New Gaia data reveals secrets of the Universe

A collage of 4 Gaia Sky maps: extinction, radial velocities, 3D motion, chemical, made using data from Gaia
Credit: ESA/Gaia/DPAC
Licence type: Attribution (CC BY 4.0)

Today, ESA’s Gaia mission released its new treasure trove of data about our home galaxy, the Milky Way. Astronomers describe stellar DNA, asymmetric motions, strange ‘starquakes’, and other fascinating insights in this most detailed survey of our galaxy to date.

Gaia is a mission to create the most accurate and complete multi-dimensional map of the Milky Way. This allows astronomers to reconstruct our home galaxy’s structure and past evolution over billions of years, and to better understand the lifecycle of stars and our place in the Universe.  

Gaia’s Data Release 3 (Gaia DR3) contains new and improved details for almost two billion stars in our galaxy. The catalogue includes new information including stellar chemical compositions, temperatures, colours, masses, ages, and the speed at which the stars move towards or away from us (radial velocity). Much of this information was revealed by the newly released spectroscopy data, a technique in which the starlight is split into its constituent colours. The data also includes special subsets of stars, like those that change brightness over time. 

Also new in this data set is the largest catalogue yet of binary stars, thousands of Solar System objects such as asteroids and moons of planets, and millions of galaxies and quasars outside the Milky Way.  

Professor Nicholas Walton (Institute of Astronomy, University of Cambridge, and member of the ESA Gaia Science Team) comments: “The Gaia: UK team at the University of Cambridge, University College London, University of Edinburgh, University of Leicester, and University of Bristol have played an essential part in this third data release providing the photometric data content – data obtained from light measurements. This release represents a major step forward in our creation of a detailed census of our Milky Way, fully characterising a significant sample of its stellar constituents. Analogous to the 100,000 Genomes project in biology, we are now able to characterise hundreds of millions of stars, which enables us to accurately determine their life cycles from birth to death, and to understand the incredible history and future of our Milky Way.” 

Dr George Seabroke (UCL Mullard Space Science Laboratory) said: “Gaia’s chemical mapping is analogous to sequencing the DNA of the human genome. The more stars we know the chemistry for, the better we can understand our galaxy as a whole. Gaia’s chemical catalogue of six million stars is ten times larger than previous ground-based catalogues, so this is really revolutionary. Gaia’s data releases are telling us where stars were located and how they are moving. Now we also know what a lot of these stars are made of.” 

“Unlike other missions that target specific objects, Gaia is a survey mission. This means that while surveying the entire sky with billions of stars multiple times, Gaia is bound to make discoveries that other more dedicated missions would miss. This is one of its strengths, and we can’t wait for the astronomy community to dive into our new data to find out even more about our galaxy and its surroundings than we could’ve imagined,” says Timo Prusti, Project Scientist for Gaia at ESA.

President of the Royal Astronomical Society, Professor Mike Edmunds, comments on the release: “The GAIA mission is based on brilliantly simple principles, used in astronomy for many years, but brought bang up-to-date by clever modern technology. The surveys it is doing are revolutionising many fields of research, for example the detailed archaeology of our own stellar system, the Milky Way. I am very much looking forward to hearing about the latest results.”

Source:  Royal Astronomical Society (RAS)/News

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Science Contacts

Dr Nicholas Walton
Institute of Astronomy
University of Cambridge
Tel: +44 (0)1223 337503
naw@ast.cam.ac.uk

Dr Francesca De Angeli
Institute of Astronomy
University of Cambridge
Tel: +44 (0)1223 337546
fda@ast.cam.ac.uk

Dr Dafydd Wyn Evans
Institute of Astronomy
University of Cambridge
Tel: +44 (0)1223 764608
dwe@ast.cam.ac.uk

Dr George Seabroke
Mullard Space Science Laboratory
University College London
Tel: +44 (0)1483 204902
g.seabroke@ucl.ac.uk

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Media Contacts:

Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7292 3979,
Mob: +44 (0)7802 877 699
massey@ras.org.uk

Gurjeet Kahlon
Mob: +44 (0)7802 877700
Royal Astronomical Society
gkahlon@ras.ac.uk

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Further information

Starquakes

• One of the most surprising discoveries coming out of the new data is that Gaia is able to detect starquakes – tiny motions on the surface of a star – that change the shapes of stars, something the observatory was not originally built for.
• Previously, Gaia already found radial oscillations that cause stars to swell and shrink periodically, while keeping their spherical shape. But Gaia has now also spotted other vibrations that are more like large-scale tsunamis. These nonradial oscillations change the global shape of a star and are therefore harder to detect.
• Gaia found strong nonradial starquakes in thousands of stars. Gaia also revealed such vibrations in stars that have seldomly been seen before. According to current ideas, these stars should not have any quakes, but Gaia did nonetheless detect them on their surfaces.

Stellar Fingerprinting: classifying the stars in our Milky Way

• What stars are made of can tell us about their birthplace and their journey afterwards, and therefore about the history of the Milky Way. With today’s data release, Gaia is revealing the largest chemical map of the galaxy coupled to the 3D motions of stars, from those in our solar neighbourhood to stars in smaller galaxies around our own.
• Some stars contain more ‘heavy metals’ than others. During the Big Bang, only light elements were formed (hydrogen and helium). All other heavier elements – called metals by astronomers – are built inside stars. When stars die, they release these metals into the gas and dust between the stars, out of which new stars form. Active star formation and death will lead to an environment that is richer in metals. A star’s chemical composition is a bit like its DNA, giving us crucial information about its origin. 
• With Gaia, we see that some stars in our galaxy are made of primordial material, while others like our Sun are made of matter enriched by previous generations of stars. Stars that are closer to the centre and plane of our galaxy are richer in metals than stars at larger distances. By looking at their chemical composition, Gaia was also able identify stars that originally came from different galaxies than our own.
  

