AstroTalk: Behind the news headlines of April June 2018

Transcription

1 AstroTalk: Behind the news headlines of April June 2018 Richard de Grijs ( 锐何思 ) (Macquarie University, Sydney, Australia) Gaia s second data release: a genuine treasure trove for star hunters In April of this year, the much-awaited second slew of data from the European Space Agency s Gaia mission was released, providing information on a phenomenal 1.7 billion stars the richest star catalogue to date. To put that vast number into context, if you were to count only to one billion at a rate of one count per second, it would take you more than 30 years. The new data will surely keep astronomers busy for even longer. A multitude of discoveries are on the horizon after this much awaited release, which is based on 22 months of charting the sky. The new data include positions, distance indicators and motions of more than one billion stars, along with highprecision measurements of asteroids within our solar system and of stars beyond our own Milky Way Galaxy. The dataset has already revealed previously unseen details about the make-up of the Milky Way s stellar population and about how stars move, essential information for investigating the formation and evolution of our Galaxy. The observations collected by Gaia are redefining the foundations of astronomy, says Günther Hasinger, ESA Director of Science. Gaia is an ambitious mission that relies on a huge human collaboration to make sense of a large volume of highly complex data. It demonstrates the need for longterm projects to guarantee progress in space science and technology and to implement even more daring scientific missions of the coming decades. The new data release pins down the positions of nearly 1.7 billion stars, and with a much greater precision than before. For some of the brightest stars in the survey, the level of precision equates to Earth-bound observers being able to spot a coin lying on the surface of the Moon. With these accurate measurements it is possible to separate the parallax of stars an apparent shift on the sky caused by Earth s annual orbit around our Sun from their true movements through the Galaxy. The new catalogue lists the parallaxes and velocities across the sky, or proper motions, for more than 1.3 billion stars. From the most accurate parallax measurements, about ten per cent of the total, astronomers can directly estimate distances to individual stars. The second Gaia data release represents a huge leap forward with respect to ESA s Hipparcos satellite, Gaia s predecessor and the first space mission for astrometry, which surveyed some 118,000 stars almost thirty years ago, says Anthony Brown of Leiden University in the Netherlands.

2 The sheer number of stars alone, with their positions and motions, would make Gaia's new catalogue already quite astonishing, adds Brown. But there is more: this unique scientific catalogue includes many other data types, with information about the properties of the stars and other celestial objects, making this release truly exceptional. As well as positions, the data include brightness information of all surveyed stars and colour measurements of nearly all, plus information on how the brightness and colour of half a million variable stars change over time. It also contains the velocities along the line of sight of a subset of seven million stars, the surface temperatures of about a hundred million and the effects of interstellar dust on 87 million. Gaia also observed objects in our solar system: the second data release comprises the positions of more than 14,000 known asteroids, which allows precise determination of their orbits. A much larger asteroid sample will be compiled in Gaia s future releases. Further afield, Gaia closed in on the positions of half a million distant quasars, bright galaxies powered by the activity of the supermassive black holes in their cores. These sources are used to define a reference frame for the celestial coordinates of all objects in the Gaia catalogue, something that is routinely done in radio waves but now for the first time is also possible at optical wavelengths. Major discoveries are expected to come once scientists start exploring Gaia s new release. The new Gaia data are so powerful that exciting results are just jumping at us, says Antonella Vallenari from the Istituto Nazionale di Astrofisica (INAF) and the Astronomical Observatory of Padova, Italy. For example, we have constructed the most detailed Hertzsprung Russell diagram of stars ever made on the full sky and we can already spot some interesting trends. It feels like we are inaugurating a new era of Galactic archaeology. Named after the two astronomers who devised it in the early twentieth century, the Hertzsprung Russell diagram compares the intrinsic brightness of stars with their colour and is a fundamental tool to study populations of stars and their evolution. A new version of this diagram, based on four million stars within five thousand light-years from the Sun selected from the Gaia catalogue, reveals many fine details for the first time. This includes the signature of different types of white dwarfs the dead remnants of stars like our Sun such that a differentiation can be made between those with hydrogen-rich cores and those dominated by helium. Combined with Gaia measurements of star velocities, the diagram enables astronomers to distinguish between various populations of stars of different ages that are located in different regions of the Milky Way, such as the disk and the halo the spherical cloud of stars surrounding the main disk and bulge of

