Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Star Formation at z~6: The UDF-Parallel ACS Fields   We report on the i-dropouts detected in two exceptionally deep ACS fields
(B_{435}, V_{606}, i_{775}, and z_{850} with 10 sigma limits of 28.8, 29.0,
28.5, and 27.8, respectively) taken in parallel with the UDF NICMOS
observations. Using an i-z>1.4 cut, we find 30 i-dropouts over 21 arcmin^2 down
to z_AB=28.1, or 1.4 i-dropouts arcmin^{-2}, with significant field-to-field
variation (as expected from cosmic variance). This extends i-dropout searches
some ~0.9^m further down the luminosity function than was possible in the GOODS
field, netting a ~7x increase in surface density. An estimate of the size
evolution for UV bright objects is obtained by comparing the composite radial
flux profile of the bright i-dropouts (z<27.2) with scaled versions of the
HDF-N + HDF-S U-dropouts. The best-fit is found with a (1+z)^{-1.57_{-0.53}
^{+0.50}} scaling in size (for fixed luminosity), extending lower redshift
(1<z<5) trends to z~6. Adopting this scaling and the brighter i-dropouts from
both GOODS fields, we make incompleteness estimates and construct a z~6 LF in
the rest-frame continuum UV (~1350 A) over a 3.5 magnitude baseline, finding a
shape consistent with that found at lower redshift. To evaluate the evolution
in the LF from z~3.8, we make comparisons against different scalings of a lower
redshift B-dropout sample. Though a strong degeneracy is found between
luminosity and density evolution, our best-fit model scales as (1+z)^{-2.8} in
number and (1+z)^0.1 in luminosity, suggesting a rest-frame continuum UV
luminosity density at z~6 which is just 0.38_{-0.07} ^{+0.09}x that at z~3.8.
Our inclusion of size evolution makes the present estimate lower than previous
z~6 estimates.

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The categories of this paper are: plasm-ph
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The categories of this paper are: atom-ph
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The categories of this paper are: cmp-lg
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The categories of this paper are: patt-sol
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The categories of this paper are: funct-an
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The categories of this paper are: adap-org
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The categories of this paper are: chem-ph
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The categories of this paper are: nucl-th
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The categories of this paper are: bayes-an
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The categories of this paper are: econ
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The categories of this paper are: hep-ph
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The categories of this paper are: ao-sci
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The categories of this paper are: nucl-ex
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The categories of this paper are: supr-con
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The categories of this paper are: cond-mat
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The categories of this paper are: hep-ex