The chemistry of our Milky Way

• What stars are made of can tell us about their birthplace and their journey afterwards, and therefore about the history of the Milky Way. With today’s data release, Gaia is bringing us a chemical map of the galaxy.
• Some stars contain more ‘heavy metals’ than others. During the Big Bang, the only stable elements to form were hydrogen and helium and traces of lithium. All other heavier elements – metals – are built inside stars. When stars die, they release these metals into the gas and dust between the stars, out of which new stars form. Active star formation and death will lead to an environment that is richer in metals. Therefore, a star’s chemical composition is a bit like its DNA, giving us crucial information about its origin.
• With Gaia we see that some stars in our galaxy are made of primordial material, while others like our Sun are made of matter enriched by previous generations of stars. Stars closer to the centre and plane of our galaxy are richer in metals than stars at larger distances. 
  

Gaia’s Milky Way in motion (3D)

• ESA’s Gaia data release 3 also shows us the speed at which more than 30 million Milky Way stars move towards or away from us. This is called ‘radial velocity’ and it is providing the third velocity dimension in the Gaia map of our galaxy. Together with the proper motions of stars (movement across the sky), we can now see how the stars move over a large portion of the Milky Way.
• This sky map shows the velocity field of the Milky Way for ~26 million stars. The colours show the radial velocities of stars along the line-of-sight. Blue shows the parts of the sky where the average motion of stars is towards us and red shows the regions where the average motion is away from us. The lines visible in the figure trace out the motion of stars projected on the sky (proper motion). These lines show how the direction of the speed of stars varies by galactic latitude and longitude. The Large and Small Magellanic Clouds (LMC and SMC) are not visible as only stars with well-defined distances were selected to make this image.

Binary stars, asteroids, quasars, and more

Other papers that are published today reflect the breadth and depth of Gaia's discovery potential. A new binary star catalogue presents the mass and evolution of more than 800 thousand binary systems, while a new asteroid survey comprising 156 thousand rocky bodies is digging deeper into the origin of our Solar System. Gaia is also revealing information about 10 million variable stars, mysterious macro-molecules between stars, as well as quasars and galaxies beyond our own cosmic neighbourhood.

Movies

Gaia sees starquakes

One of the surprising discoveries coming out of Gaia data release 3, is that Gaia is able to detect starquakes – tiny motions on the surface of a star – that change the shapes of stars, something the observatory was not originally built for.

Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO.

The chemistry of our Milky Way

What stars are made of can tell us about their birthplace and their journey afterwards, and therefore about the history of the Milky Way. With today’s data release, Gaia is bringing us a chemical map of the galaxy.

Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO.

Milky Way in motion

ESA’s Gaia data release 3 shows us the speed at which more than 30 million Milky Way stars move towards or away from us. This is called ‘radial velocity’ and it is providing the third velocity dimension in the Gaia map of our galaxy. We can now see how the stars move over a large portion of the Milky Way’s disc.

Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO.

Asteroid populations

By far the largest group of Solar System objects in Gaia's data release 3 are 154 741 asteroids for which Gaia has determined their orbits. Depending on their orbits one can distinguish different groups of asteroids.

Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO.

Notes for editors

The UK team involved in the Gaia mission is supported by the UK Space Agency and the Science and Technology Facilities Council.

More details on Gaia’s data releases 3 can be found here: https://www.cosmos.esa.int/web/gaia/data-release-3 

From 13 June 2022, 12:00 CEST onwards, the new Gaia data can be accessed at https://gea.esac.esa.int/archive/

This media kit from ESA summarises the data in a series of infographics: ​​https://www.esa.int/Science_Exploration/Space_Science/Gaia/Gaia_data_release_3_media_kit 

More in-depth stories on the new Gaia data can be found here: https://www.cosmos.esa.int/web/gaia/dr3-stories

A series of scientific papers describing the data and their validation process will appear in a special issue of the journal Astronomy & Astrophysics: https://www.cosmos.esa.int/web/gaia/dr3-papers 

Gaia’s data release 3 will be presented on Monday 13 June during a virtual media briefing at a Royal Astronomical Society hosted event at Goonhilly Earth Station, and will be available on the RAS YouTube channel at https://youtu.be/rjCaRSJMIhc (The UK event presentation slides will be available at https://www.gaia.ac.uk/gaia-dr3-uk-event ).

The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises scientific meetings, publishes international research and review journals, recognises outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 4,000 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.

The RAS accepts papers for its journals based on the principle of peer review, in which fellow experts on the editorial boards accept the paper as worth considering. The Society issues press releases based on a similar principle, but the organisations and scientists concerned have overall responsibility for their content. Keep up with the RAS on Twitter, Facebook, Instagram, LinkedIn, and YouTube.

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Is the nearest star cluster to the Sun being destroyed?

The Hyades and their tidal tails

The true extent of the Hyades tidal tails have been revealed for the first time by data from the ESA’s Gaia mission. The Gaia data has allowed the former members of the star cluster (shown in pink) to be traced across the whole sky. Those stars are marked in pink, and the shapes of the various constellations are traced in green. The image was created using Gaia Sky. ESA/Gaia/DPAC, CC BY-SA 3.0 IGO; acknowledgement: S. Jordan/T. Sagrista

Data from ESA’s Gaia star mapping satellite have revealed tantalising evidence that the nearest star cluster to the Sun is being disrupted by the gravitational influence of a massive but unseen structure in our galaxy.

If true, this might provide evidence for a suspected population of ‘dark matter sub-halos’. These invisible clouds of particles are thought to be relics from the formation of the Milky Way, and are now spread across the galaxy, making up an invisible substructure that exerts a noticeable gravitational influence on anything that drifts too close.

