One of the challenges in exploring the early universe is that it is so far from us, as we peer billions of light years away to see it as it was all those billion of years ago. That means small faint objects, like dwarf galaxies, that we suspect do the main job of reionising the universe are nearly impossible to measure. It's therefore a challenge to constrain the DRAGONS universe; one way is to wait until little things build into bigger things that you then can see and test those. The other is to constrain the Semi-Analytic Models against the hydro simulations of Smaug. In this astounding exhaustive and thorough review of the two techniques my student Yuxiang Qin explores the connections and learns what to modify in one to mimic the other. Just being on top of one of these techniques would considered impressive in a PhD, to do both is truly exceptional.
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One of the oldest questions in the study of Reionisation, the few hundred million years in which almost all of the hydrogen in the Universe was ionised effectively at once, is simple - where does the UV light to ionise the gas come from? One very popular idea is blackholes, or rather the accretion disks around them, where material swirls around the gravitational plughole become incredibly hot and bright in UV / X-ray emission. This fantastic work by Yuxiang Qin used DRAGONS universes to show that there simply isn't enough of these sources, known as AGN or Quasars, to do the job - or at least not if you want to match the number of blackholes that exist today. That's because to be bright, and reionise the universe, they have to feed a lot and in the process grow too large relative to what we see today. This work undoubtedly disappointed some Quasar fans out there, but that's the beauty of science, the facts don't care what you might hope and you have to follow the results to their conclusion.
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A lovely piece of work by my student Yuxiang Qin, and amazingly rapid turn around of a paper using the DRAGONS series of supercomputer models. The newly discovered galaxy ZF-Cosmos-20115 had some remarkably strange properties that at first glance seemed to bend the laws of galaxy formation to be so large so soon after the Big Bang. This work instead revealed that the rapid stellar mass gain, and the resulting quiescence thereafter, can be naturally explained by significant mergers of smaller objects that created the large stellar nucleus - but this large central bulge itself then inhibited future star formation. This was then tracked back in time in the DRAGONS universe to reveal that the rapidly growing black holes of the earlier universe could indeed by housed in what then becomes these strange quiescent galaxies at later times.
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