This is the last paper from the thesis of my amazing PhD student (now Dr!) Yuxiang Qin, which was published in the Monthly Notices of the Royal Astronomical Society, and explored the modifications to semi-analytic models that best describe the nature and impact of star formation and stellar feedback (i.e. when stars explode!) on the early galaxies. He created an entire new paradigm, with accompanying model/code, that others can incorporate into their own simulated universes. The preprint version of the paper is available freely.
Read More
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.
Read More
One of my favourite human beings, colleagues and collaborators - Dr Paul Geil - also came up with one of my favourite paper titles of all time. The work using DRAGONS focusses on the structure of the Epoch of Reionisation. In the first billion years after the Big Bang growing galaxies shone with ionising UV light, lighting up the Universe itself. This light ionised the hydrogen gas lying around the galaxies, creating cavities or bubbles in the intergalactic medium. the exact structure of the bubbles, how many of a given size and their rate of growth, is intimately tied to the nature of dark matter and the physics of galaxy formation. Paul predicted how new telescopes like the Square Kilometre Array can explore these bubbles, and exactly how awesome they will be at constraining all sorts of physics. A huge piece of work that will be years in the testing, so forward looking is its predictions.
Read More
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.
Read More
Where do galaxies form? how big can they be? Do galaxies 'prefer' to lie closer to one another or further apart? And how does all of this change across Cosmic Time? These are just some of the questions Jaehong Park asked within the DRAGONS team in this paper. To explore how galaxies grow near one another, known as clustering, and how they grow within the larger dark matter halos, aka bias, Jaehong analysed countless thousands of simulated galaxies. Compared against one another, at different outputs from the simulated universe of DRAGONS, the overall distributions were robustly analysed with statistical tools that then permitted comparison with images from Hubble. The work is a staggering scale and one I'm sure Jaehong will be proud of for many years to come.
Read More
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.
Read More
This is one of the most fun papers I have ever written (and not just because of the title). The picture astronomers have of the early universe is one of galaxies growing rapidly, turning vast quantities of gas rich clouds into stars in a boom-time of star formation. By using the Smaug simulations of this period I and my DRAGONS colleagues were able to explore this picture. We found that cold gas is indeed consumed rapidly, in just 300 million years irrespective of how stars explode or that gas can cool. However, theres so much material pouring into the galaxies at this time that they simply can't consume it all! A system where demand (gas turing into stars) can't raise to meet supply (of new primary material flowing in) is a recession.
Far from a booming bull-market, the early Universe was a recessionary bear-market and that's why I love this paper...
Read More
This is a spectacular study by my Yuxiang Qin, one of my PhD students I am fortunate to co-supervise with Dr Simon Mutch and Prof Stuart Wyithe as part of DRAGONS. In this work Yuxiang compares the growth of dark matter structures in the early universe both with and without the impact of gas physics (in particular the fact that giant clouds of atoms have a pressure that prevents them collapsing unlike dark matter). Most simulations ignore that effect to save computational time. Yuxiang showed that's potentially a disastrous step for first structures where the gas prevents the halo from collapsing and through its gravitational pull can also slow the collapse of dark matter itself meaning simulations that take a computational shortcut can produce early haloes that are far larger than they should otherwise be.
Read More
A lovely prediction paper from Chuanwu Liu of the DRAGONS collaboration showing the expected sizes for the most distant galaxies that current (and future) telescopes are trying to observe. The tentative existing detections appear to be well explained by our model of galaxy formation with the effective radius (i.e. the size of the disk of the galaxy) being larger for brighter objects but only with a power law scaling of 0.25! In other words a galaxy ten thousand times more luminous will be a disk galaxy only ten times wider. Finally, we make clear that the successor to the Hubble Space Telescope (the James Webb Space Telescope) will be unlikely to see these tiny disks and instead we will have to wait for ground based extremely large telescopes like the Giant Magellan Telescope (and critically one in which Australia is heavily invested).
