ROSAT PSPC spectra of X-ray selected Narrow Emission Line Galaxies

Abstract

We analyse the ROSAT PSPC spectrum of 19 X-ray selected Narrow Emission Line Galaxies (NELGs) discovered during the optical identification of sources in the ROSAT UK Deep Survey. Their properties are compared to those of broad line Active Galactic Nuclei (AGN) in the same sample. Counts in three spectral bands have been extracted for all the sources, and have been fitted with a power-law model assuming the Galactic value for NH. The average slope of NELGs is α=0.45+-0.09, whilst for the AGN it is α=0.96+-0.03. The power-law model is a good fit for ~90% of NELGs and ~75% of AGN. Recent work shows that the fractional surface density of NELGs increases with respect to AGN at faint fluxes. Thus they are expected to be an important component of the residual soft (<2 keV) X--ray background. The slope of the X--ray background (α~0.4, 1-10 keV) is harder than that of AGN (α~1) but our results show that it is consistent with the summed spectrum of the NELGs in the deep survey (α~0.4). This may finally reconcile the spectrum of the background with the properties of the sources that constitute it.