Tag Archives: #sagittarius

GAIA’S New Data Takes Us To The Milky Way’s Anticenter And Beyond (Astronomy)

The motion of stars in the outskirts of our galaxy hints at significant changes in the history of the Milky Way. This and other equally fascinating results come from a set of papers that demonstrate the quality of ESA’s Gaia Early third Data Release (EDR3), which is made public today.

Gaia’s stellar motion for the next 400 thousand years. Credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO. Acknowledgement: A. Brown, S. Jordan, T. Roegiers, X. Luria, E. Masana, T. Prusti and A. Moitinho

Astronomers from the Gaia Data Processing and Analysis Consortium (DPAC) saw the evidence of the Milky Way’s past by looking at stars in the direction of the galaxy’s ‘anticentre’. This is in the exact opposite direction on the sky from the centre of the galaxy.

The results on the anticentre come from one of the four ‘demonstration papers’ released alongside the Gaia data. The others use Gaia data to provide a huge extension to the census of nearby stars, derive the shape of the Solar System’s orbit around the centre of the galaxy, and probe structures in two nearby galaxies to the Milky Way. The papers are designed to highlight the improvements and quality of the newly published data.


Gaia EDR3 contains detailed information on more than 1.8 billion sources, detected by the Gaia spacecraft. This represents an increase of more than 100 million sources over the previous data release (Gaia DR2), which was made public in April 2018. Gaia EDR3 also contains colour information for around 1.5 billion sources, an increase of about 200 million sources over Gaia DR2. As well as including more sources, the general accuracy and precision of the measurements has also improved.

“The new Gaia data promise to be a treasure trove for astronomers,” says Jos de Bruijne, ESA’s Gaia Deputy Project Scientist.

The colour of the sky from Gaia’s Early Data Release 3. Credit: ESA/Gaia/DPAC; CC BY-SA 3.0 IGO. Acknowledgement: A. Moitinho


The new Gaia data have allowed astronomers to trace the various populations of older and younger stars out towards the very edge of our galaxy – the galactic anticentre. Computer models predicted that the disc of the Milky Way will grow larger with time as new stars are born. The new data allow us to see the relics of the 10 billion-year-old ancient disc and so determine its smaller extent compared to the Milky Way’s current disc size.

The new data from these outer regions also strengthen the evidence for another major event in the more recent past of the galaxy.

The data show that in the outer regions of the disc there is a component of slow-moving stars above the plane of our galaxy that are heading downwards towards the plane, and a component of fast-moving stars below the plane that are moving upwards. This extraordinary pattern had not been anticipated before. It could be the result of the near-collision between the Milky Way and the Sagittarius dwarf galaxy that took place in our galaxy’s more recent past.

The Sagittarius dwarf galaxy contains a few tens of millions of stars and is currently in the process of being cannibalised by the Milky Way. Its last close pass to our galaxy was not a direct hit, but this would have been enough so that its gravity perturbed some stars in our galaxy like a stone dropping into water.

Using Gaia DR2, members of DPAC had already found a subtle ripple in the movement of millions of stars that suggested the effects of the encounter with Sagittarius sometime between 300 and 900 million years ago. Now, using Gaia EDR3, they have uncovered more evidence that points to its strong effects on our galaxy’s disc of stars.

The patterns of movement in the disc stars are different to what we used to believe,” says Teresa Antoja, University of Barcelona, Spain, who worked on this analysis with DPAC colleagues. Although the role of the Sagittarius dwarf galaxy is still debated in some quarters, Teresa says, “It could be a good candidate for all these disturbances, as some simulations from other authors show.


The history of the galaxy is not the only result from the Gaia EDR3 demonstration papers. DPAC members across Europe have performed other work to demonstrate the extreme fidelity of the data and the unique potential for unlimited scientific discovery.

In one paper, Gaia has allowed scientists to measure the acceleration of the Solar System with respect to the rest frame of the Universe. Using the observed motions of extremely distant galaxies, the velocity of the Solar System has been measured to change by 0.23 nm/s every second. Because of this tiny acceleration the trajectory of the Solar System is deflected by the diameter of an atom every second, and in a year this adds up to around 115 km. The acceleration measured by Gaia shows a good agreement with the theoretical expectations and provides the first measurement of the curvature of the Solar System’s orbit around the galaxy in the history of optical astronomy.


