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Gaia’s living and breathing Galaxy

The recent data release from the Gaia space observatory is already changing our understanding of the evolution of our Galaxy.

Refers to European Space Egency’s Gaia space observatory Data Release 2, (2018).

The Milky Way came alive earlier this year with the European Space Agency’s Gaia space observatory Data Release 2 (DR2) (see Fig. 1). Whereas the first data release, made available 2 years ago, was meant mostly to whet the community’s appetite, DR2 (freely available at finally provides astronomers with what was promised a quarter of a century ago: one billion stars with unprecedented astrometry. Even though one billion is only 1% of the estimated Galactic stellar census, the new data set represents a giant leap in our ability to trace the motion of stars in the sky. These movements are vanishingly small and previously had been tracked only for a small number of the brightest and closest stars. But stellar kinematics are fundamental to our understanding of our Galaxy’s past and present, because velocities are half of the star’s phase–space coordinates: the initial conditions required to predict its orbit. Assuming that gravity remains unchanged throughout the Milky Way, orbits are the most direct probe of the distribution of matter, both visible and invisible. Reconstructing the details of the Galaxy’s gravitational potential is the overarching goal of the Gaia mission.

Fig. 1: The Milky Way and neighbouring galaxies as seen by the European Space Agency’s Gaia space observatory.

Reprinted with permission from ©ESA/Gaia/DPAC.

Before embarking on such a reconstruction, a few key assumptions had to be verified. One of them is that of equilibrium, in other words, the conjecture that the state of the Galaxy does not change appreciably with time. Before Gaia DR2, the steady state of our Galaxy’s stellar disk was only postulated. After the release, it could be quantified. For example, Teresa Antoja and colleagues1 have recently demonstrated that the phase–space density distribution of the disk stars is highly structured. When the azimuthal component of velocity is plotted against the galacto-centric radius (distance from the star to the centre of the galaxy), many discontinuities become immediately apparent, visible as sharp diagonal ridges (see also ref.2). Even more striking is the spiral-like pattern clearly discernible in the behaviour of the vertical stellar velocity as a function of height above the disk. The exact processes producing these inhomogeneities have yet to be fully understood, but Antoja et al. provide a rather convincing hypothesis: the apparent fluctuations in the coarse-grained density are a telltale sign of a system out of equilibrium. More precisely, the striation in the radial dependence of the azimuthal velocity could be caused by the interactions with the Galactic bar or the spiral arms, whereas the vertical velocity spiral is likely the consequence of a perturbation caused by a dwarf galaxy satellite punching through the Milky Way’s disk. Think of a massive perturber imparting a velocity kick to a large number of stars around the impact point. What ensues is the familiar demonstration of Liouville’s theorem: following distinct orbital frequencies, the affected stars will start to phase-mix, shearing the phase–space region they occupy. The preservation of the phase–space density requires the spatial dimension to shrink if the velocity extent increases owing to the stars’ evolution in the gravitational potential. Gradually, the originally regular phase–space volume will stretch, thin out and wind up into a spiral. Antoja and collaborators suggest that by analysing the properties of the spiral, the details of the interaction can be deciphered. They make an estimate of the time of the fly-by and arrive at a number in agreement with the epoch of the disk crossing of the Sagittarius dwarf galaxy. Sagittarius is the third largest satellite of the Milky Way and has long been suspected to wreak havoc in our Galaxy’s disk3,4.

Such interactions between our Galaxy and its satellites were much more common and frighteningly more dramatic in the past. The memory of these events is preserved in the orbital structure of the Milky Way’s stellar halo and can today be read off Gaia’s measurements. One particular record stands out most clearly, that of the spectacular head-on collision between the young Milky Way and a massive dwarf galaxy some 8–11 billion years ago5,6. As the two objects smashed into each other, the dwarf’s debris sprayed over our Galaxy, populating the halo with stars on highly eccentric orbits. The exact shape of these orbits could not have been identified without Gaia’s data. This is because for an eccentric orbit observed near the Sun, most of the velocity is in the line-of-sight component, yielding a minuscule proper motion on the sky, smaller than the typical astrometric uncertainty in the pre-Gaia era. The merger rearranged the Milky Way into the Galaxy we observe today. The most fragile Galactic component, the disk, suffered the most: it was likely truncated, puffed up and scattered. But at the same time, the intruder probably brought with it a fresh supply of gas, which, once accreted, may have been used later to rebuild the Milky Way’s disk. We are only beginning to comprehend the implications of this violent encounter. One of the first illuminating insights is the discovery of a large number of globular clusters likely deposited into the Milky Way by the dwarf galaxy in the last throws of disruption7.

Despite the extraordinary quality and the exhilarating scope of the Gaia DR2 data, they are very incomplete. For example, for the vast majority of stars, the distances are yet to be measured accurately. This is because the parallactic ellipse (the wobble in the stellar motion in the sky due to the Earth’s rotation around the Sun) is a small periodic perturbation of the more obvious proper motion-induced displacement of the stellar position. As such, the measurement accuracy improves more slowly with time compared with that of the proper motion and depends sensitively on how often and how regularly Gaia comes back to observe the given star. In addition, Gaia can only measure spectra for the brightest subset of the sources it sees. Thus, most of the one billion stars will lack line-of-sight velocities at the end of the mission. However, this particular gap is being addressed — albeit partially — by three major ground-based spectroscopic surveys coming online in the very near future. The European 4MOST and WEAVE and the United States-led DESI will attempt to collect spectra for some of the faintest stars in the Gaia library.


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    Antoja, T. et al. A dynamically young and perturbed Milky Way disk. Nature 561, 360–362 (2018).

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    Kawata, D. et al. Radial distribution of stellar motions in Gaia DR2. Mon. Not. R. Astron. Soc. Lett. 479, L108–L112 (2018).

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    Purcell, C. W. et al. The Sagittarius impact as an architect of spirality and outer rings in the Milky. Nature 477, 301–303 (2011).

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    Laporte, C. F. P. et al. The influence of Sagittarius and the Large Magellanic Cloud on the stellar disc of the Milky Way Galaxy. Mon. Not. R. Astron. Soc. 481, 286–306 (2018).

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    Belokurov, V. et al. Co-formation of the disc and the stellar halo. Mon. Not. R. Astron. Soc. 478, 611–619 (2018).

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    Helmi, A. et al. The merger that led to the formation of the Milky Way’s inner stellar halo and thick disk. Nature 563, 85–88 (2018).

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    Myeong, G. C. et al. The Sausage Globular Clusters. Mon. Not. R. Astron. Soc. 478, 5449–5459 (2018).

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Correspondence to Vasily Belokurov.

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