5.2 Formation rate
The formation rate of Population III stars can be indirectly estimated from the re-ionization fraction of
the early universe, which was re-ionized by light from these stars [117, 47
]. Using estimates of the
ultraviolet light emission from Population III stars, their ionization efficiency, and the re-ionization fraction
of the early universe, one can determine that about 0.01% - 1% of the universe’s baryonic matter was found
in these very massive stars. This corresponds to
Population III stars in a
galaxy
and thus a collapse rate that is
. Thus, a reasonable occurrence rate can be found for an
observation (luminosity) distance of
(which corresponds to a redshift of
, in the
cosmology used by [86
]). However, uncertainties in the assumptions make this formation rate uncertain by a
few orders of magnitude [1
, 96
, 2, 86
]. Indeed, the latest results suggest that the initial mass function
begins to fall off dramatically above
and the number of stars fomred with masses
above
may be many orders of magnitude below this rate[189]. Instead, Population III
stars may be dominated by the collapse of stars in the
. These stars may also
collapse ultimately to black holes. For discussion of these objects, we refer the reader back to
Section 4.