Research Fellow and Associate Professor @ Swinburne University
I study the formation of the First Galaxies and the Epoch of Reionisation as part of the DRAGONS team led by Professor Stuart Wyithe. This uses a (SPH) hydrodynamical simulation series Smaug and a larger volume N-body (i.e. dark matter) simulation Tiamat with a new semi-analytic model Meraxes to predict what telescopes will see reionisation.
I am a Chief Investigator in the world's first dark matter detector in the Southern Hemisphere called SABRE based at the bottom of a gold mine at SUPL (Stawell Underground Physics Laboratory) in Victoria, Australia.
Beginning in 2017 I am an Associate Investigator in the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (CAASTRO 3D) and also an Associate Investigator in the ARC Centre of Excellence for Gravitational Wave Discovery (OzGRav).
As a member of two leading surveys on the next-generation Australia Square Kilometre Array Pathfinder telescope I create local universe simulations that can be used to test our galaxy formation and dark matter theories when compared with observations from the WALLABY and DINGO surveys.
This CV contains all my various activities.
My Research Papers
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...
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.
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).
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.
An unusual opportunity came up to speak at the International Mining and Resources Conference housed at the Melbourne Convention and Exhibition Centre to explore the possibilities of spin off tech from our underground dark matter searches. I focussed on the science of SABRE, the possibilities of an X-ray like scan for gold in the mine around using Muon Tomography and other underground science such as understanding how life grows without radiation / astrobiology. Finally I discussed the possible future for mining, in space! Key technologies such as automation and refinement have been deployed by the giants in the resource extraction sector and could find a home for their advanced technologies in the final frontier.
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.
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!
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!
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!
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!
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.
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.
I’m CI of the dark matter detector SABRE at the Stawell Underground Physics Laboratory and can proudly announce that we've been funded by the ARC! Australia will now join an international search for the nature of dark matter as the first site in the Southern Hemisphere.
My student's third paper of a stunning 3-part series on the growth of dark matter structures. In this paper Camila finally demonstrated the long studied concentration of dark matter haloes was tied to the growth history of that halo and hence, through her other works, the basic cosmology of the universe. Reference: Correa, Wyithe, Schaye and Duffy 2015 MNRAS 452, 1217C
My student's second paper of a stunning 3-part series on the growth of dark matter structures. In this paper Camila tied the distribution of dark matter in haloes (i.e. the density profile) and initial power spectrum of the universe. This used detailed N-body simulations that Camila herself ran several of on supercomputer. Reference: Correa, Wyithe, Schaye and Duffy 2015 MNRAS 450, 1521C
My student's first paper of her PhD was a stunning 3-part series on the growth of dark matter structures. In this paper Camila set up the analytic machinery that tied the mass accretion history of haloes to the underlying cosmology of the universe using linear structure formation theory. In particular she showed that the rapid exponential growth of haloes in the early universe slows to become a slower power law at late times thanks to Dark Energy. Reference: Correa, Wyithe, Schaye and Duffy 2015 MNRAS 450, 1514-1520
With the excitement of our funding secured to build the world's first dark matter detector in the Southern Hemisphere in Stawell, Victoria we hosted VIPs and a film crew from 7's Sunrise Weekend. It was 30+ degrees and 100% humidity a km underground but that's where you need to go to search for dark matter!
The first paper from the "DRAGONS" team led by Prof Stuart Wyithe, investigating how the First Galaxies formed. Using a series of hydrodynamical simulations series known as Smaug we show that the first billion years after the Big Bang is a very exciting time, with the entire universe lit by a hidden population of small galaxies that current telescopes have yet to see. Reference: Duffy, Wyithe, Mutch, Poole 2014 MNRAS 443, 3435D.
I analysed the ability of radio and optical (visible light) telescopes to probe the nature of Dark Energy. I showed that radio telescopes are rapidly improving in capability and although starting from a low base they will rival the best optical telescopes by the time of the Square Kilometre Array (SKA). One issue is that the SKA demands such a large supercomputer that Moore's law might not get us such a machine by the time SKA is built. Reference: Duffy 2014 Annalen der Physik 526, 283D for the Special Issue "The Accelerating Universe".