Gaia EDR3 has also allowed a new census of stars in the solar neighbourhood to be obtained. The Gaia Catalogue of Nearby Stars contains 331 312 objects, which is estimated to be 92 percent of the stars within 100 parsecs (326 light years) of the Sun. The previous census of the solar neighbourhood, called the Gliese Catalogue of Nearby stars, was carried out in 1957. It possessed just 915 objects initially, but was updated in 1991 to 3803 celestial objects. It was also limited to a distance of 82 light years: Gaia’s census reaches four times farther and contains 100 times more stars. It also provides location, motion, and brightness measurements that are orders of magnitude more precise than the old data.


A fourth demonstration paper analysed the Magellanic Clouds: two galaxies that orbit the Milky Way. Having measured the movement of the Large Magellanic Cloud’s stars to greater precision than before, Gaia EDR3 clearly shows that the galaxy has a spiral structure. The data also resolve a stream of stars that is being pulled out of the Small Magellanic Cloud, and hints at previously unseen structures in the outskirts of both galaxies.

Bridge of stars between the Magellanic clouds. Credit: ESA/Gaia/DPAC; CC BY-SA 3.0 IGO. Acknowledgements: S. Jordan, T. Sagristà, X. Luri et al (2020)

At 12:00 CET on 3 December, the data produced by the many scientists and engineers of the Gaia DPAC Consortium become public for anyone to look at and learn from. This is the first of a two-part release; the full Data Release 3 is planned for 2022.

Gaia EDR3 is the result of a huge effort from everyone involved in the Gaia mission. It’s an extraordinarily rich data set, and I look forward to the many discoveries that astronomers from around the world will make with this resource,” says Timo Prusti, ESA’s Gaia Project Scientist. “And we’re not done yet; more great data will follow as Gaia continues to make measurements from orbit.

Reference: Gaia Collaboration, A. G. A. Brown, A. Vallenari, T. Prusti, J.H.J. de Bruijne, et al., “Gaia Early Data Release 3. Summary of the contents and survey properties”, A&A, 2020. https://doi.org/10.1051/0004-6361/202039657 https://www.aanda.org/articles/aa/pdf/forth/aa39657-20.pdf

Provided by ESA

Earth Faster, Closer to Black Hole in New Map of Galaxy (Astronomy)

Earth just got 7 km/s faster and about 2000 light-years closer to the supermassive black hole in the center of the Milky Way Galaxy. But don’t worry, this doesn’t mean that our planet is plunging towards the black hole. Instead the changes are results of a better model of the Milky Way Galaxy based on new observation data, including a catalog of objects observed over the course of more than 15 years by the Japanese radio astronomy project VERA.

Position and velocity map of the Milky Way Galaxy. Arrows show position and velocity data for the 224 objects used to model the Milky Way Galaxy. The solid black lines show the positions of the Galaxy’s spiral arms. The colors indicate groups of objects belonging the same arm. The background is a simulation image. (Credit: NAOJ)

VERA (VLBI Exploration of Radio Astrometry, by the way “VLBI” stands for Very Long Baseline Interferometry) started in 2000 to map three-dimensional velocity and spatial structures in the Milky Way. VERA uses a technique known as interferometry to combine data from radio telescopes scattered across the Japanese archipelago in order to achieve the same resolution as a 2300 km diameter telescope would have. Measurement accuracy achieved with this resolution, 10 micro-arcseconds, is sharp enough in theory to resolve a United States penny placed on the surface of the Moon.

Because Earth is located inside the Milky Way Galaxy, we can’t step back and see what the Galaxy looks like from the outside. Astrometry, accurate measurement of the positions and motions of objects, is a vital tool to understand the overall structure of the Galaxy and our place in it. This year, the First VERA Astrometry Catalog was published containing data for 99 objects.

Based on the VERA Astrometry Catalog and recent observations by other groups, astronomers constructed a position and velocity map. From this map they calculated the center of the Galaxy, the point that everything revolves around. The map suggests that the center of the Galaxy, and the supermassive black hole which resides there, is located 25800 light-years from Earth. This is closer than the official value of 27700 light-years adopted by the International Astronomical Union in 1985. The velocity component of the map indicates that Earth is travelling at 227 km/s as it orbits around the Galactic Center. This is faster than the official value of 220 km/s.

Now VERA hopes to observe more objects, particularly ones close to the central supermassive black hole, to better characterizes the structure and motion of the Galaxy. As part of these efforts VERA will participate in EAVN (East Asian VLBI Network) comprised of radio telescope located in Japan, South Korea, and China. By increasing the number of telescopes and the maximum separation between telescopes, EAVN can achieve even higher accuracy.

“The First VERA Astrometry Catalog” by VERA collaboration et al. appeared in Publications of the Astronomical Society of Japan in August 2020.

