  Radiative cooling by metals in shocked gas mediates the formation of
ionization front instabilities in the galaxy today that are responsible for a
variety of phenomena in the interstellar medium, from the morphologies of
nebulae to triggered star formation in molecular clouds. An important question
in early reionization and chemical enrichment of the intergalactic medium is
whether such instabilities arose in the H II regions of the first stars and
primeval galaxies, which were devoid of metals. We present three-dimensional
numerical simulations that reveal both shadow and thin-shell instabilities
readily formed in primordial gas. We find that the hard UV spectra of
Population III stars broadened primordial ionization fronts, causing H2
formation capable of inciting violent thin- shell instabilities in D-type
fronts, even in the presence of intense Lyman-Werner flux. The high post- front
gas temperatures associated with He ionization sustained and exacerbated shadow
instabilities, unaided by molecular hydrogen cooling. Our models indicate that
metals eclipsed H2 cooling in I-front instabilities at modest concentrations,
from 0.001- 0.01 solar. We conclude that ionization front instabilities were
prominent in the H II regions of the first stars and galaxies, influencing the
escape of ionizing radiation and metals into the early universe.