ESA Research Fellow Tereza Jerabkova and colleagues from ESA and the European Southern Observatory made the discovery while studying the way a nearby star cluster is merging into the general background of stars in our galaxy. This discovery was based on Gaia’s Early third Data Release (EDR3) and data from the second release.

The team chose the Hyades as their target because it is the nearest star cluster to the Sun. It is located just over 153 light years away, and is easily visible to skywatchers in both northern and southern hemispheres as a conspicuous ‘V’ shape of bright stars that marks the head of the bull in the constellation of Taurus. Beyond the easily visible bright stars, telescopes reveal a hundred or so fainter ones contained in a spherical region of space, roughly 60 light years across.

A star cluster will naturally lose stars because as those stars move within the cluster they tug at each other gravitationally. This constant tugging slightly changes the stars’ velocities, moving some to the edges of the cluster. From there, the stars can be swept out by the gravitational pull of the galaxy, forming two long tails.

One tail trails the star cluster, the other pulls out ahead of it. They are known as tidal tails, and have been widely studied in colliding galaxies but no one had ever seen them from a nearby open star cluster, until very recently.

Locating the Hyades tidal tails
Access the video

The key to detecting tidal tails is spotting which stars in the sky are moving in a similar way to the star cluster. Gaia makes this easy because it is precisely measuring the distance and movement of more than a billion stars in our galaxy. “These are the two most important quantities that we need to search for tidal tails from star clusters in the Milky Way,” says Tereza.

Previous attempts by other teams had met with only limited success because the researchers had only looked for stars that closely matched the movement of the star cluster. This excluded members that left earlier in its 600–700 million year history and so are now travelling on different orbits.

To understand the range of orbits to look for, Tereza constructed a computer model that would simulate the various perturbations that escaping stars in the cluster might feel during their hundreds of millions of years in space. It was after running this code, and then comparing the simulations to the real data that the true extend of the Hyades tidal tails were revealed. Tereza and colleagues found thousands of former members in the Gaia data. These stars now stretch for thousands of light years across the galaxy in two enormous tidal tails.

But the real surprise was that the trailing tidal tail seemed to be missing stars. This indicates that something much more brutal is taking place than the star cluster gently ‘dissolving’. 

Evolution of Hyades star cluster from ~ 650 million years ago until now
Access the video

Running the simulations again, Tereza showed that the data could be reproduced if that tail had collided with a cloud of matter containing about 10 million solar masses. “There must have been a close interaction with this really massive clump, and the Hyades just got smashed,” she says.

But what could that clump be? There are no observations of a gas cloud or star cluster that massive nearby. If no visible structure is detected even in future targeted searches, Tereza suggests that object could be a dark matter sub-halo. These are naturally occurring clumps of dark matter that are thought to help shape the galaxy during its formation. This new work shows how Gaia is helping astronomers map out this invisible dark matter framework of the galaxy.

“With Gaia, the way we see the Milky Way has completely changed. And with these discoveries, we will be able to map the Milky Way’s sub-structures much better than ever before,” says Tereza. And having proved the technique with the Hyades, Tereza and colleagues are now extending the work by looking for tidal tails from other, more distant star clusters.

Note For Editor

“The 800pc long tidal tails of the Hyades star cluster: Possible discovery of candidate epicyclic over-densities from an open star cluster” by Tereza Jerabkova et al. will be published online by Astronomy and Astrophysics on 24 March 2021. https://www.aanda.org/10.1051/0004-6361/202039949

For more information, please contact:
ESA Media Relations
Email: media@esa.int

Source: ESA/Space Science

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Pinpointing Gaia to Map the Milky Way

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Pinpointing Gaia to Map the Milky Way

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Pinpointing Gaia to Map the Milky Way (Annotated)

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Surveying the skies

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The Gaia Spacecraft

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Gaia’s View of the Milky Way

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Videos

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ESOcast 200 Light: ESO helps map the Galaxy

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Animation of Gaia's Orbit

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ESO’s VST helps determine the spacecraft’s orbit to enable the most accurate map ever of more than a billion stars

This image, a composite of several observations captured by ESO’s VLT Survey Telescope (VST), shows the ESA spacecraft Gaia as a faint trail of dots across the lower half of the star-filled field of view. These observations were taken as part of an ongoing collaborative effort to measure Gaia’s orbit and improve the accuracy of its unprecedented star map.

Gaia, operated by the European Space Agency (ESA), surveys the sky from orbit to create the largest, most precise, three-dimensional map of our Galaxy. One year ago, the Gaia mission produced its much-awaited second data release, which included high-precision measurements — positions, distance and proper motions — of more than one billion stars in our Milky Way galaxy. This catalogue has enabled transformational studies in many fields of astronomy, addressing the structure, origin and evolution the Milky Way and generating more than 1700 scientific publications since its launch in 2013.

In order to reach the accuracy necessary for Gaia’s sky maps, it is crucial to pinpoint the position of the spacecraft from Earth. Therefore, while Gaia scans the sky, gathering data for its stellar census, astronomers regularly monitor its position using a global network of optical telescopes, including the VST at ESO’s Paranal Observatory [1]. The VST is currently the largest survey telescope observing the sky in visible light, and records Gaia’s position in the sky every second night throughout the year.

“Gaia observations require a special observing procedure,” explained Monika Petr-Gotzens, who has coordinated the execution of ESO’s observations of Gaia since 2013. “The spacecraft is what we call a ‘moving target’, as it is moving quickly relative to background stars — tracking Gaia is quite the challenge!”

“The VST is the perfect tool for picking out the motion of Gaia,” elaborated Ferdinando Patat, head of the ESO’s Observing Programmes Office. “Using one of ESO’s first-rate ground-based facilities to bolster cutting-edge space observations is a fine example of scientific cooperation.”

