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

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Abstract
We present the results of an intermediate-resolution (1.5 Å) spectroscopic study of 17 X-ray-luminous narrow emission-line galaxies previously identified in the Cambridge–Cambridge ROSAT Serendipity Survey and the 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 Hα emission lines lie in the range 170–460km s–1. Five of the objects show clear evidence for asymmetry in the [OIII]λ5007 emission-line profile. Broad Hα emission is detected in six of the Seyfert galaxies, which range in type from Seyfert 1.5 to 2. Broad Hβ emission is only detected in one Seyfert galaxy. The mean full width at half maximum for the broad lines in the Seyfert galaxies is FWHM = 3900±1750km s–1. Broad (FWHM = 2200±pm 600km s–1) Hα emission is also detected in three of the starburst galaxies. This emission could originate from stellar winds or supernovae remnants. The mean Balmer decrement for the sample is Hα/Hβ = 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 ∼10 per cent of the 2-keV X-ray background.
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