9.3. Observed Line Profiles
The large diversity in line profiles has already been demonstrated in chapter 2 and several more examples are given in Fig. 23. The broad line profiles of many objects are smooth and symmetric. Their shape is well fitted, in many cases, by a logarithmic function (E -log | - 0|, see Fig. 23). Profile differences can be large or small. There are cases of almost identical profiles for lines of very different excitation. A likely explanation here is that the emission line spectrum of all clouds is the same. There are opposite examples too, and a clear trend is still to be found. It has been claimed that the high excitation lines are systematically broader than the low excitation lines, but there are definitely some exceptions to that. One common trend is observed between the H and H line profiles, where the latter is relatively stronger away from the line center (see diagram). Another trend is for the HeI5876 line to be somewhat broader than H.
Figure 23. Observed broad line profiles. Bottom: A symmetric CIV1549 line (solid curve) fitted with a logarithmic profile (dashed line)(courtesy of B. Wilkes). Top left: H, H and their ratio, showing the larger width of H (courtesy of G. Stirpe). Top right: Emission line redshift differences (Corbin 1990).
Broad, asymmetric lines are seen in many objects, but no clear tendency for the blue or the red wing to be stronger. Some line profiles are bumpy, and their shape varies in time. The best known examples are some steep spectrum radio sources whose line profiles are extremely broad and disturbed. It has been suggested that the lines are emitted in an edge-on, disk-like system, with axis parallel to the direction of the radio jet.
Finally, there is a clear tendency for the high excitation lines in luminous AGNs to be systematically blue-shifted relative to the low excitation lines. The effect can be large, up to 3000 km s-1, and seems to increase with luminosity. The few detailed studies of this phenomenon suggest that the systemic redshift is given by the low excitation lines. An example is shown in Fig. 23.
As for the narrow lines, the profiles are smooth with a clear tendency for the blue wing of some lines to be stronger. The blue asymmetry is most noticeable in lines of higher excitation and/or critical de-excitation density. This part of the line originates in high density, fast moving NLR clouds. The phenomenon is observed in most Seyfert 1 and Seyfert 2 galaxies.