“This is an exciting ground-space collaboration, using one of ESO’s world-class telescopes to anchor the trailblazing observations of ESA’s billion star surveyor,” commented Timo Prusti, Gaia project scientist at ESA.

The VST observations are used by ESA’s flight dynamics experts to track Gaia and refine the knowledge of the spacecraft’s orbit. Painstaking calibration is required to transform the observations, in which Gaia is just a speck of light among the bright stars, into meaningful orbital information. Data from Gaia’s second release was used to identify each of the stars in the field of view, and allowed the position of the spacecraft to be calculated with astonishing precision — up to 20 milliarcseconds.

“This is a challenging process: we are using Gaia’s measurements of the stars to calibrate the position of the Gaia spacecraft and ultimately improve its measurements of the stars,” explains Timo Prusti

“After careful and lengthy data processing, we have now achieved the accuracy required for the ground-based observations of Gaia to be implemented as part of the orbit determination,” says Martin Altmann, lead of the Ground Based Optical Tracking (GBOT) campaign at the Centre for Astronomy of Heidelberg University, Germany.

The GBOT information will be used to improve our knowledge of Gaia’s orbit not only in observations to come, but also for all the data that have been gathered from Earth in the previous years, leading to improvements in the data products that will be included in future releases.

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Notes

[1] This collaboration between ESO and ESA is just one of several cooperative projects which have benefitted from the expertise of both organisations in progressing astronomy and astrophysics. On 20 August 2015, the ESA and ESO Directors General signed a cooperation agreement to facilitate synergy through projects such as these.

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More Information

In order to foster exchanges between astrophysics-related spaceborne missions and ground-based facilities, as well as between their respective communities, ESA and ESO are joining forces to organise a series of international astronomy meetings. The first ESA-ESO joint workshop will take place in November 2019 at ESO and a call for proposals for the second workshop, to take place in 2020 at ESA, is currently open.

ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It has 16 Member States: Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile and with Australia as a Strategic Partner. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its world-leading Very Large Telescope Interferometer as well as two survey telescopes, VISTA working in the infrared and the visible-light VLT Survey Telescope. Also at Paranal ESO will host and operate the Cherenkov Telescope Array South, the world’s largest and most sensitive gamma-ray observatory.

ESO is also a major partner in two facilities on Chajnantor, APEX and ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre Extremely Large Telescope, the ELT, which will become “the world’s biggest eye on the sky”.

The European Space Agency (ESA) is Europe’s gateway to space. Its mission is to shape the development of Europe’s space capability and ensure that investment in space continues to deliver benefits to the citizens of Europe and the world.

ESA is an international organisation with 22 Member States. By coordinating the financial and intellectual resources of its members, it can undertake programmes and activities far beyond the scope of any single European country.

ESA's Gaia satellite was launched in 2013 to create the most precise three-dimensional map of more than one billion stars in the Milky Way. The mission has released two lots of data thus far: Gaia Data Release 1 in 2016 and Gaia Data Release 2 in 2018. More releases will follow in the coming years.

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Links

• ESOblog: How ESO collaborates with ESA

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Contacts 

Calum Turner
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6670
Email: pio@eso.org

Source: ESO/News

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Hubble & Gaia accurately weigh the Milky Way

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Globular clusters surrounding the Milky Way (artist’s impression)

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ESA’s Gaia satellite

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Globular cluster NGC 4147

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Videos

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Hubblecast 117 Light: Hubble & Gaia weigh the Milky Way

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Globular clusters surrounding the Milky Way (artist’s impression)

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In a striking example of multi-mission astronomy, measurements from the NASA/ESA Hubble Space Telescope and the ESA Gaia mission have been combined to improve the estimate of the mass of our home galaxy the Milky Way: 1.5 trillion solar masses.

The mass of the Milky Way is one of the most fundamental measurements astronomers can make about our galactic home. However, despite decades of intense effort, even the best available estimates of the Milky Way’s mass disagree wildly. Now, by combining new data from the European Space Agency (ESA) Gaia mission with observations made with the NASA/ESA Hubble Space Telescope, astronomers have found that the Milky Way weighs in at about 1.5 trillion solar masses within a radius of 129 000 light-years from the galactic centre.

Previous estimates of the mass of the Milky Way ranged from 500 billion to 3 trillion times the mass of the Sun. This huge uncertainty arose primarily from the different methods used for measuring the distribution of dark matter — which makes up about 90% of the mass of the galaxy.

“We just can’t detect dark matter directly,” explains Laura Watkins (European Southern Observatory, Germany), who led the team performing the analysis. “That’s what leads to the present uncertainty in the Milky Way’s mass — you can’t measure accurately what you can’t see!”

Given the elusive nature of the dark matter, the team had to use a clever method to weigh the Milky Way, which relied on measuring the velocities of globular clusters — dense star clusters that orbit the spiral disc of the galaxy at great distances [1].

“The more massive a galaxy, the faster its clusters move under the pull of its gravity” explains N. Wyn Evans (University of Cambridge, UK). “Most previous measurements have found the speed at which a cluster is approaching or receding from Earth, that is the velocity along our line of sight. However, we were able to also measure the sideways motion of the clusters, from which the total velocity, and consequently the galactic mass, can be calculated.” [2]

The group used Gaia’s second data release as a basis for their study. Gaia was designed to create a precise three-dimensional map of astronomical objects throughout the Milky Way and to track their motions. Its second data release includes measurements of globular clusters as far as 65 000 light-years from Earth.

"Global clusters extend out to a great distance, so they are considered the best tracers astronomers use to measure the mass of our galaxy" said Tony Sohn (Space Telescope Science Institute, USA), who led the Hubble measurements.