Read More
The recent discovery by Oesch et al. (2016) of a far-off galaxy seen just 400 million years after the Big Bang but already having accumulated a billion Sun's worth of stars was considered a bizarre object. Yet the simulated DRAGONS universe apparently contains several analogues as shown in this beautiful work by my colleague Dr Simon Mutch. We show that such a monstrously oversized baby galaxy is possible if it grows rapidly but consistently throughout time and not as a result of cannibalising neighbouring objects through galaxy mergers as is oft suspected.
Read More
A key goal of the DRAGONS investigation was to predict how growing galaxies in the early universe would ionise the neutral hydrogen around them. This is the long-sought after signal of Reionisation (also known as Cosmic Dawn) when the Universe was filled with light, lifting this dark opaque fog. It is the target for telescopes like the Square Kilometre Array to characterise that early universe when ionised bubbles of gas around the galaxies resembles a swiss cheese model. This beautiful work by Dr Paul Geil calculated how our simulated galaxies would impact that material around them finding that the galaxy formation that resulted in the biggest impact was the nature of how stars exploded. This both ionised gas around it but more importantly limited how new stars could form and hence limit the amount of ionising radiation and therefore the size and number of the ionised bubbles. This is however not a unique signature and instead even when we find the swiss cheese universe we have a lot of work ahead to tease out its lessons. Depressing but beautifully analysed science by Paul.
Read More
A mammoth effort by my long-time collaborator Dr Simon Mutch explaining the semi-analytic model Meraxes that `paints' the galaxies onto the background dark matter structures formed in the huge simulated DRAGONS universe. This work has so many critical lessons on key physics that grows a galaxy that matches what we see in the distant universe (and hence seeing those objects as they were long ago when the light first left them). Perhaps the key is that the fraction of energetic light that can escape forming galaxies (and hence ionise the neutral hydrogen atoms in the vast distances between them) has to increase towards earlier times. Somehow galaxies trap evermore of this radiation as they grow up. A mystery that we will hopefully solve in this series of works!
Read More
The first paper by Chuanwu Liu in his PhD with DRAGONS showed that we can explain observations of distant galaxies glowing in ultraviolet (UV) light. This light is responsible for ionising the neutral hydrogen between the galaxies, ending the Cosmic Dark Ages in a process known as Reionisation. Chuanwu's work showed that our simulated galaxies can recreate the current observations, but that we can then predict what future observations may see as our simulations form much smaller objects at this time than even Hubble can find. The main finding was that dwarf galaxies are responsible for providing most of the ionising radiation that lights up the universe; in agreement with my entirely complimentary and independent technique in Duffy et al. (2014b). Promising start to your academic career Chuanwu with such a careful and expansive analysis on this hot topic!
Read More
The first paper in the DRAGONS series, by my long time collaborator Dr Greg Poole, explaining the enormous dark matter simulation Tiamat that underlines the entire project. This is an epic work detailing the challenges involved in correctly identifying dark matter structures within which galaxies are expected to form. This is particularly challenging at early times in the universe's history when so many dark matter haloes were colliding and merging, causing a nightmare for basic book-keeping or cataloguing of such messy objects. Beautiful work and one that sets the stage for the rest of the DRAGONS papers!
Read More
The first paper by Paul Angel for his PhD as part of DRAGONS and it's enormous. A careful phenomenological study and characterisation of the structure of dark matter haloes in the early universe. In particular Paul focussed on the concentration of the dark matter haloes as measured by fitting the halo with the NFW and Einasto profiles. At the current age of the universe works such as Duffy et al. (2008) show small mass haloes are typical denser (that is more concentrated) that more massive ones. This is because smaller objects form earlier than large objects in our hierarchical universe, earlier times in an expanding universe implies that it was overall smaller and hence denser as are then the objects that form. Paul discovered that the universe in DRAGONS is so young that essentially everything is forming at nearly the same time and hence density so the concentration-mass relation is flat!
Read More
