Metal Absorption Lines as Probes of the Intergalactic Medium Prior to the Reionization Epoch

Winds from star-forming galaxies provide the most promising explanation for the enrichment of the intergalactic medium with heavy elements. Theoretical and observational arguments indicate that the pollution may have occurred at z ≳ 6; however, direct observational tests of such a scenario are needed. We model starburst winds in the high-redshift universe and find that the fraction of space filled by enriched material varies strongly with the assumed star formation efficiency f_* and the fraction of supernova energy powering each wind f_esc. We show that metals carried by these winds can be seen in absorption against bright background sources, such as quasars or gamma-ray bursts, in narrow lines with characteristic equivalent widths ~0.5 Å ≲ W ≲ 5 Å. We argue that a substantial fraction of the metals in high-redshift winds are likely to reside in low-ionization states (C II, O I, Si II, and Fe II), but higher ionization states (C IV and Si IV) could also provide useful probes of the winds. The number of such lines can constrain both f_* and f_esc. Statistics of metal absorption lines can also be used to identify whether H₂ is an efficient coolant in the early universe and to study the initial mass function of stars at high redshifts.