The team combined these data with Hubble’s unparalleled sensitivity and observational legacy. Observations from Hubble allowed faint and distant globular clusters, as far as 130 000 light-years from Earth, to be added to the study. As Hubble has been observing some of these objects for a decade, it was possible to accurately track the velocities of these clusters as well.

“We were lucky to have such a great combination of data,” explained Roeland P. van der Marel (Space Telescope Science Institute, USA). “By combining Gaia’s measurements of 34 globular clusters with measurements of 12 more distant clusters from Hubble, we could pin down the Milky Way’s mass in a way that would be impossible without these two space telescopes.”

Until now, not knowing the precise mass of the Milky Way has presented a problem for attempts to answer a lot of cosmological questions. The dark matter content of a galaxy and its distribution are intrinsically linked to the formation and growth of structures in the Universe. Accurately determining the mass for the Milky Way gives us a clearer understanding of where our galaxy sits in a cosmological context.

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Notes

[1] Globular clusters formed prior to the construction of the Milky Way’s spiral disk, where our Sun and the Solar System later formed. Because of their great distances, globular star clusters allow astronomers to trace the mass of the vast envelope of dark matter surrounding our galaxy far beyond the spiral disk. 

[2] The total velocity of an object is made up of three motions — a radial motion plus two defining the sideway motions. However, in astronomy most often only line-of-sight velocities are available. With only one component of the velocity available, the estimated masses depend very strongly on the assumptions for the sideway motions. Therefore measuring the sideway motions directly significantly reduces the size of the error bars for the mass.

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More Information

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

ESA's Gaia satellite was launched in 2013 to create the most precise three-dimensional map of more than one billion stars in the Milky Way. The mission has release two lots of data thus far: Gaia Data Release 1 in 2016 and Gaia Data Release 2 in 2018. More releases will follow in the coming years.

The study was presented in the paper “Evidence for an Intermediate-Mass Milky Way from Gaia DR2 Halo Globular Cluster Motions”, which will be published in The Astrophysical Journal.

The international team of astronomers in this study consists of Laura L. Watkins (European Southern Observatory, Germany), Roeland P. van der Marel (Space Telescope Science Institute, USA, and Johns Hopkins University Center for Astrophysical Sciences, USA), Sangmo T. Sohn (Space Telescope Science Institute, USA), and N. Wyn Evans (University of Cambridge, UK).

Image credit: ESA/Hubble, L. Watkins, L. Calçada

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Links

• Hubblesite release
• Science paper
• Gaia mission page

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Contacs

Laura Watkins
European Southern Observatory
Garching, Germany
Tel: +49 89 3200 6257
Email: lwatkins@eso.org

N. Wyn Evans
University of Cambridge
Cambridge, United Kingdom
Tel: +44-01223-765847
Email: nwe@ast.cam.ac.uk

Roeland P. van der Marel
Space Telescope Science Institute
Baltimore, USA
Tel: +1-410-338-4931
Email: marel@stsci.edu

Bethany Downer
ESA/Hubble, Public Information Officer
Garching, Germany
Email: bethany.downer@partner.eso.org

Source: ESA/Hubble/News

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Gaia clocks new speeds for Milky Way Andromeda collision

Future motions of the Milky Way, Andromeda and Triangulum galaxies

Copyright Orbits: E. Patel, G. Besla (University of Arizona), R. van der Marel (STScI);

Images: ESA (Milky Way); ESA/Gaia/DPAC (M31, M33) 

ESA’s Gaia satellite has looked beyond our Galaxy and explored two nearby galaxies to reveal the stellar motions within them and how they will one day interact and collide with the Milky Way – with surprising results. 

Our Milky Way belongs to a large gathering of galaxies known as the Local Group and, along with the Andromeda and Triangulum galaxies – also referred to as M31 and M33, respectively – makes up the majority of the group’s mass. 

Astronomers have long suspected that Andromeda will one day collide with the Milky Way, completely reshaping our cosmic neighbourhood. However, the three-dimensional movements of the Local Group galaxies remained unclear, painting an uncertain picture of the Milky Way’s future.

Gaia’s new map of star density 

Copyright: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO

 

“We needed to explore the galaxies’ motions in 3D to uncover how they have grown and evolved, and what creates and influences their features and behaviour,” says lead author Roeland van der Marel of the Space Telescope Science Institute in Baltimore, USA.  

“We were able to do this using the second package of high-quality data released by Gaia.”

Gaia is currently building the most precise 3D map of the stars in the nearby Universe, and is releasing its data in stages. The data from the second release, made in April 2018, was used in this research. 

Previous studies of the Local Group have combined observations from telescopes including the NASA/ESA Hubble Space Telescope and the ground-based Very Long Baseline Array to figure out how the orbits of Andromeda and Triangulum have changed over time. The two disc-shaped spiral galaxies are located between 2.5 and 3 million light-years from us, and are close enough to one another that they may be interacting. 

Two possibilities emerged: either Triangulum is on an incredibly long six-billion-year orbit around Andromeda but has already fallen into it in the past, or it is currently on its very first infall. Each scenario reflects a different orbital path, and thus a different formation history and future for each galaxy.

The sharpest view ever of the Triangulum Galaxy

Copyright: NASA, ESA, and M. Durbin, J. Dalcanton, and B. F. Williams (University of Washington); CC BY 4.0

While Hubble has obtained the sharpest view ever of both Andromeda and Triangulum, Gaia measures the individual position and motion of many of their stars with unprecedented accuracy.

“We combed through the Gaia data to identify thousands of individual stars in both galaxies, and studied how these stars moved within their galactic homes,” adds co-author Mark Fardal, also of Space Telescope Science Institute. 

