applications
to compare many observations of such galaxies with low resolution. Te resolved large-scale behaviour is then derived and the team compares this with the large-scale behaviour in the simulations. If the large-scale behaviours agree, the team can deduce that its numerical model of the internal physics is coherent with observations. Roos added: ‘Numerical simulations are test-beds for current theories and only the careful comparison with observations can provide insight on whether a theory holds or not.’ Te team studied three galaxy masses, which
span the whole range of masses in typical star-forming galaxies of the early universe, and three feedback configurations. Feedback is how a supermassive black hole (SMBH) or a star affects the host galaxy that ‘feeds’ it with its gas. Typically, active SMBHs release energy in the centre of their host, leading to high velocity winds expelling gas, and heating or ionising their surroundings. Stellar feedback is mainly composed of winds
from young stars and supernova explosions. To disentangle the effects of SMBH feedback and stellar feedback, the researchers used three configurations: only SMBH feedback, only stellar feedback and both feedback. With different resolutions (from 12 parsecs down to 1.5
www.scientific-computing.com l
STATISTICAL
COMPARISON IS USED TO COMPARE MANY OBSERVATIONS OF SUCH GALAXIES WITH LOW RESOLUTION
parsecs), this makes a total of 24 simulations that were run during 11 million core hours. A parsec is an astronomical unit of length used
to measure large distances to objects outside of our solar system. One parsec is equal to more than three light years. Active SMBHs have long been accused of
killing the galaxies in which they live because of the huge amount of energy they release into it. However, the researchers found that moderately active SMBHs are not able to rip the gas out of their host and they are not able to suppress or delay in-place star formation, even when the interplay between supermassive black hole physics and stellar feedback physics is accounted for. Tey are, therefore, not galactic quenchers.
Roos explained: ‘What is even more surprising is that the amount of gas swept out of the host by both the active supermassive black hole and the
@scwmagazine
stars is lower at higher mass, whereas black holes are more massive and more powerful in them.’ ‘Terefore, even though the energy is there, the
coupling with interstellar matter is so inefficient that the gas of the galaxy is not abruptly ripped out (it is to some extent, but not enough to kill the host suddenly). Further investigation is thus needed in order to explain why and how galaxies suddenly die,’ Roos added.
Our universe’s structure Why is the universe structured in the way it is? Stars, quasars, galaxies and clusters of galaxies all exist in our current universe – but linking this structured universe to the smooth matter distribution of the early universe is not yet fully realised. If we can understand the cosmic reionisation
process, which refers to a period in the early universe where predominantly neutral intergalactic material was ionised by the first luminous sources, then we could start to understand how structures such as stars and galaxies formed, and why our structured universe exists. Researchers from the University of Sussex
are using simulations to model the cosmic reionisation process over a huge range of length scales. Dr. IIian Iliev, reader in astronomy at
OCTOBER/NOVEMBER 2016 25 ➤
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