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Blog

Filtering by Tag: Simulation

"The impact of feedback and the hot halo on the rates of gas accretion on to galaxies" - Correa et al. (2018)

Alan Duffy

My former student, and now high flying postdoctoral researcher at Leiden University, Dr Camila Correa answered one of the basic questions in galaxy formation in this paper - how does gas get to the galaxy from the larger Universe? The simple answer is, it depends. Essentially the bigger you are the more gas you can pull in, until you get to something the size of our Milky Way, when the `accretion'  rate of material infalling then flattens out. This picture is complicated as the hot gas halo surrounding a galaxy is responsible for preventing new material from infalling as it shocks against the hot halo. The amount of the hot halo depends on the type of energetic events within a galaxy, be it exploding stars (supernovae) or accreting black holes (AGN). A beautiful bit of work that will inform theorists and observers for years to come!

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"Dark-ages reionization and galaxy formation simulation IX: Economics of Reionizing Galaxies" - Duffy et al. (2017)

Alan Duffy

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...

 

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"Dark-ages reionization and galaxy formation simulation VIII: Suppressed growth of dark matter haloes during the Epoch of Reionization" - Qin et al. (2017)

Alan Duffy

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. 

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"Dark-ages reionization and galaxy-formation simulation VII: The sizes of high-redshift galaxies" - Chuanwu et al. (2017)

Alan Duffy

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).

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"Dark-ages reionization and galaxy-formation simulation VI: The origins and fate of the highest known redshift galaxy" - Mutch et al. (2016b)

Alan Duffy

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.

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"Dark-ages reionization and galaxy formation simulation V: morphology and statistical signatures of reionization" - Geil et al. (2016)

Alan Duffy

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.

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"Dark-ages reionization and galaxy formation simulation III: Modelling galaxy formation and the epoch of reionization" - Mutch et al. (2016a)

Alan Duffy

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!

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"Dark-ages reionization and galaxy formation simulation - IV. UV luminosity functions of high-redshift galaxies" - Chuanwu et al. (2016)

Alan Duffy

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! 

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"Dark-ages Reionization and Galaxy formation simulation - I. The dynamical lives of high-redshift galaxies" - Poole et al. (2016)

Alan Duffy

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! 

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"Dark-ages reionization and galaxy formation simulation - II. Spin and concentration parameters for dark matter haloes during the epoch of reionization" - Angel et al. (2016)

Alan Duffy

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! 

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"The Theoretical Astrophysical Observatory: Cloud-based Mock Galaxy Catalogs" - Bernyk et al. (2016)

Alan Duffy

TAO is an outrageously ambitious project spearheaded by Swinburne's Prof Darren Croton to bridge the gap between observations of our universe and those we simulate (such as the ones I create). Ideally astronomers log onto TAO and select their favourite telescope and strategy for viewing (stare for a long time at a small region, or briefly over a wide path of sky, the former lets you see fainter objects while the latter gives you only the brightest ones). Then you get an output that is identical in format to the one you took with that telescope in reality (including all known issues with signal to noise and interference etc). This makes the comparison between what we predict and observe as close as possible and hence maximise the lessons we can learn from seeing out into the universe.

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DRAGONS unleashed!

Alan Duffy

DRAGONS is out! Our first six collaboration papers are on the arXiv and submitted to the journals. Can’t describe what a relief this is for myself and the team..! Led by U.Melb’s Professor Stuart Wyithe it's been a few hard years of science, simulating the first galaxies after the Big Bang and trying to figure out what these look like from telescopes on Earth, 13 billion years later (and 40 billion light years distant) but finally the results are in and they’re amazing. 

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