Surveying the Galaxy The Gaia satellite was launched in December 2013, and over its five-year mission it will measure the positions, motions and distance indicators of more than a billion stars. The ESSOG project aims to exploit this huge body of information, helping to build a more detailed picture of our galaxy, as Professor James Binney explains. Astrometry, the measurement
of the movement of stars across the sky, lies at the heart of astronomy. Upon this foundation astrophysicists build our physical picture of the universe. The deployment of the European Space Agency’s (ESA) Hipparcos satellite in the ‘90s revolutionised astrometry by, for the first time, making astrometric measurements in space and by pioneering a novel technology. “Doing it from space is a game-changer, as the Earth’s atmosphere does two very bad things from the perspective of measuring stars. First, it dithers the image of a star, which makes it difficult to identify the centre of the image. Then light is also refracted by the atmosphere, so it actually moves the apparent position of a star,” explains James Binney, Professor of Physics at the University of Oxford. Moreover, from space it’s possible to look in two quite different directions at once, and doing this resolves the classical difficulty in measuring `parallax’, the tiny variation over the seasons in the direction to a star as the Earth moves around the Sun. “When you look in one direction with a telescope, the parallactic motions of all stars occur in phase. So the angles between stars, which is what you can measure, do not change very much – the stars kind of dance in formation,” outlines Professor Binney. The Hipparcos satellite solved this problem by looking simultaneously in two directions separated by more than 90 degrees, and imaging these two star fields on a single detector. The parallactic motions of the stars in one field were out of phase with those in
the other field, so the distances between their images on the detector changed significantly, and the annual variation of these distances could be measured with precision. Hipparcos was a great success, so soon after its mission was complete, ESA funded a second astrometric satellite called Gaia, which will gather huge volumes of data. “Gaia started its five-year programme of activity in the summer of 2014, and it is measuring the motion of over a billion stars with a precision that is in the order of a hundred times greater than what Hipparcos achieved,” says Professor Binney. Moreover, Gaia works in a different way to
ESSOG project April 2018 saw the release of the first significant set of data from the Gaia mission. The release contains the parallaxes and sky motions of more than a billion stars, and Doppler shifts for several million stars. As Principal Investigator of the ESSOG project, Professor Binney aims to extract scientific insights by combining this trove of data from Gaia with results from massive spectroscopic surveys using large ground-based telescopes. “The goal in ESSOG was to develop the conceptual tools required to exploit this enormous body of information on the kinematics and chemical compositions
Gaia sweeps its two telescopes systematically over the skies, in a complicated pattern, and finds what stars are there. It then measures their positions repeatedly – almost 70 times over the 5 year-period - and from those positions it figures out how they are moving. Hipparcos. “Hipparcos was sent into orbit with a list of roughly 100,000 stars. It was instructed to measure the parallaxes and motions of these stars,” says Professor Binney. “Gaia was not sent with a list, it simply monitors everything in the sky brighter than a faint threshold. It sweeps its two telescopes systematically over the sky, in a complicated pattern, and finds what stars are there. It then measures their positions repeatedly – almost 70 times over the 5 year-period – and from those positions it figures out how they are moving.”
of these stars,” he explains. “We do that by fitting the data into a dynamical model of the galaxy. Such a model specifies the distribution of the mysterious dark matter that holds together galaxies and clusters of galaxies. We have shown that just over half the force that holds the Sun in its orbit around the Galaxy comes from dark matter rather than stars or interstellar gas. The model also specifies how different types of star are distributed in `phase space’ – where the coordinates are position and velocity. Some stars in the galaxy have a
Gaia’s all-sky view of our Milky Way Galaxy and neighbouring galaxies, based on measurements of nearly 1.7 billion stars. The map shows the total brightness and colour of stars observed by the ESA satellite in each portion of the sky between July 2014 and May 2016. Copyright: ESA/Gaia/DPAC.
ESSOG has developed a new type of perturbation theory that yields accurate fits to complex orbits. Left/ top; an orbit in real space; right/ bottom a cross section of the orbit is phase space.
