Narrow-emission-line galaxies and the origin of the unresolved X-ray background

Recently, from deep observations utilizing the ROSAT PSPC in the 0.5-2 keV X-ray band, it has become apparent that there is the possibility of the existence of a large (1000-2000 deg(-2)) population of narrow-emission-line galaxies (NELGs) lying below the source counts of AGN at faint fluxes. Here it is assumed that these NELGs are a mix of Seyfert 2 and starburst galaxies that may be the higher redshift counterparts of the IRAS warm galaxies and are associated with the submJy radio population. Using luminosity functions defined in the infrared and assumptions about the average log (L-x/L-60) ratio, an attempt is made to fit the new NELG source counts with a model incorporating pure luminosity evolution of the form L(z) = L(0)(1+z)(k). Previous determinations of the local Seyfert 2 and starburst log (L-x/L-60) ratio predict contributions to the NELG population of 26 and 5 per cent respectively by S (0.5-2 keV) = 3 x 10(-15) erg s(-1) cm(-2), but are found to be too low to fit the observed counts. A higher ratio, closer to that of broad-line AGN, is needed. The two populations are initially treated separately due to the present lack of a large statistical sample in current X-ray surveys. It is found that using a model with k = 3.1, as previously used for QSOs and starburst galaxies, the source counts to the faintest limits can be fitted independently of the choice of energy index, alpha(x). Decreasing the evolution to a lower rate of k = 2.5, we find that models utilizing a softer index (alpha(x) = 1.0) cannot fit the source counts at the faint fluxes. The contribution of the NELG population to the 1-2 keV X-ray background (XRB) is calculated. It is found that if the published value of the log (L-x/L-60) ratio is used, both Seyfert 2 and starburst galaxies are expected to contribute between 4 and 10 per cent each of the 1-2 keV soft XRB. Fits of the NELG model to the source count observations produce a contribution of between 30 and 50 per cent and between 10 and 27 per cent for starburst and Seyfert 2 galaxies respectively, depending on the model parameters, implying that the NELG population can supply the rest of the unresolved soft XRB that remains unexplained by QSOs. A combination of the two NELG candidates has the effect of lowering the log (L-x/L-60) of each component. In this case the overall contribution to the soft XRB would be between 39 and 48 per cent, depending on the model parameters.