| Redshifted intergalactic 3He+ 8.7 GHz hyperfine absorption (2009) | |||||||||
Abstract | |||||||||
| 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^+. We show that this transition is a strongly linear observable of the ionization history of intergalactic helium (for which HeII to HeIII reionization is believed to complete at z ~ 3) and of the abundance of ^3He over cosmic time. 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 show that the spin temperature of this 8.7 GHz line is approximately the cosmic microwave background temperature, such that this transition is best observed in absorption against high-redshift, radio-bright quasars. Instruments must reach ~1 \mu Jy RMS noise in bands of 1 MHz on a 1 Jy source to directly resolve this absorption. However, in combination with HI Ly\alpha forest measurements, an instrument can statistically detect this absorption from z > 3 with 30 \mu Jy RMS noise in 0.1 MHz spectral bands over 100 MHz, which may be within the reach of present instruments.. Comment: 14 pages, 3 figures; v2 matches version submitted to PRD with a degeneracy factor correction and clarifications | |||||||||
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