Delayed Enrichment by Unseen Galaxies: Explaining the Rapid Rise in IGM
  CIV Absorption from z = 6-5
  In the near future, measurements of metal absorption features in the
intergalactic medium (IGM) will become an important constraint on models of the
formation and evolution of the earliest galaxies, the properties of the first
stars, and the reionization and enrichment of the IGM. The first measurement of
a metal abundance in the IGM at a redshift approaching the epoch of
reionization already offers intriguing hints. Between z=5.8 and 4.7 (a 0.3 Gyr
interval only 1 Gyr after the big bang), the measured density of CIV absorbers
in the IGM increased by a factor of ~ 3.5 (Ryan-Weber et al. 2009; Becker,
Rauch & Sargent 2009). If these values prove to be accurate, they pose two
puzzles: (1) The total amount of CIV at z=5.8 implies too little star formation
to reionize the IGM by z=6 or to match the WMAP electron scattering optical
depth (tau). (2) The rapid growth from z = 6-5 is faster than the buildup of
stellar mass or the increase in the star formation rate density over the same
interval. We show that a delay of ~ 0.4-0.7 Gyr between the instantaneous
production of ionizing photons and the later production of metal absorption
features (added to the delay due to stellar lifetimes) can provide the full
explanation for both puzzles. We calculate the delay in metal production due to
finite stellar lifetimes alone and find that it is too short to explain the
rapid CIV density increase. The additional delay could naturally be explained
as the result of ~ 200 km/s outflows carrying carbon to distances of ~ 100 kpc,
the typical distance between galaxies and CIV absorbers in enrichment
simulations, and the typical outflow or absorption region scale observed at z ~
2-3.
