Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Spontaneous and Stimulated Star Formation in Galaxies   We present recent results from several on-going studies: The first addresses
the question of gas-density thresholds for star formation, as probed by the
outer disks of normal nearby galaxies. The second concerns the observational
evidence for the existence of gravitating non-luminous (GNL) galaxies, as
predicted by most recent simulations of galaxy formation in Lambda-CDM
cosmologies. We find that (1) If star formation is traced by far-ultraviolet
light, then there is no evidence for a threshold to star formation at any gas
density so far probed, and (2) there is no evidence for GNL galaxies
gravitationally interacting with known optical systems based on the
observations (a) that there are no ring galaxies without plausible optically
visible intruders, (b) all peculiar galaxies in the Arp Atlas that are bodily
distorted have nearby plausibly interacting companions, and (c) there are no
convincingly distorted/peculiar galaxies within Karachentsev's sample of more
than 1,000 apparently/optically isolated galaxies.

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The categories of this paper are: dg-ga
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The categories of this paper are: hep-th
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The categories of this paper are: solv-int
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The categories of this paper are: nucl-th
5
The categories of this paper are: gr-qc
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The categories of this paper are: patt-sol
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The categories of this paper are: comp-gas
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The categories of this paper are: alg-geom
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The categories of this paper are: ao-sci
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The categories of this paper are: bayes-an
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The categories of this paper are: nucl-ex
3
The categories of this paper are: physics
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The categories of this paper are: plasm-ph
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The categories of this paper are: math-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: acc-phys