Oxygen pumping: Probing intergalactic metals at the epoch of reionization

We consider the pumping of the 63.2 μm fine-structure line of neutral O I in the high-redshift intergalactic medium (IGM), in analogy with the Wouthuysen-Field effect for the 21 cm line of cosmic H I. We show that the soft UV background at ~1300 Å can affect the population levels, and if a significant fraction of the IGM volume is filled with "fossil H II regions" containing neutral O I, then this can produce a nonnegligible spectral distortion in the cosmic microwave background (CMB). O I from redshift z is seen in emission at (1 + z)63.2 μm, and in the range 7 < z < 10 produces a mean spectral distortion of the CMB with y = (10^-9 to 3 × 10^-8)(Z/10^-3 Z☉)(IUV), where Z is the mean metallicity of the IGM and IUV is the UV background at 1300 Å in units of 10^-20 ergs s-1 Hz^-1 cm-2 sr^-1. A measurement of this signature can trace the metallicity at the end of the dark ages, prior to the completion of cosmic reionization, and is complementary to cosmological 21 cm studies. While future CMB experiments such as Planck could constrain the metallicity to the 10^-2 Z☉ level, specifically designed experiments could potentially achieve a detection.