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The Cambridge-Cambridge ROSAT Serendipity Survey - II. Classification of X-ray Luminous Galaxies

We present the results of an intermediate-resolution (1.5Å) spectroscopic study of 17 X-ray luminous narrow emission-line galaxies previously identified in the Cambridge-Cambridge {\it ROSAT} Serendipity Survey and the {\it Einstein} Extended Medium Sensitivity Survey. Emission-line ratios reveal that the sample is composed of ten Seyfert and seven starburst galaxies. Measured linewidths for the narrow H$α$ emission lines lie in the range $170-460\,$km$\,$s$^{-1}$. Five of the objects show clear evidence for asymmetry in the [OIII]$λ$5007 emission-line profile. Broad H$α$ emission is detected in six of the Seyfert galaxies, which range in type from Seyfert 1.5 to 2. Broad H$β$ emission is only detected in one Seyfert galaxy. The mean full width at half maximum for the broad lines in the Seyfert galaxies is ${\rm FWHM = 3900\pm 1750\,km\,s^{-1}}$. Broad (${\rm FWHM = 2200\pm 600\,km\,s^{-1}}$) $Hα$ emission is also detected in three of the starburst galaxies, which could originate from stellar winds or supernovae remnants. The mean Balmer decrement for the sample is H$α$/H$β=3$, consistent with little or no reddening for the bulk of the sample. There is no evidence for any trend with X-ray luminosity in the ratio of starburst galaxies to Seyfert galaxies. Based on our previous observations, it is therefore likely that both classes of object comprise $\sim 10\,$per cent of the $2\,$keV X-ray background.