Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Upper Limits on Metals in Quasar Lyman-Alpha Forest Clouds: Absence of C
  IV Lines in Echelle Spectra   Recently Lu presented tentative evidence for C IV lines in QSO Lyman-alpha
forest systems with strong lines. We have performed a similar search for C IV
in our 10 km/sec echelle spectra of the bright QSO HS~1946+7658. We shifted the
spectra to align the expected positions of the C IV lines in 65 Lyman-alpha
systems, then added them. The resulting composite spectrum, equivalent to 390
hours of exposure time on the Lick 3-m telescope, has a signal-to-noise ratio
of 80 per 0.025 Angstrom in the rest frame of the absorbers. We do not see any
C IV lines down to a $2\sigma$ limit of W$(1548) \leq 1.4 $~m\AA, about
one-fifth of the strength of the lines seen by Lu. The C IV lines which Lu saw
must be restricted to rare Lyman-alpha systems with large H I column densities
$\geq 10^{14}$~cm$^{-2}$, which are too rare to show C IV in our sample. More
common Lyman-alpha systems with H I column densities of $10^{13}$ --
$10^{14}$~cm$^{-2}$ do not show C IV lines. If their ionization is H/H~I $=
10^4$ then they have [C/H] $\leq -2.0$.

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The categories of this paper are: hep-ph
36
The categories of this paper are: cmp-lg
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The categories of this paper are: adap-org
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The categories of this paper are: bayes-an
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The categories of this paper are: hep-lat
35
The categories of this paper are: mtrl-th
7
The categories of this paper are: hep-th
1
The categories of this paper are: cs
32
The categories of this paper are: atom-ph
29
The categories of this paper are: funct-an
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The categories of this paper are: econ
27
The categories of this paper are: comp-gas
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The categories of this paper are: gr-qc
20
The categories of this paper are: acc-phys
10
The categories of this paper are: q-bio
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The categories of this paper are: stat