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

TL;DR

This paper explores the potential detection of primordial $^3$HeII hyperfine line emissions from ionized regions around high-redshift black holes, analyzing brightness, size, and detectability with SKA1-MID.

Contribution

It models the evolution of $^3$HeII hyperfine line brightness around early black holes and assesses the feasibility of detecting these signals with future radio telescopes.

Findings

Brightness temperature peaks at 0.2-0.5 μK during recombination period.

Maximum emission region size is about 0.5 arcminutes.

Collective flux may reach 100 nJy, potentially detectable by SKA1-MID.

Abstract

We consider the possibility of the detection of $^3$HeII hyperfine line (rest frequency, $8.67 \, \rm GHz$) emission from ionized zones around accreting black holes (BHs) formed at high redshifts, $z=15\hbox{--}30$. 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 $\simeq 0.2\hbox{--}0.5 \mu$K, 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.