Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Decline of the space density of quasars between z=2 and z=4   We define a new complete sample of 13 optically-luminous radio quasars
M_AB(1450 Angstrom) < -26.9 mag and log P_1.4 GHz(W/Hz) > 25.7 with redshift
3.8 < z < 4.5, obtained by cross-correlating the FIRST radio survey and the APM
catalogue of POSS-I. We measure the space density to be 1.0 +/- 0.3 /Gpc^3, a
factor 1.9 +/- 0.7 smaller than the space density of similar quasars at z=2.
Using a new measurement of the radio-loud fraction of quasars we find that at
z=4 the total space density of quasars with M_AB(1450 Angstrom) < -26.9 is 7.4
+/- 2.6/Gpc^3. This is a factor 1.8 +/- 0.8 less than the space density at z=2,
found by the 2dF quasar survey. This (z=2)/(z=4) ratio, consistent with that of
the radio-loud quasars, is significantly different from the ratio of about 10
found for samples including lower-luminosity quasars. This suggests that the
decline of the space density beyond z=2 is slower for optically-luminous
quasars than for less-luminous ones.

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The categories of this paper are: alg-geom
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The categories of this paper are: hep-lat
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The categories of this paper are: patt-sol
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The categories of this paper are: dg-ga
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The categories of this paper are: econ
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The categories of this paper are: math
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The categories of this paper are: solv-int
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The categories of this paper are: chao-dyn
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The categories of this paper are: bayes-an
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The categories of this paper are: cs
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The categories of this paper are: ao-sci
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The categories of this paper are: hep-ph
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The categories of this paper are: supr-con
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The categories of this paper are: hep-th
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The categories of this paper are: nucl-th
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The categories of this paper are: gr-qc