3 our Galaxy and that formed in different ways. Further scrutiny suggests that the fast-moving stars thought to belong to the halo encompass two stellar populations that originated via two different formation scenarios, calling for more detailed investigations. Gaia will greatly advance our understanding of the Universe on all cosmic scales, says Timo Prusti, Gaia project scientist at ESA. Even in the neighbourhood of the Sun, which is the region we thought we understood best, Gaia is revealing new and exciting features. For a subset of stars within a few thousand light-years of the Sun, Gaia has measured the velocity in all three dimensions, revealing patterns in the motions of stars that are orbiting the Galaxy at similar speeds. In fact, the Milky Way has likely formed in part from the merging of many smaller systems. How exactly that happened, is still a puzzle, however. To learn more about the history of formation of the Milky Way, astronomers from the University of Groningen in the Netherlands and the University of California at Riverside in the USA inspected the motions of stars in the Galactic halo. Stars in the halo are more pristine than those in the central regions and the Galactic disk, and they spend most of their time outside of the disk-like structure that gives the Milky Way its name. It is thought that these halo stars are the stars that joined the Milky Way onboard of small galaxies. For this study, a team led by my colleague Amina Helmi, professor of dynamics, structure and formation of the Milky Way at the University of Groningen, combined the vast Gaia dataset with data from the RAVE survey. The researchers discovered that a large fraction of the halo stars travel in groups. Helmi: This indicates that the stars indeed originate from small galaxies that were cannibalised by the Milky Way a very long time ago. The astronomers describe these groups as large flows of stars like flocks of birds traveling together through the Milky Way. We believe there might be tens or even hundreds such flocks. At the moment, we only see small groups with just a few stars, but that is probably because we do not yet have all the necessary data. The team were bewildered of the behaviour of halo stars that spend most of the time in the outskirts of the Milky Way. Surprisingly, more than 70% of those stars appear to be moving in the opposite sense than the vast majority of stars in the Milky Way. Such a high fraction is unexpected in current models. Helmi: One may compare stars from the outer halo with commuters that drive the wrong way. We do not yet quite understand why.

4 These discoveries were made using halo stars that, in their journey through the Milky Way, are by chance currently close to the Sun. Gaia will provide us with data from stars from all over the Milky Way. Helmi: With such data we will get many new insights on how the Milky Way formed and be able to reconstruct its genealogy tree. The team discovered relics of merger events in the Milky Way halo. Five small groups of stars appear to represent mergers with smaller galaxies, while a big blob comprising hundreds of stars appears to be the remnant of a large merger event. Our aim is to study how the Milky Way has evolved, says Helmer Koppelman, a Ph.D. student in Helmi s research group. The idea is that smaller galaxies merge to form larger ones. One of the questions is whether a lot of small galaxies merge, or a few large ones. As most stars in the Milky Way s halo are thought to be remnants of merger events, Koppelman and his colleagues focused on halo stars in the Gaia data. We collected information from stars within 3000 light years of the Sun, as the accuracy of the position and movement is highest for stars that are near us, Koppelman explains. The first step was to filter out the stars from the Milky Way disk. These stars move around the centre of the disk, so are easily identified. What remained were about 6000 halo stars. By calculating their trajectory, Koppelman was able to identify stars with a shared origin. We discovered five small clusters which we believe are remnants of five merger events. However, many of the remaining stars also appeared to have a shared history. These stars form a huge blob with a retrograde movement compared to the disk. This suggests they are the result of a merger with a large galaxy. In fact, we believe that this merger event must have remodelled the disk in our Milky Way. A more detailed study of the nature of this merger is now underway. At this point in time, we can say that our Milky Way was shaped by a massive merger event and some smaller mergers. Koppelman also looked for stars belonging to the Helmi stream, which is named after his Ph.D. supervisor who identified it back in 1999 as the remnant of a merger event. Up to now, fewer than 20 stars belonging to the Helmi stream had been identified. The Gaia data has added over 100 new stars. Further analysis should clarify the nature of the galaxy that produced this stream. We will also be looking at stars beyond 3000 light-years to discover more members

