Probing primordial 3He from hyperfine line afterglows around supercritical black holes

Abstract

Abstract We consider the possibility of the detection of 3HeII hyperfine line (rest frequency, $8.67 \, \rm GHz$) emission from ionized zones around accreting black holes (BHs) formed at high redshifts, z = 15–30. We show that the brightness temperature in 8.67GHz line increases and reaches a peak value after the accretion onto the BH exhausts and HeIII recombines into HeII. This period of brightening last up to 40 million years. We find that during this period the maximum brightness temperature reaches ≃ 0.2–0.5μK, depending on the epoch when such a black hole starts growing. The maximum angular size of the region emitting in the hyperfine line is around 0.5′. The flux from such a region ($\simeq 0.3 \, \rm nJy$) is too small to be detected by SKA1-MID. The RMS of the collective flux from many emitting regions from a volume bounded by the synthesized beam and the band-width of SKA1-MID might reach 100 nJy, which is potentially detectable by SKA1-MID.