Where Are the Missing Cosmic Metals?

The majority of the heavy elements produced by stars 2 billion years after the big bang (redshift z ≈ 3) are presently undetected at those epochs. We propose a solution to this cosmic "missing metals" problem in which such elements are stored in gaseous halos produced by supernova explosions around star-forming galaxies. By using data from the ESO VLT Large Program, we find that (1) only 5%-9% of the produced metals reside in the cold phase, the rest being found in the hot (T = 105.8-106.4 K) phase, and (2) 1%-6% (3%-30%) of the observed C IV (O VI) is in the hot phase. We conclude that at z ≳ 3, more than 90% of the metals produced during the star formation history can be placed in a hot phase of the intergalactic medium (IGM), without violating any observational constraint. The observed galaxy mass-metallicity relation and the IGM and intracluster medium metallicity evolution are also naturally explained by this hypothesis.