluminosity 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 issignificantly 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 luminositiessuch that at Lx(z=0) < 10^{42} erg s^{-1} the spatial density of NELGs is veryclose to that of BLAGN. The implications of these results for the contributionof this class of objects to the cosmic X-ray background are discussed.Comment: To be published in M.N.R.A.S., 10 pages, 2 Tables. Figures are  available in the anonymous ftp account on ftp://mds.pha.jhu.edu/pub/nl_pape

A Deep ROSAT Survey — X. X-ray-luminous narrow-emission-line galaxies