Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A main sequence for quasars   The last 25 years saw a major step forward in the analysis of optical and UV
spectroscopic data of large quasar samples. Multivariate statistical approaches
have led to the definition of systematic trends in observational properties
that are the basis of physical and dynamical modeling of quasar structure. We
discuss the empirical correlates of the so-called "main sequence" associated
with the quasar Eigenvector 1, its governing physical parameters and several
implications on our view of the quasar structure, as well as some luminosity
effects associated with the virialized component of the line emitting regions.
We also briefly discuss quasars in a segment of the main sequence that includes
the strongest FeII emitters. These sources show a small dispersion around a
well-defined Eddington ratio value, a property which makes them potential
Eddington standard candles.

29
The categories of this paper are: funct-an
4
The categories of this paper are: cond-mat
12
The categories of this paper are: nucl-th
7
The categories of this paper are: hep-th
25
The categories of this paper are: patt-sol
28
The categories of this paper are: alg-geom
31
The categories of this paper are: ao-sci
0
The categories of this paper are: hep-ph
19
The categories of this paper are: eess
26
The categories of this paper are: bayes-an
13
The categories of this paper are: quant-ph
5
The categories of this paper are: gr-qc
21
The categories of this paper are: adap-org
17
The categories of this paper are: q-fin
11
The categories of this paper are: hep-lat
9
The categories of this paper are: nlin