High-redshift surveys have so far discovered over a hundred quasars above redshift of six. This number will likely increase significantly in the coming years, due to ongoing and planned deep, wide-field surveys, such as eROSITA in X-rays, the Vera Rubin Observatory at optical wavelengths as well as Euclid in infrared. Such observations will also push these discoveries to lower luminosities too, giving a more complete picture of the build up of the black hole population in the early Universe. In fact it could even be possible to detect 105 solar mass black holes at z = 10 with mega second exposures from NIRCam on JWST or from the proposed Lynx X-ray telescope.
Focusing on the most extreme examples, supermassive black holes in excess of billion solar masses have been detected above z = 7, challenging theoretical models of the growth of such objects. The current record holder in terms of luminosity, and hence black hole mass is SDSS J010013.02+ 280225.8, with an inferred mass exceeding 1010 solar masses at z = 6.3. While there have been a number of theoretical studies of these objects, they still struggle to form some of the most massive observed black holes at z > 6.
As a part of our DiRAC project dp012 (Bennett et al., MNRAS, to be submitted), using the computational facilities in Cambridge and Durham, we have investigated the most promising scenarios for building up the most massive known supermassive black holes in the early Universe. Allowing for mildly super-Eddington accretion and earlier seeding redshift which is still entirely compatible with the direct collapse seeding model, we have found that it is possible to assemble a 1010 solar masses black hole by z = 6.
As shown in Fig. 1 the mass growth of our simulated black hole is consistent with observations, hinting that episodic super-Eddington accretion may be required in the early Universe to grow the most extreme mass black holes within a Gyr of cosmic time. Importantly we found that the feedback impact of this black hole is very significant.
This is shown in Fig. 2 where we plot the combined thermal and kinetic SZ decrement (top) and gas radial velocity (bottom) for the original FABLE model (left) and our new simulations (right). Hot, fast outflows are pushing gas way beyond the virial radius, diminishing the SZ signal on small scales and enchanting it on large scales. Moreover, by studying the redshift evolution of the simulated hot halo we make detailed predictions for mock maps, from radio to X-rays, that should help us disentangle how mass is accumulated onto these gargantuan black holes over cosmic time e.g. an early (sustained) growth and feedback scenario vs. a late and rapid growth and feedback scenario.