3.1 Observational Methods
The broad emission lines are quite distinctive features of AGN spectra, and since their generation involves well-understood processes (atomic physics), a wealth of information has been produced, using standard astronomical methods. The basic assumption of current BLR studies is that the line emission is due to photoionization by the continuum. 83 Since the BLR resides well outside the central engine, the observed effects may be regarded as somewhat secondary, being due to reprocessing.
The emission lines can be divided into two varieties: high
ionization lines (HIL) such as L
and CIV
1550, and
low ionization
lines (LIL) such as the Balmer and FeII lines. They are normally
studied with respect to three different qualities, each with its own
method of analysis 84:
I. Line intensities
These yield information about various physical gas quantities, such as the chemical composition, the electron density and temperature, and the ionization structure. Ratios between various elements are often used. An important fact is that these ratios seem independent of the source luminosity, indicating similar physical conditions (or mechanisms) in both faint and bright objects. Both HIL and LIL are often present simultaneously. About 20 broad (and as many narrow) lines are available for study in a typical AGN.
II. Line profiles
Typical widths of the broad emission lines correspond to gas velocities ~ 1.5 x 103-104 km s-1. The narrow emission lines are also broad enough to exclude thermal motion. Some profiles are smooth and symmetric, whereas others show clear asymmetries. The latter case may indicate an emission region in radial motion, which is directed inwards if the red wing responds to a continuum change before the blue wing, and outwards in the opposite case. 85 If both wings respond simultaneously, chaotic or circular motion may dominate. 86 Chaotic motion means here that the matter moves along open trajectories in the gravitational field of the central object.
III. Line variability
Spectrophotometric observations have shown that most, perhaps all, Seyfert 1 galaxies have variable broad emission lines. Some QSOs have also shown this behaviour, even though systematic observations are scarce. The line variability usually lags behind the continuum one, in a correlated manner. This constitutes strong evidence in favour of the photoionization hypothesis. The lag is correlated with the luminosity, indicating a more extended BLR in powerful sources.