Population Studies of Active Galactic Nuclei

Active Galactic Nuclei (or `AGN') are the most distant and luminous phenomena in the Universe. The essential properties of Seyfert 1 nuclei and quasars may be found in ourBeginner's Guide to AGN

This page very briefly describes recent work on AGN discovered as part of `RIXOS'; the `ROSAT International X-ray/Optical Survey'. These are relatively faint X-ray emitting AGN (Seyfert 1s to 1.9s and quasars), spanning redshifts from 0.1 to 3.4. We have measured the X-ray and optical continuum slopes and luminosities for this sample (about 160 objects); we have also derived the parameters of UV and optical emission lines, including luminosities, equivalent widths (EWs) and full widths at half maximum (FWHM).


Optical to X-ray continuum.
We find that the optical and X-ray slopes are anti-correlated, ie. they rise and fall towards the EUV together; also when the X-ray slope is soft, the optical to X-ray flux ratio is high and vice-versa. This is consistent with the effects of absorption by a cold, dusty gas and we estimate that for the sample as a whole, the RIXOS AGN have a range of absorbing column densities from 0 to 3e21 /cm^2.This gas must lie beyond the broad line region, and a possible source for it is in the molecular torus . Full details are published in Puchnarewicz et al (1996), Monthly Notices of the Royal Astronomical Society, volume 281, page 1243.

Optical and UV emission line parameters

Optical emission line strength.
 While the distribution of the forbidden [OIII]5007 equivalent widths (EWs) in the RIXOS AGN is typical of optically-selected samples, the EWs of the Balmer lines (Halpha and Hbeta) are relatively low. The diagram compares the RIXOS AGN (purple) with the samples of Stephens (1989; green) and Boroson and Green (1992; red). This is consistent with the presence of a dust absorber between the broad and narrow line regions (eg. the molecular torus), and intrinsically weak optical line emission.

The relationship between line strength and velocity.
 There is a strong correlation between the EW and FWHM of MgII2798 (see the figure opposite), which may be similar to that seen in other samples for Hbeta. We demonstrate that this is consistent with models that suggest two line-emitting zones in the BLR, a `very broad line region' (VBLR) and `intermediate line region' (ILR). The correlation between EW and FWHM in MgII may be a physical characteristic of the ILR: eg. the radiative acceleration of the line-emitting clouds; or it may reflect a geometric dependence, eg. anisotropies in the line or continuum emission, or a smaller covering factor of the ILR at large distances.

A two-zone model for the broad line region
 Wills et al. (1993) and Francis et al. (1992) proposed the VBLR/ILR two-zone model for the high-ionization lines. The emission line profile is made up two components - one relatively constant underlying component from the VBLR, and a more dynamic core from the ILR. This model is illustrated in Figure (a); as the core increases relative to the broad underlying VBLR component, the EW increases, but the line becomes narrower at FWHM, leading to an anti-correlation between EW and FWHM in high-ionization lines such as Ly-alpha and CIV1549.

We have extended this model by assuming that there is an intrinsic correlation in the component of emission lines emitted from the ILR. The predicted FWHM of the composite profile of an arbitrary line, plotted as a function of the ILR/VBLR ratio, is shown in Figure (b). It demonstrates that when the relative contribution from the VBLR is large, an anti-correlation between FWHM and EW is observed, but as the ILR contribution increases, a correlation is seen. This suggests that the high-ionization lines, which generally have higher FWHM than the low-ionization species, are dominated by emission from the VBLR, while the low-ionization lines are emitted mostly from the ILR.

Balmer line width and the X-ray spectral slope.
 Looking at the Balmer lines, we find that the Halpha FWHM is also dependent on the X-ray slope, and increases as the X-ray slope hardens (shown opposite; the outlier highlighted by the square has double-peaked Balmer lines - it may contain a binary black hole or line-emitting jets). Since the X-ray slope probably indicates the level of cold gas absorption (see above), this links the velocity of the Balmer line-emitting regions in the outer broad line region with the amount of absorption. This may be an orientation effect, consistent with the basic geometry of the Unified Model, or it may indicate that the position of the Balmer line region, relative to the central supermassive black hole , depends on the amount of gas which lies beyond the central regions.

Full details of the properties of the optical and UV emission lines for the RIXOS AGN have been published in Monthly Notices of the Royal Astronomical Society, Vol 291, p177.

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This page written by Liz Puchnarewicz (emp@mssl.ucl.ac.uk)
Last modified 6th October 1999