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

Abstract

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$\alpha$ emission lines lie in the range $170-460$kms${}^{-1}$. Five of the objects show clear evidence for asymmetry in the [OIII]$\lambda$5007 emission-line profile. Broad H$\alpha$ emission is detected in six of the Seyfert galaxies, which range in type from Seyfert 1.5 to 2. Broad H$\beta$ emission is only detected in one Seyfert galaxy. The mean full width at half maximum for the broad lines in the Seyfert galaxies is $\mathrm{FWHM}=3900\pm 1750\mathrm{km}{\mathrm{s}}^{-1}$. Broad ($\mathrm{FWHM}=2200\pm 600\mathrm{km}{\mathrm{s}}^{-1}$) $H\alpha$ 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$\alpha$/H$\beta =3$, 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.

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