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See detailFifty Years of Quasars. Quasars Classes and Their Relationships
D'Onofrio, Mauro; Marziani, Paola; Sulentic, Jack W. et al

in D'Onofrio, M. (Ed.) Fifty Years of Quasars: From Early Observations and Ideas to Future Research (2012)

Most of the questions in this chapter deal with sources that do not show the "classical" broad emission-line spectrum that characterizes the majority of known AGNs at high and low redshift. If that ... [more ▼]

Most of the questions in this chapter deal with sources that do not show the "classical" broad emission-line spectrum that characterizes the majority of known AGNs at high and low redshift. If that majority represents a "parent population" of AGNs, is the apparent absence of broad lines a result of obscuration, orientation, or different physical conditions? Can all of the subclasses be unified under the AGN umbrella? With the unification scheme set in place and assumed to be fundamentally correct, there are at least four overarching questions: do all type-2 AGNs possess an obscured broad-line region? Or how can we distinguishing type-2 AGNs without a broad-line region if they exist? Where is the low end of quasar activity? The least luminous AGNs are the so-called low-ionization nuclear emission-line regions (LINERs), but it is legitimate to ask if they are all true AGNs. [less ▲]

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