Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Cosmic Metal Production and the Contribution of QSO Absorption Systems
  to the Ionizing Background   The recent discovery by Cowie \etal and Tytler \etal of metals in the \Lya
clouds shows that the intergalactic medium (IGM) at high redshift is
contaminated by the products of stars, and suggests that ionizing photons from
massive star formation may be a significant contributor to the UV background
radiation at early epochs. We assess the validity of the stellar
photoionization hypothesis. Based on recent computations of metal yields and
O-star Lyman continuum (LyC) fluxes, we find that 0.2\% of the rest-mass energy
of the metals produced is radiated as LyC. By modeling the transfer of ionizing
radiation through the IGM and the rate of chemical enrichment, we demonstrate
that the background intensity of photons at 1 Ryd that accompanies the
production of metals in the \Lya forest clouds may be significant, approaching
$0.5\times 10^{-21}\uvunits$ at $z\approx 3$ if the LyC escape fraction is
$\gta 0.25$. Together with quasars, massive stars could then, in principle,
provide the hydrogen and helium LyC photons required to ionize the universe at
high redshifts. We propose that observations of the \HeII Gunn-Peterson effect
and of the metal ionization states of the \Lya forest and Lyman-limit absorbers
should show the signature of a stellar spectrum. We also note that the stellar
photoionization model fails if a large fraction of the UV radiation emitted
from stars cannot escape into the IGM, as suggested by the recent {\it Hopkins
Ultraviolet Telescope} observations by Leitherer \etal of low-redshift
starburst galaxies, or if most of the metals observed at $z\approx 3$ were
produced at much earlier epochs.

28
The categories of this paper are: alg-geom
11
The categories of this paper are: hep-lat
23
The categories of this paper are: solv-int
34
The categories of this paper are: chem-ph
20
The categories of this paper are: acc-phys
14
The categories of this paper are: math-ph
29
The categories of this paper are: funct-an
19
The categories of this paper are: eess
13
The categories of this paper are: quant-ph
30
The categories of this paper are: q-alg
4
The categories of this paper are: cond-mat
31
The categories of this paper are: ao-sci
37
The categories of this paper are: supr-con
35
The categories of this paper are: mtrl-th
15
The categories of this paper are: nucl-ex
3
The categories of this paper are: physics