X-ray properties of a sample of NLS1 selected from the SDSS

Abstract

Galaxy surveys have revealed the existence of Narrow Emission Line Galaxies (NELG) which are spectroscopically classified as starforming (SF), but whose X-ray luminosity exceeds 1042 erg s?1 which unambiguously signals the presence of an AGN. The origin for such a discrepancy is not understood. In order to addres this issue, we present a study using a large sample of 239 NELG (FWHM(H? ) < 1200 km/s) from the SDSS, for which we can construct the BPT diagram and that are detected in the hard (2?10 keV) X-ray band within the 2XMM X-ray source catalogue. We find that 33 of these objects are diagnosed as starforming galaxies but their X-ray luminosity exceeds 1042 erg s?1 . We conducted a detailed analysis of this population, using other optical spectral features,like the FWHM of the H? emission line and the X-ray spectral properties. We find that the bulk of this misclassified population, which represents over 10% of our full sample of hard X-ray detected NELGs, is constituted by NLS1 galaxies.