Blackhole Highlight: The environment of bright QSO’s at z ~ 6

Figure 1. The distribution of massive black holes in different patches of the 900 million year old Universe according to the recent simulations carried out at the IoA/KICC. Black holes are both more massive and more numerous as the overdensity increases.

Observations of nearby galaxies and high redshift quasars suggest that black holes are present in the majority of galaxies. The SDSS, UKIDSS and VISTA surveys have revealed numerous high-redshift QSOs harbouring black holes with masses in excess of a billion solar masses at a time when the Universe was less than one billion years old (about 10% of it’s current age). The fact that such massive black holes have managed to grow by this time provides a significant challenge for theoretical models of the formation and growth of supermassive black holes.

Researchers at the IoA/KICC have performed a suite of state-of-the-art numerical simulations which self-consistently follow the growth of black holes from very early times to the present day. By simulating a large volume of the Universe, they investigated the growth of massive black hole in a variety of cosmological environments. The images shown in Figure 1 illustrate the distribution of gas in different patches of the 900 million year old Universe. Black holes are far more numerous and massive in the most overdense regions of the Universe. The biggest black circles in the bottom row correspond to black holes with masses close to one billion solar masses. Only very massive black holes located in highly overdense regions were found to give rise to peak luminosities in agreement with observational estimates for the brightest, high redshift quasars, as shown in Figure 2. This finding indicates that these quasars should reside in some of the rarest dark matter halos at the time. Members of the IoA/KICC also used these simulations to study the so called AGN feedback, the interaction between massive black holes and gas in their surroundings. They found that the energy released due to accretion onto the most massive black holes gives rise to powerful outflows that push gas at speeds exceeding a thousand km/s. The comparison of simulated quasar outflows with observations will provide an essential tool to pin down the physical mechanisms underlying the interaction of massive black holes and their host galaxies.