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chemical composition similar to the Sun, while others have a different composition, with less iron or more elements like oxygen and magnesium.” In the context of a chemo-dynamical model, these chemical distinctions between stars reveal where and when each star was born, and thus give insights into the evolution of the Galaxy. As the Galaxy has aged, dying stars have polluted the once pristine interstellar medium with elements heavier than lithium. “This enrichment – or pollution – of the interstellar medium of the Galaxy by heavy elements is an integral part of the evolution of the Galaxy. By studying the correlations between the motion of stars and their chemical composition, we expect to be able to explain – to a large extent – how our Galaxy was assembled, and how it has arrived at its present configuration,” outlines Professor Binney. The role of ESSOG is to develop the conceptual tools, algorithms and computer codes required to achieve this goal. “We have developed a couple of novel approaches to building dynamical models,” continues Professor Binney. “These models give you dynamically consistent fantasy galaxies, which can be compared with the observational data coming from both Gaia and ground-based surveys. By adjusting the fantasy model until it gives us a decent fit, we can map the Galaxy’s gravitational field and determine how stars are distributed in phase space,” he outlines. “The stars in catalogues tend to be relatively nearby, or very luminous, so they are easy to measure. By ‘observing’ the fantasy galaxy in a computer, we can relate the biased contents of the catalogues to what is actually out there.” Early work on the Gaia data will centre on analysing the data for the several million stars for which Gaia has obtained spectra, in addition to measuring astrometry. Later, a slightly different modelling process will be applied to data for the much greater number of stars for which we don’t have a spectrum from which a Doppler shift can be measured. “There are different approaches available – we use different groups of stars in different ways,” says Professor Binney. Since the interstellar medium contains smoke (‘dust’) that absorbs light, it’s necessary to model the interstellar medium in parallel with the Galaxy’s stellar and darkmatter content. “The ESSOG project produced a new tool for mapping the interstellar medium using measurements of how strongly light from individual stars has been absorbed. We are now applying this tool to the Gaia data – a very challenging task computationally. ESSOG also involved modelling dynamically the flow of interstellar gas within the Galactic disc. This
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study led to a significant decrease in the rate at which the Galaxy’s central bar is thought to rotate. The recently released data will allow us to test whether this revision was correct.”
Cosmological paradigm The improved understanding of our Galaxy that will flow from ESSOG will test the prevailing cosmological paradigm in greater depth than previously possible. “Since about 2000, we’ve had the Lambda-CDM model, which has been tremendously successful in explaining the large-scale distribution of galaxies and differences between the appearance of galaxies observed at ever greater distances and therefore at ever earlier cosmic epochs. In fact, although many details remain uncertain, we believe that we now understand in broad outline how galaxies formed,” explains Professor Binney. The Lambda-CDM model includes an accelerating expansion of the universe. “Lambda stands for the cosmological constant, which drives the acceleration of the cosmic expansion. On the largest scales, gravity seems to be acting repulsively, causing the expansion of the universe to speed up,” says Professor Binney. The Gaia data will allow researchers to test the Lambda-CDM model and potentially identify any areas where it could be refined and improved, which Professor Binney says is an important aspect of the project’s research. “In principle we could predict from the Lambda-CDM model and well established physics how galaxies are structured, but only via computations that are unfeasibly complex. So researchers have been guessing what the results of these computations would be. Using the Gaia data and the tools we’ve produced in ESSOG, we can test predictions based on these guesses,” he says. Professor Binney says Gaia will deepen our understanding of all extra-galactic astronomy. “Our understanding of galaxies and the largescale structure of the universe rests heavily on our understanding of how stars work and evolve,” he says. “This understanding will be made more precise and more secure by having precise distances to stars in our own galaxy that are like those you can see in other galaxies. Gaia is going to make this possible.” There is also a wider dimension to the ESSOG project. While the Gaia mission is a central part of the ESA’s long-term research programme, it was decided that the data it gathered would be made publically available, so it’s not a given that the scientific payoff from these data will enhance Europe’s science base. “It’s important that there are groups in Europe that are well-prepared for the release of these data,” stresses Professor Binney.
ESSOG Extracting science from surveys of our Galaxy Project Objectives
To develop the tools required to extract science from surveys of our Galaxy. The tools to include chemodynamical models of the stellar and gaseous components and a procedure for mapping interstellar dust. To use preliminary versions of these tools to analyse data from ground-based surveys in anticipation of the arrival of data from Gaia in 2018.
Project Funding
FP7-IDEAS-ERC / Funded under ERC-AG-PE9 - ERC Advanced Grant - Universe sciences/ Maximum ERC Funding € 1 954 460. Start date: 2013-04-01, End date: 2018-03-31.
Contact Details
Project Coordinator, Professor James Binney Rudolf Peierls Centre for Theoretical Physics Clarendon Laboratory Parks Road Oxford OX1 3PU T: +44 01865 (2)73979 E: James.Binney@physics.ox.ac.uk W: https://www2.physics.ox.ac.uk/research/ galactic-dynamics
Host Institution
The Chancellors, Masters and Scholars of The University of Oxford Wellington Square University Offices Oxford OX1 2JD United Kingdom Professor James Binney
Professor James Binney studied in Cambridge, Freiburg and Oxford, had postdocs in Oxford and Princeton and joined the Oxford faculty in 1981. His books include graduate-level texts on galactic astronomy and dynamics, books on critical phenomena and quantum mechanics, and `A Very Short Introduction to Astrophysics’. He has received medals from learned societies in France, Italy, the USA and the UK. He has been a Fellow of the Royal Society since 2000.
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