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Redshifted intergalactic 3He+ 8.7 GHz hyperfine absorption

Motivated by recent interest in redshifted 21 cm emission of intergalactic hydrogen, we investigate the 8.7 GHz ^2S_{1/2} F=0-1 hyperfine transition of ^3He^+. While the primordial abundance of 3He relative to hydrogen is 10^-5, the hyperfine spontaneous decay rate is 680 times larger. Furthermore, the antenna temperature is much lower at the frequencies relevant for the ^3He^+ transition compared to that of z>6 21 cm emission. We find that the spin temperature of this 8.7 GHz line in the intergalactic medium is approximately the cosmic microwave background temperature, such that this transition is best observed in absorption against high-redshift, radio-bright quasars. We show that intergalactic 8.7 GHz absorption is a promising, unsaturated observable of the ionization history of intergalactic helium (for which HeII->HeIII reionization is believed to be complete at z~3) and of the primordial 3He abundance. Instruments must reach ~1 μJy RMS noise in bands of 1 MHz on a 1 Jy source to directly resolve this absorption. However, in combination with HI Lyαforest measurements, an instrument can statistically detect this absorption from z > 3 with 30 μJy RMS noise in 0.1 MHz spectral bands over 100 MHz, which may be within the reach of present instruments.