A fascinating technique using the outbursts of supermassive blackholes as barometers to measure the pressure of the gas around the galaxies, as the outbursts inflate 'bubbles' of ionised gas. These pressures were then compared with the hydrodynamical simulations and found to be significantly rarer, over-pressurised regions than normal. Reference: Malarecki, Staveley-Smith, Saripalli, Subrahmanyan, Jones, Duffy, Rioja 2013 MNRAS 432 200M.
Sarah and myself started investigating the shape and spin properties of Dark Matter haloes just after I left Jodrell Bank. This then increased in scope when she started to consider the actions of the baryons (as featured in Duffy et al 2010) in changing these key properties of the collapsed Dark Matter structures. This work showed that the baryons strongly influence the halo, making it more spherical and rotating faster than the Dark Matter only predictions. This is a key result for Gravitational lensing and X-ray temperature mass estimates. Reference: Bryan, Kay, Duffy, Schaye, Dalla Vecchia, Booth 2013 MNRAS 429 3316B.
Using one of the largest simulated universes in existence (the Millennium Simulation) we populated the Dark Matter haloes with detailed Neutral Hydrogen gas (which radio telescopes can detect). By 'observing' these objects with the expected performance of the Australian SKA Pathfinder telescope we created a series virtual surveys that ASKAP can be expected to detect. These catalogues are as detailed and real as we can hope to have until we turn the telescope on. Some incredible fly through movies and images are available (be warned they can be pretty large). Reference: Duffy, Meyer, Staveley-Smith et al 2012 MNRAS 426 3385D
An analysis of the ability of the forthcoming Australian Square Kilometre Array Pathfinder to investigate the nature of Dark Energy. It will likely be the first radio telescope to make these kind of observations and is an exciting precursor to the type of science that the full Square Kilometre Array (SKA) can accomplish. Reference: Duffy, Moss, Staveley-Smith 2012 PASA 29 202D
Following on from Duffy et al. (2010) we considered the same simulated haloes when "Modelling neutral hydrogen in galaxies using cosmological hydrodynamical simulations". This studied the baryonic properties of simulated haloes; focussing on Neutral Hydrogen, but also Molecular Hydrogen and Stellar masses as a function of cosmic time, halo mass and baryonic physics. With this paper I made the Victorian State Finals for the Fresh Science Award. Reference: Duffy, Kay, Battye, Booth, Dalla Vecchia, Schaye 2012 MNRAS 420 2799
My first SPH simulation paper on the "Impact of baryon physics on dark matter structures: a detailed simulation study of halo density profiles". It demonstrated that the physics of galaxy formation can (surprisingly) strongly affect the dark matter distribution. It won the "Best Paper by a UWA Early Career Researcher" award. Reference: Duffy, Schaye, Kay, Dalla Vecchia, Battye, Booth 2010 MNRAS 405 2161D
This was the simulation paper for the OverWhelmingly Large Simulations (OWLS) effort that I was involved with during my PhD. This paper in particular focusses on the impact that different baryonic processes can have on the global star-formation rate, amongst many other effects. Reference: Schaye, Dalla Vecchia, Booth, Wiersma, Theuns, Haas, Bertone, Duffy, McCarthy, vd Voort 2010 MNRAS 402 1536-1560
My first N-body simulation paper (as well as Letter) on the topic of "Dark matter halo concentrations in the Wilkinson Microwave Anisotropy Probe year 5 cosmology" when I was based at Leiden Observatory. It demonstrated a fascinating inverse relation between the concentration (compactness) of a dark matter object and the mass of said object. Reference: Duffy, Schaye, Kay, Dalla Vecchia 2008 MNRAS 390L 64D
My first publication in a journal, on the subject of "Galaxy redshift surveys selected by neutral hydrogen using the Five-hundred metre Aperture Spherical Telescope". The telescope will be a fantastic survey instrument, capable of detecting millions to hundreds of millions of galaxies around us for billions of light years. Reference: Duffy, Battye, Davies, Moss, Wilkinson 2008 MNRAS 383 150D
My first scientific presentation at an international radio conference "From Planets to Dark Energy: the Modern Radio Universe" back in October 1-5 2007. Hosted at the The University of Manchester, UK. Published online at SISSA, Proceedings of Science, p.91 Reference: Duffy, Battye, Davies, Moss, Wilkinson 2007 MRU ConfE 91D