The Metal Contents of Very Low Column Density Lyman-alpha Clouds: Implications for the Origin of Heavy Elements in the Intergalactic Medium

Abstract

We investigate the metal contents of Lyman-alpha clouds at 2.2<z<3.6 with 10^13.5<N(HI)<10^14 cm-2 using high resolution, high S/N spectra of 9 quasars obtained with the 10m Keck I telescope. Using an composite spectrum analysis technique, we deduce a mean metallicity of [C/H]<-3.5 for such clouds, which is a factor of more than 10 less than that inferred for Lyman-alpha clouds with N(HI)>10^14.5 cm-2. This result rules out the suggestion that a generation of Population III stars could have polluted the entire universe to a (nearly) uniform metallicity level of [C/H]=-2.5. Cosmological simulations involving gas hydrodynamics indicate that Lyman-alpha absorption with N(HI)>10^14.5 cm-2 mostly occur in the filamentary gaseous regions surrounding and connecting collapsed objects, while those with N(HI)<10^14 cm-2 are preferentially found in void regions further away from collapsed objects. These results, coupled with the simulation results of Ostriker and Gnedin (1996) and Gnedin and Ostriker (1997) for Pop III star formation and enrichment, strongly suggest that most of the heavy elements in Lyman-alpha clouds with N(HI)>10^14.5 cm-2 (i.e., gas in the filaments) were probably prodcued in situ by Pop II stars, in the sense that they were either made by stars within the clouds or were ejected from nearby star-forming galaxies. Within this context, clouds with N(HI)<10^14 cm-2 (i.e., gas in void regions) may only have e experienced pollution from Pop III stars.