OPTICAL AND X-RAY-PROPERTIES OF THE RIXOS AGN .2. EMISSION-LINES

We present the optical and UV emission-line properties of 160 X-ray-se lected active galactic nuclei (AGN) taken from the RIXOS survey (inclu ding H alpha, H beta, [O III] lambda 5007, Mg II lambda 2798 and C III ] lambda 1909). This sample is believed to contain a mixture of absorb ed and unabsorbed objects, with column densities up to similar to 4 x 10(21) cm(-2). Although the distribution of the [O III] lambda 5007 EW for the RIXOS AGN is typical of optically selected samples, the Balme r Line EWs are relatively low. This is consistent with the presence of a dust absorber between the broad- and narrow-line regions (e.g. a mo lecular torus), and intrinsically weak optical line emission. We find Baldwin effects in C III] lambda 1909 and Mg II lambda 2798, and a pos itive response of the Mg II line to its ionizing continuum. There is a strong correlation between the EW and FWHM of Mg II, which may be sim ilar to that seen in other samples for H beta. We demonstrate that thi s is consistent with models that suggest two line-emitting zones, a 'v ery broad-line region' (VBLR) and an 'intermediate-line region' (ILR). The correlation between EW and FWHM in Mg II may be a physical charac teristic of the ILR: e.g. the radiative acceleration of the line-emitt ing clouds; or it may reflect a geometric dependence, e.g. anisotropie s in the Line or continuum emission, or a smaller covering factor of t he ILR at large distances. We found no correlation between the H beta FWHM and the slope of the X-ray spectrum, despite reports of correlati ons in other samples. However, this may be due to the effects of dust absorption, which suppresses the broad H beta component, masking any r elationship. The H alpha FWHM does tend to be narrow when alpha(x) is soft, and broadens as alpha(x) hardens, although the formal probabilit y for this correlation is low (91 per cent). If the distribution of al pha(x) in the RIXOS sample reflects the level of intrinsic absorption in these AGN, the data suggest a possible link between the velocity of the Balmer line-emitting region and the amount of absorbing material beyond.