“While Gaia primarily aims to study the Milky Way, it’s powerful enough to spot especially massive and bright stars within nearby star-forming regions – even in galaxies beyond our own.” 

The stellar motions measured by Gaia not only reveal how each of the galaxies moves through space, but also how each rotates around its own spin axis. 

A century ago, when astronomers were first trying to understand the nature of galaxies, these spin measurements were much sought-after, but could not be successfully completed with the telescopes available at the time.

Stellar motions in the Andromeda galaxy

Copyright: ESA/Gaia (star motions); NASA/Galex (background image); R. van der Marel, M. Fardal, J. Sahlmann (STScI)
“It took an observatory as advanced as Gaia to finally do so,” says Roeland.

“For the first time, we’ve measured how M31 and M33 rotate on the sky. Astronomers used to see galaxies as clustered worlds that couldn’t possibly be separate ‘islands’, but we now know otherwise.

“It has taken 100 years and Gaia to finally measure the true, tiny, rotation rate of our nearest large galactic neighbour, M31. This will help us to understand more about the nature of galaxies.” 

By combining existing observations with the new data release from Gaia, the researchers determined how Andromeda and Triangulum are each moving across the sky, and calculated the orbital path for each galaxy both backwards and forwards in timefor billions of years. 

 

“The velocities we found show that M33 cannot be on a long orbit around M31,” says co-author Ekta Patel of the University of Arizona, USA. “Our models unanimously imply that M33 must be on its first infall into M31.” 

While the Milky Way and Andromeda are still destined to collide and merge, both the timing and destructiveness of the interaction are also likely to be different than expected.  

As Andromeda’s motion differs somewhat from previous estimates, the galaxy is likely to deliver more of a glancing blow to the Milky Way than a head-on collision. This will take place not in 3.9 billion years’ time, but in 4.5 billion – some 600 million years later than anticipated.

Sharpest ever view of the Andromeda Galaxy

Copyright:  NASA, ESA, J. Dalcanton (University of Washington, USA), B. F. Williams (University of Washington, USA), L. C. Johnson (University of Washington, USA), the PHAT team, and R. Gendler.

“This finding is crucial to our understanding of how galaxies evolve and interact,” says Timo Prusti, ESA Gaia Project Scientist.

“We see unusual features in both M31 and M33, such as warped streams and tails of gas and stars. If the galaxies haven’t come together before, these can’t have been created by the forces felt during a merger. Perhaps they formed via interactions with other galaxies, or by gas dynamics within the galaxies themselves. 

“Gaia was designed primarily for mapping stars within the Milky Way — but this new study shows that the satellite is exceeding expectations, and can provide unique insights into the structure and dynamics of galaxies beyond the realm of our own. The longer Gaia watches the tiny movements of these galaxies across the sky, the more precise our measurements will become.”

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Notes for Editors

“First Gaia Dynamics of the Andromeda System: DR2 Proper Motions, Orbits, and Rotation of M31 and M33” by R. P. van der Marel et al. is published in Astrophysical Journal. 

ESA’s Gaia satellite was launched in 2013 to create the most precise three-dimensional map of one billion of the stars within the Milky Way. The mission has released two lots of data so far: Gaia Data Release 1 on 14 September 2016, and Gaia Data Release 2 on 25 April 2018 (the latter of which was used in this study). More releases will follow in coming years.

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Contacts

Roeland P. van der Marel
Space Telescope Science Institute
Baltimore, USA
Email: marel@stsci.edu

Mark Fardal
Space Telescope Science Institute
Baltimore, USA
Email: fardal@stsci.edu

Ekta Patel
Steward Observatory
University of Arizona, USA
Email: ektapatel@email.arizona.edu

Timo Prusti
ESA Gaia Project Scientist
Email: tprusti@cosmos.esa.int

Markus Bauer
ESA Science Programme Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer@esa.int

Source: ESA/GAIA

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Infant exoplanet weighed by Hipparcos and Gaia

Beta Pictoris system

Copyright ESO/A-M. Lagrange et al.

The mass of a very young exoplanet has been revealed for the first time using data from ESA’s star mapping spacecraft Gaia and its predecessor, the quarter-century retired Hipparcos satellite. 

Astronomers Ignas Snellen and Anthony Brown from Leiden University, the Netherlands, deduced the mass of the planet Beta Pictoris b from the motion of its host star over a long period of time as captured by both Gaia and Hipparcos.

The planet is a gas giant similar to Jupiter but, according to the new estimate, is 9 to 13 times more massive. It orbits the star Beta Pictoris, the second brightest star in the constellation Pictor.

The planet was only discovered in 2008 in images captured by the Very Large Telescope at the European Southern Observatory in Chile. Both the planet and the star are only about 20 million years old – roughly 225 times younger than the Solar System. Its young age makes the system intriguing but also difficult to study using conventional methods.

“In the Beta Pictoris system, the planet has essentially just formed,” says Ignas. “Therefore we can get a picture of how planets form and how they behave in the early stages of their evolution. On the other hand, the star is very hot, rotates fast, and it pulsates.”

This behaviour makes it difficult for astronomers to accurately measure the star’s radial velocity – the speed at which it appears to periodically move towards and away from the Earth. Tiny changes in the radial velocity of a star, caused by the gravitational pull of planets in its vicinity, are commonly used to estimate masses of exoplanets. But this method mainly works for systems that have already gone through the fiery early stages of their evolution.

In the case of Beta Pictoris b, upper limits of the planet’s mass range had been arrived at before using the radial velocity method. To obtain a better estimate, the astronomers used a different method, taking advantage of Hipparcos’ and Gaia’s measurements that reveal the precise position and motion of the planet’s host star in the sky over time.

Astrometric measurements to detect exoplanets

Copyright ESA 

“The star moves for different reasons,” says Ignas. “First, the star circles around the centre of the Milky Way, just as the Sun does. That appears from the Earth as a linear motion projected on the sky. 

