|QSO hosts||Searching for H-R diagram of QSO||The redshift of BL Lacertae objects||The core fundamental plane of radio galaxies|
The Hertzsprung-Russell (H-R) diagram discriminates between the principal classes of normal stars and isolates important states of stellar evolution. Most of this is accomplished with a simple 2 parameter space. We are searching an equivalent diagram that might help clarify the phenomenology of active galactic nuclei (AGN) and lead to better understanding the physics that drives them. It was expected that more than two dimensions would be required to remove the degeneracy between physics and source orientation. Do we need an H-R diagram for broad-line AGN or will an average quasar spectrum tell us all that we need to know? We find that AGN exhibit a large phenomenological diversity, thus parameter space equivalent to the H-R diagram is invaluable (see, for a review, Marziani et al. 2006). We introduced a 4D "Eigenvector1" (4DE1) parameter space as a surrogate H-R diagram for AGN. The results are very encouraging and virtually every observational conundrum that has arisen in the past decades concerning the structure, physical condition and dynamics of the broad line emitting region (BLR) of AGN is simplified in the 4DE1 context (Sulentic et al. 2007).
|Figure 1: Physical trends along the sequence occupied by low-z AGN in the optical plane of the 4DE1 space. The abscissa reports the prominence of FeII optical emission, measured as the intensity ratio between the FeII emission blend at λ4570 and the intensity of the broad component of Hβ. The ordinate is the FWHM of the Hβ broad component. Almost all low-z quasars are located in the sketched area, which defines an observational sequence of decreasing line width and increasing FeII prominence. Theoretical analysis indicates that electron density of the emitting regions and Eddingon ratio increase along the sequence, while black hole mass and ionization parameter decrease. In other words, at the left upper corner of the diagram we see massive black holes, radiation at low Eddington ratio, and showing a high ionization spectrum; on the contrary at the lower rights we have sources like Narrow Line Seyfert 1s with prominent low ionization emission and high (~ 1). The sequence can be understood in terms of evolution from young or rejuvenated sources radiating close to the Eddington limit toward more massive and evolved systems.|
We have shown in the last 10 years that
several BLR physical and kinematic properties seem to be governed primarily by the Eddington ratio (see the figure).
Low-ionization emission line properties are remarkably similar over a very wide range
of luminosity, even at very high redshift, and broad line profile shapes show only second-order effects that can be
directly ascribed to the black hole mass. We have also verified that the BLR appears to be transparent
to radio loudness. This does not mean that radio-loud and radio-quiet samples of quasars always show similar spectra.
On the contrary only a subset of radio-quiet quasars and lobe dominated RL quasar show very similar spectra. However, what makes them
radio-different is an as yet unknown parameter that has little, if any, effect on their BLR. Radio-quiet and radio-loud
quasars may well follow a parallel evolution.
We are still left with many questions concerning the actual origin of the BLR gas, its dynamics and spatial disposition. For example, high-ionization gas (an accretion disk wind?) showing non-radial, outward motion may decrease in importance from Narrow Line Seyfert 1s,(the youngest/rejuvenated AGN), to Fanaroff-Riley II radio-loud sources (very evolved sources). The central engine sustains a prominent outflow only if the Eddington ratio is relatively high, as in the case of Narrow Line Seyfert 1 sources.
Placing these considerations on a firmer observational basis requires renewed efforts, involving nebular diagnostics line profile analysis, and the ability to understand how the viewing angle affects observed BLR parameters for each source (we cannot count on two dimensional reverberation mapping). At this time, 4DE1 is promising for decoupling the source orientation from physics. We are actively pursuing the study of high-redshift AGN with unprecedented resolution and S/N with the help of VLT and HST observations.