5 of the different streams we identified. Together with simulations of galaxy evolution, this should give us exciting new insights into the evolution of the Milky Way. At Gaia s precision, it is also possible to see the motions of stars within some globular clusters ancient systems of stars bound together by gravity and found in the halo of the Milky Way and within our neighbouring galaxies, the Small and Large Magellanic Clouds. Gaia data were used to derive the orbits of 75 globular clusters and 12 dwarf galaxies that revolve around the Milky Way, providing all-important information to study the past evolution of our Galaxy and its environment, the gravitational forces that are at play, and the distribution of the elusive dark matter that permeates galaxies. Measuring the proper motion of several million stars in the Large Magellanic Cloud, astronomers were able to see an imprint of the stars rotating clockwise around the centre of the galaxy. Gaia is astronomy at its finest, says Fred Jansen, Gaia mission manager at ESA. Scientists will be busy with this data for many years, and we are ready to be surprised by the avalanche of discoveries that will unlock the secrets of our Galaxy. Figure 1: 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 Brighter regions indicate denser concentrations of especially bright stars, while darker regions correspond to patches of the sky where fewer bright stars are observed. The colour representation is obtained by combining the total amount of light with the amount of blue and red light recorded by Gaia in each patch of the sky. The bright horizontal structure that dominates the image is the Galactic plane, the flattened disk that hosts most of the stars in our Galaxy. In the middle of the image, the Galactic centre appears vivid and teeming with stars. Darker regions across the Galactic plane correspond to foreground clouds of interstellar gas and

6 dust, which absorb the light of stars located further away, behind the clouds. Many of these conceal stellar nurseries where new generations of stars are being born. Sprinkled across the image are also many globular and open clusters groupings of stars held together by their mutual gravity, as well as entire galaxies beyond our own. The two bright objects in the lower right of the image are the Large and Small Magellanic Clouds, two dwarf galaxies orbiting the Milky Way. (Credit: Gaia Data Processing and Analysis Consortium (DPAC); A. Moitinho / A. F. Silva / M. Barros / C. Barata, University of Lisbon, Portugal; H. Savietto, Fork Research, Portugal)

7 Figure 2: Gaia s all-sky view of our Milky Way Galaxy and neighbouring galaxies. The maps show the total brightness and colour of stars (top), the total density of stars (middle) and the interstellar dust that fills the Galaxy (bottom). (Credit: Gaia Data Processing and Analysis Consortium (DPAC); Top and middle: A. Moitinho / A. F. Silva / M. Barros / C. Barata, University of Lisbon, Portugal; H. Savietto, Fork Research, Portugal;Bottom: Gaia Coordination Unit 8; M. Fouesneau / C. Bailer-Jones, Max Planck Institute for Astronomy, Heidelberg, Germany)

8 Figure 3: Left panel: different star streams (coloured dots), the Milky Way disk (blue) and in black the rest of the halo stars, in which the horizontal cigar-shaped blob is visible. Right panel: same data, now seen form a 90 degree rotated angle. (Credit: Koppelman et al.) Figure 4: The Milky Way disk is embedded in a roundish halo of stars. The stars (in purple) are from a computer simulation of the remains from a merger with a small galaxy. The arrows indicate the motion of these stars that are now part of the halo. Larger arrows indicate faster motion. The astronomers suspect that tens to hundreds of such flows of stars are criss-crossing the Milky Way. (Credit: Amina Helmi/Jovan Veljanoski/Maarten Breddels/University of Groningen)

9 Figure 5: This image of the Large Magellanic Cloud combines the total density of stars detected by Gaia in each pixel with information about the proper motion of stars their velocity across the sky which is represented as the texture of the image, giving it a fingerprint-like appearance. Measuring the proper motion of several million stars, astronomers were able to see an imprint of the stars rotating clockwise around the centre of the galaxy. The impression of motion is evoked by the swirling nature of the line texture. (Credit: ESA/Gaia/DPAC)

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