Firm observational evidence indicates that supermassive black holes are present at the centre of the majority of galaxies already from very early cosmic times all the way to the present day Universe. Feedback from these black holes in the form of large scale outflows is believed to be one of the key ingredients shaping the evolution and properties of galaxies in our Universe.
Recent observations (Cicone et al. 2014) have detected the emission from spatially extended cold gas around a bright quasar that existed when the Universe was less than 10% of its current age. This cold gas is moving at a very high velocity of the order of 1000km/s and has been detected up to 90,000 light years away from the rapidly accreting black hole that is powering the observed quasar. While this high velocity gas has been interpreted as the signature of a powerful black hole outflow, this observation is in tension with simple theoretical expectations which suggest that while rapidly moving, gas should be very hot instead of cold.
Using the moving-mesh code AREPO, researches at IoA/KICC, Cambridge have performed some of the most realistic cosmological simulations that follow black hole growth and feedback in the early Universe. In simulations, a galaxy hosting a bright quasar undergoes extremely high levels of star formation as it is located at the intersection of cosmic web filaments which deliver copious amount of gas to it. This eventually drives a “starburst phase” where a large number of stars explode as supernovae and their joint effect triggers a galaxy-scale wind. Thus, as the even more powerful black hole-driven outflow leaves the central region of galaxy and propagates outwards, it sweeps over the gas that has been polluted by metals, initially produced within stars and subsequently released by supernova blast waves. These “pockets” of metal enriched gas cause part of the hot quasar-driven outflow to cool to low temperatures. This is illustrated in the figure above where the hot and rapidly moving outflow originating from the black hole is shown with a black contour, cold gas pockets containing up to a billion Solar masses and moving together with the hot outflow are depicted with grey pixels, and inflowing cosmic web filaments are illustrated in orange hues. The black hole and its host galaxy are situated at the centre of the image.
Thanks to the simulations performed at the DiRAC-2 facility in Cambridge, researchers at IoA/KICC have found that self-consistent modelling of both supernovae and black hole outflows lead to the formation of cold gas pockets moving with 1000 km/s which are spatially extended over 90,000 light years (30kpc) in remarkable agreement with the puzzling observations of Cicone et al. 2014.