A universal 21 cm signature of growing massive black holes in the early Universe

Monthly Notices of the Royal Astronomical Society. 2019. Vol. 489. No. 1. P. 1127–1138.

Sazonov S.

There is a hope that looking into the early Universe with next-generation telescopes, one will be able to observe the early accretion growth of supermassive black holes (BHs) when their masses were ∼104-106 M☉. According to the standard accretion theory, the bulk of the gravitational potential energy released by radiatively efficient accretion of matter on to a BH in this mass range is expected to be emitted in the extreme UV-ultrasoft X-ray bands. We demonstrate that such a `miniquasar' at z ∼ 15 should leave a specific, localized imprint on the 21 cm cosmological signal. Namely, its position on the sky will be surrounded by a region with a fairly sharp boundary of several arcmin radius, within which the 21 cm brightness temperature quickly grows inwards from a background value of (∼-250) mK to (∼+30) mK. The size of this region is only weakly sensitive to the BH mass, so that the flux density of the excess 21 cm signal is expected to be ∼0.1-0.2 mJy at z ∼ 15 and should be detectable by the Square Kilometer Array. We argue that an optimal strategy would be to search for such signals from high-z miniquasar candidates that can be found and localized with a next-generation X-ray mission such as Lynx. A detection of the predicted 21 cm signal would provide a measurement of the growing BH's redshift to within ∆z/(1 + z) ≲ 0.01.

Keywords: black holes, accretion, quasars