We call it proper motion. And then there is the parallax effect, which is caused by the Earth orbiting around the Sun. Because of this, over the year, we see the star from slightly different angles.” 

And then there is something that the astronomers describe as ‘tiny wobbles’ in the trajectory of the star across the sky – minuscule deviations from the expected course caused by the gravitational pull of the planet in the star’s orbit. This is the same wobble that can be measured via changes in the radial velocity, but along a different direction – on the plane of the sky, rather than along the line of sight.

“We are looking at the deviation from what you expect if there was no planet and then we measure the mass of the planet from the significance of this deviation,” says Anthony. “The more massive the planet, the more significant the deviation.”

To be able to make such an assessment, astronomers need to observe the trajectory of the star for a long period of time to properly understand the proper motion and the parallax effect.

The Gaia mission, designed to observe more than one billion stars in our Galaxy, will eventually be able to provide information about a large amount of exoplanets. In the 22 months of observations included in Gaia’s second data release, published in April, the satellite has recorded the star Beta Pictoris about thirty times. That, however, is not enough.

“Gaia will find thousands of exoplanets, that’s still on our to-do list,” says Timo Prusti, ESA’s Gaia project scientist. “The reason that the exoplanets can be expected only late in the mission is the fact that to measure the tiny wobble that the exoplanets are causing, we need to trace the position of stars for several years.”

Combining the Gaia measurements with those from ESA’s Hipparcos mission, which observed Beta Pictoris 111 times between 1990 and 1993, enabled Ignas and Anthony to get their result much faster. This led to the first successful estimate of a young planet’s mass using astrometric measurements.

“By combining data from Hipparcos and Gaia, which have a time difference of about 25 years, you get a very long term proper motion,” says Anthony.

“This proper motion also contains the component caused by the orbiting planet. Hipparcos on its own would not have been able to find this planet because it would look like a perfectly normal single star unless we had measured it for a much longer time.

“Now, by combining Gaia and Hipparcos and looking at the difference in the long term and the short term proper motion, we can see the effect of the planet on the star.”

The result represents an important step towards better understanding the processes involved in planet formation, and anticipates the exciting exoplanet discoveries that will be unleashed by Gaia’s future data releases.

 Note for Editors

“The mass of the young planet Beta Pictoris b through the astrometric motion of its host star,” by I. Snellen and A. Brown is published in Nature Astronomy, 20 August 2018.  

Source: ESA/GAIA

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Gaia's first year of scientific observations

Stellar density map

Copyright: ESA/Gaia – CC BY-SA 3.0 IGO

Last Friday, 21 August, ESA’s billion-star surveyor, Gaia, completed its first year of science observations in its main survey mode. 

After launch on 19 December 2013 and a six-month long in-orbit commissioning period, the satellite started routine scientific operations on 25 July 2014. Located at the Lagrange point L2, 1.5 million km from Earth, Gaia surveys stars and many other astronomical objects as it spins, observing circular swathes of the sky. By repeatedly measuring the positions of the stars with extraordinary accuracy, Gaia can tease out their distances and motions through the Milky Way galaxy. 

For the first 28 days, Gaia operated in a special scanning mode that sampled great circles on the sky, but always including the ecliptic poles. This meant that the satellite observed the stars in those regions many times, providing an invaluable database for Gaia’s initial calibration.

At the end of that phase, on 21 August 2014, Gaia commenced its main survey operation, employing a scanning law designed to achieve the best possible coverage of the whole sky.

Since the start of its routine phase, the satellite recorded 272 billion positional or astrometric measurements 54.4 billion brightness or photometric data points, and 5.4 billion spectra.

The Gaia team have spent a busy year processing and analysing these data, en route towards the development of Gaia’s main scientific products, consisting of enormous public catalogues of the positions, distances, motions and other properties of more than a billion stars. Because of the immense volumes of data and their complex nature, this requires a huge effort from expert scientists and software developers distributed across Europe, combined in Gaia’s Data Processing and Analysis Consortium (DPAC).

“The past twelve months have been very intense, but we are getting to grips with the data, and are looking forward to the next four years of nominal operations,” says Timo Prusti, Gaia project scientist at ESA. “We are just a year away from Gaia's first scheduled data release, an intermediate catalogue planned for the summer of 2016. With the first year of data in our hands, we are now halfway to this milestone, and we’re able to present a few preliminary snapshots to show that the spacecraft is working well and that the data processing is on the right track.”

 The parallax method of measuring a star's distance
Copyright: ESA/Medialab

As one example of the ongoing validation, the Gaia team has been able to measure the parallax for an initial sample of two million stars.

Parallax is the apparent motion of a star against a distant background observed over the period of a year and resulting from the Earth's real motion around the Sun; this is also observed by Gaia as it orbits the Sun alongside Earth. But parallax is not the only movement seen by Gaia: the stars are also really moving through space, which is called proper motion.

Gaia has made an average of roughly 14 measurements of each star on the sky thus far, but this is generally not enough to disentangle the parallax and proper motions.

To overcome this, the scientists have combined Gaia data with positions extracted from the Tycho-2 catalogue, based on data taken between 1989 and 1993 by Gaia's predecessor, the Hipparcos satellite.

This restricts the sample to just two million out of the more than one billion that Gaia has observed so far, but yields some useful early insights into the quality of its data.

The nearer a star is to the Sun, the larger its parallax, and thus the parallax measured for a star can be used to determine its distance. In turn, the distance can be used to convert the apparent brightness of the star into its true brightness or ‘absolute luminosity’.

Astronomers plot the absolute luminosities of stars against their temperatures – which are estimated from the stars' colours – to generate a ‘Hertzsprung-Russell diagram’, named for the two early 20th century scientists who recognised that such a diagram could be used as a tool to understand stellar evolution. 

