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Metal Enrichment and Ionization Balance in the Lyman $alpha$ Forest at $z = 3$ | | Antoinette Songaila
; Lennox L. Cowie
; | | Date: |
17 May 1996 | | Journal: | Astron.J. 112 (1996) 335-351 | | Abstract: | The recent discovery of carbon in close to half of the low neutral hydrogen column density [$N({
m H~I}) > 3 en{14}cm2$] Lyman forest clouds toward $z sim 3$ quasars has challenged the widely held view of this forest as a chemically pristine population uniformly distributed in the intergalactic medium, but has not eliminated the possibility that a primordial population might be present as well. Using extremely high signal-to-noise observations of a sample of quasars we now show that ion{C}{4} can be found in 75% of clouds with $N({
m H~I}) > 3 en{14}cm2$ and more than 90% of those with $N({
m H~I}) > 1.6 en{15}cm2$. Clouds with $N({
m H~I}) > 10^{15}cm2$ show a narrow range of ionization ratios, spanning less than an order of magnitude in ion{C}{4}/ion{H}{1}, ion{C}{2}/ion{C}{4}, ion{Si}{4}/ion{C}{4} and ion{N}{5}/ion{C}{4}, and their line widths require that they be photoionized rather than collisionally ionized. This in turn implies that the systems have a spread of less than an order of magnitude in both volume density and metallicity. Carbon is seen to have a typical abundance of very approximately $10^{-2}$ of solar and Si/C about three times solar, so that the chemical abundances of these clouds are very similar to those of Galactic halo stars. ion{Si}{4}/ion{C}{4} decreases rapidly with redshift from high values ($> 0.1$) at $z > 3.1$, a circumstance which we interpret as a change in the ionizing spectrum as the intergalactic medium becomes optically thin to He$^+$ ionizing photons. Weak clustering is seen in the ion{C}{4} systems for $Delta v < 250kms$, which we argue provides an upper limit to the clustering of ion{H}{1} clouds. If the clouds are associated with galaxies, this requires a rapid evolution in galaxy clustering between $z = 3$ and $z = 0$. | | Source: | arXiv, astro-ph/9605102 | | Services: | Forum | Review | PDF | Favorites | |
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