References: VERA collaboration, Tomoya Hirota, Takumi Nagayama, Mareki Honma, Yuuki Adachi, Ross A Burns, James O Chibueze, Yoon Kyung Choi, Kazuya Hachisuka, Kazuhiro Hada, Yoshiaki Hagiwara, Shota Hamada, Toshihiro Handa, Mao Hashimoto, Ken Hirano, Yushi Hirata, Takanori Ichikawa, Hiroshi Imai, Daichi Inenaga, Toshio Ishikawa, Takaaki Jike, Osamu Kameya, Daichi Kaseda, Jeong Sook Kim, Jungha Kim, Mi Kyoung Kim, Hideyuki Kobayashi, Yusuke Kono, Tomoharu Kurayama, Masako Matsuno, Atsushi Morita, Kazuhito Motogi, Takeru Murase, Akiharu Nakagawa, Hiroyuki Nakanishi, Kotaro Niinuma, Junya Nishi, Chung Sik Oh, Toshihiro Omodaka, Miyako Oyadomari, Tomoaki Oyama, Daisuke Sakai, Nobuyuki Sakai, Satoko Sawada-Satoh, Katsunori M Shibata, Makoto Shizugami, Jumpei Sudo, Koichiro Sugiyama, Kazuyoshi Sunada, Syunsaku Suzuki, Ken Takahashi, Yoshiaki Tamura, Fumie Tazaki, Yuji Ueno, Yuri Uno, Riku Urago, Koji Wada, Yuan Wei Wu, Kazuyoshi Yamashita, Yuto Yamashita, Aya Yamauchi, Akito Yuda, The First VERA Astrometry Catalog, Publications of the Astronomical Society of Japan, Volume 72, Issue 4, August 2020, 50, https://doi.org/10.1093/pasj/psaa018 https://academic.oup.com/pasj/article-abstract/72/4/50/5824859?redirectedFrom=fulltext

Provided by NAOJ / VERA

Astronomers Precisely Measure Distance to Magnetar (Astronomy)

XTE J1810−197 (J1810), a magnetar located in the constellation of Sagittarius, was the first magnetar identified to emit radio pulses, and has been extensively studied during a radio-bright phase in 2003–2008. It is estimated to be relatively nearby compared to other Galactic magnetars, and provides a useful prototype for the physics of high magnetic fields, magnetar velocities, and the plausible connection to extragalactic fast radio bursts. Upon the re-brightening of the magnetar at radio wavelengths in late 2018, researchers of current study resumed an astrometric campaign on J1810 with the Very Long Baseline Array, and sampled 14 new positions of J1810 over 1.3 years and has made the direct geometric measurement of the distance to XTE J1810-197, a magnetar located in the constellation of Sagittarius.

An artist’s impression of a magnetar emitting a burst of radiation. Image credit: Sophia Dagnello, NRAO / AUI / NSF.

Magnetars are a variety of neutron stars — the superdense remains of massive stars that exploded as supernovae — with extremely strong magnetic fields.

A typical magnetar magnetic field is a trillion times stronger than the Earth’s magnetic field, making magnetars the most magnetic objects in the Universe.

They can emit strong bursts of X-rays and gamma rays, and recently have become a leading candidate for the sources of fast radio bursts (FRBs).

Researchers performed the phase calibration for the new observations with two phase calibrators that are quasi-colinear on the sky with J1810, enabling substantial improvement of the resultant astrometric precision. They then, combine their new observations with two archival observations from 2006 and have refined the proper motion and reference position of the magnetar and have measured its annual geometric parallax, the first such measurement for a magnetar.

This effect, called parallax, allows astronomers to use geometry to directly calculate the object’s distance. And what they found?

They found that the parallax of 0.40 ± 0.05 mas corresponds to a most probable distance 2.5 (↑+0.4 ↓−0.3) kpc for J1810. Their new astrometric results confirm an unremarkable transverse peculiar velocity of ≈200 km s−¹ km for J1810, which is only at the average level among the pulsar population. The magnetar proper motion vector points back to the central region of a supernova remnant (SNR) at a compatible distance at ≈70 kyr ago, but a direct association is disfavored by the estimated SNR age of ∼3 kyr.

References: H Ding, A T Deller, M E Lower, C Flynn, S Chatterjee, W Brisken, N Hurley-Walker, F Camilo, J Sarkissian, V Gupta, A magnetar parallax, Monthly Notices of the Royal Astronomical Society, , staa2531, https://doi.org/10.1093/mnras/staa2531