Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Evolution since z = 0.5 of the Morphology-Density relation for Clusters
  of Galaxies   Using traditional morphological classifications of galaxies in 10
intermediate-redshift (z~0.5) clusters observed with WFPC-2 on the Hubble Space
Telescope, we derive relations between morphology and local galaxy density
similar to that found by Dressler for low-redshift clusters. Taken
collectively, the `morphology-density' relationship, M-D, for these more
distant, presumably younger clusters is qualitatively similar to that found for
the local sample, but a detailed comparison shows two substantial differences:
(1) For the clusters in our sample, the M-D relation is strong in centrally
concentrated ``regular'' clusters, those with a strong correlation of radius
and surface density, but nearly absent for clusters that are less concentrated
and irregular, in contrast to the situation for low redshift clusters where a
strong relation has been found for both. (2) In every cluster the fraction of
elliptical galaxies is as large or larger than in low-redshift clusters, but
the S0 fraction is 2-3 times smaller, with a proportional increase of the
spiral fraction. Straightforward, though probably not unique, interpretations
of these observations are (1) morphological segregation proceeds
hierarchically, affecting richer, denser groups of galaxies earlier, and (2)
the formation of elliptical galaxies predates the formation of rich clusters,
and occurs instead in the loose-group phase or even earlier, but S0's are
generated in large numbers only after cluster virialization.

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The categories of this paper are: nucl-th
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The categories of this paper are: alg-geom
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The categories of this paper are: nucl-ex
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The categories of this paper are: quant-ph
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The categories of this paper are: hep-ex
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The categories of this paper are: q-bio
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The categories of this paper are: cs
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The categories of this paper are: adap-org
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The categories of this paper are: math-ph
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The categories of this paper are: solv-int
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The categories of this paper are: q-fin
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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: funct-an
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The categories of this paper are: hep-th
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The categories of this paper are: stat