Gaia's first Hertzsprung-Russell diagram

Copyright: ESA/Gaia/DPAC/IDT/FL/DPCE/AGIS

“Our first Hertzsprung-Russell diagram, with absolute luminosities based on Gaia’s first year and the Tycho-2 catalogue, and colour information from ground-based observations, gives us a taste of what the mission will deliver in the coming years,” says Lennart Lindegren, professor at the University of Lund and one of the original proposers of the Gaia mission.

As Gaia has been conducting its repeated scans of the sky to measure the motions of stars, it has also been able to detect whether any of them have changed their brightness, and in doing so, has started to discover some very interesting astronomical objects.

Gaia has detected hundreds of transient sources so far, with a supernova being the very first on 30 August 2014. These detections are routinely shared with the community at large as soon as they are spotted in the form of ‘Science Alerts’, enabling rapid follow-up observations to be made using ground-based telescopes in order to determine their nature.

One transient source was seen undergoing a sudden and dramatic outburst that increased its brightness by a factor of five. It turned out that Gaia had discovered a so-called ‘cataclysmic variable’, a system of two stars in which one, a hot white dwarf, is devouring mass from a normal stellar companion, leading to outbursts of light as the material is swallowed. The system also turned out to be an eclipsing binary, in which the relatively larger normal star passes directly in front of the smaller, but brighter white dwarf, periodically obscuring the latter from view as seen from Earth.

Unusually, both stars in this system seem to have plenty of helium and little hydrogen. Gaia’s discovery data and follow-up observations may help astronomers to understand how the two stars lost their hydrogen.

The Cat's Eye Nebula

Copyright: NASA/ESA/HEIC/The Hubble Heritage Team/STScI/AURA (background image); 

ESA/Gaia/DPAC/UB/IEEC (blue points)

Gaia has also discovered a multitude of stars whose brightness undergoes more regular changes over time. Many of these discoveries were made between July and August 2014, as Gaia performed many subsequent observations of a few patches of the sky close to the ecliptic poles. This closely sampled sequence of observations made it possible to find and study variable stars located in these regions.

Located close to the south ecliptic pole is the famous Large Magellanic Cloud (LMC), a dwarf galaxy and close companion of our own galaxy, the Milky Way. Gaia has delivered detailed light curves for dozens of RR Lyrae type variable stars in the LMC, and the fine details revealed in them testify to the very high quality of the data. 

Another curious object covered during the same mission phase is the Cat’s Eye Nebula, a planetary nebula also known as NGC 6543, which lies close to the north ecliptic pole.

Planetary nebulae are formed when the outer layers of an aging low-mass star are ejected and interact with the surrounding interstellar medium, leaving behind a compact white dwarf. Gaia made over 200 observations of the Cat’s Eye Nebula, and registered over 84 000 detections that accurately trace out the intricate gaseous filaments that such objects are famous for. As its observations continue, Gaia will be able to see the expansion of the nebular knots in this and other planetary nebulae.  

Gaia's asteroid detections

Copyright: ESA/Gaia/DPAC/CU4, L. Galluccio, F. Mignard, P. Tanga (Observatoire de la Côte d'Azur)

Closer to home, Gaia has detected a wealth of asteroids, the small rocky bodies that populate our solar system, mainly between the orbits of Mars and Jupiter. Because they are relatively nearby and orbiting the Sun, asteroids appear to move against the stars in astronomical images, appearing in one snapshot of a given field, but not in images of the same field taken at later times. 

Gaia scientists have developed special software to look for these ‘outliers’, matching them with the orbits of known asteroids in order to remove them from the data being used to study stars. But in turn, this information will be used to characterise known asteroids and to discover thousands of new ones. 

Finally, in addition to the astrometric and photometric measurements being made by Gaia, it has been collecting spectra for many stars. The basic use of these data is to determine the motions of the stars along the line-of-sight by measuring slight shifts in the positions of absorption lines in their spectra due to the Doppler shift. But in the spectra of some hot stars, Gaia has also seen absorption lines from gas in foreground interstellar material, which will allow the scientists to measure its distribution. 

 “These early proof-of-concept studies demonstrate the quality of the data collected with Gaia so far and the capabilities of the processing pipeline. The final data products are not quite ready yet, but we are working hard to provide the first of them to the community next year. Watch this space,” concludes Timo.

About Gaia

Gaia is an ESA mission to survey one billion stars in our galaxy and local galactic neighbourhood in order to build the most precise 3D map of the Milky Way and answer questions about its origin and evolution.

The mission’s primary scientific product will be a catalogue with the positions, motions, brightnesses, and colours of the surveyed stars. An intermediate version of the catalogue will be released in 2016. In the meantime, Gaia's observing strategy, with repeated scans of the entire sky, is allowing the discovery and measurement of many transient events across the sky, which are shared with the community at large in the form of Science Alerts.

The nature of the Gaia mission leads to the acquisition of an enormous quantity of complex, extremely precise data, and the data-processing challenge is a huge task in terms of expertise, effort and dedicated computing power. A large pan-European team of expert scientists and software developers, the Data Processing and Analysis Consortium (DPAC), located in and funded by many ESA member states, is responsible for the processing and validation of Gaia's data, with the final objective of producing the Gaia Catalogue. Scientific exploitation of the data will only take place once they are openly released to the community.

For further information, please contact:

Markus Bauer 



ESA Science and Robotic Exploration Communication Officer




Tel: +31 71 565 6799





Mob: +31 61 594 3 954





Email: markus.bauer@esa.int




Timo Prusti



Gaia Project Scientist


Email: timo.prusti@esa.int

Source: ESA/GAIA