Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: QSO Lifetimes   The QSO lifetime t_Q is one of the most fundamental quantities for
understanding black hole and QSO evolution, yet it remains uncertain by several
orders of magnitude. If t_Q is long, then only a small fraction of galaxies
went through a luminous QSO phase. In contrast, a short lifetime would require
most galaxies today to have undergone a QSO phase in their youth. The current
best estimates or constraints on t_Q from black hole demographics and the
radiative properties of QSOs vary from at least 10^6 to 10^8 years. This broad
range still allows both possibilities: that QSOs were either a rare or a common
stage of galaxy evolution. These constraints also do not rule out the
possibility that QSO activity is episodic, with individual active periods much
shorter than the total active lifetime.
  In the next few years a variety of additional observational constraints on
the lifetimes of QSOs will become available, including clustering measurements
and the proximity effect. These new constraints can potentially determine t_Q
to within a factor of 3 and therefore answer one of the most fundamental
questions in black hole evolution: Do they shine as they grow? This precision
will also test the viability of our current model for accretion physics,
specifically the radiative efficiency and need for super-Eddington luminosities
to explain the black hole population.

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The categories of this paper are: hep-ph
4
The categories of this paper are: cond-mat
35
The categories of this paper are: mtrl-th
10
The categories of this paper are: q-bio
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The categories of this paper are: q-fin
36
The categories of this paper are: cmp-lg
12
The categories of this paper are: nucl-th
20
The categories of this paper are: acc-phys
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The categories of this paper are: cs
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The categories of this paper are: stat
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The categories of this paper are: nucl-ex
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The categories of this paper are: hep-lat
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The categories of this paper are: comp-gas
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The categories of this paper are: chao-dyn
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The categories of this paper are: plasm-ph
34
The categories of this paper are: chem-ph