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A 43,000 light year bridge between galaxies

Last Updated on Friday, 10 February 2017 17:40
Published on Friday, 10 February 2017 17:30

The Magellanic Clouds, the two largest satellite galaxies of the Milky Way, appear to be connected by a bridge stretching across 43,000 light years, according to an international team of astronomers led by Dr Vasily Belokurov from the University of Cambridge. The discovery is reported in the journal Monthly Notices of the Royal Astronomical Society and is based on the Galactic stellar census being conducted by the European Space Agency’s Gaia satellite (see Further Information).

 

thumb Magellanic cloudsSeveral tens of photographs were taken at the Paranal observatory in Chile and stitched together to produce this panorama. The Milky Way is arching across the entire picture. The LMC and the SMC can be seen just above the horizon. Credit: D. Erkal (Cambridge, UK). Click for a larger imageThe Cambridge team concentrated on the area around the Magellanic Clouds, satellite galaxies to our own Milky Way, located in the sky of the southern hemisphere. They used the Gaia data to pick out pulsating stars of a particular type: so-called RR Lyrae variables, which are very old and chemically un-evolved. As these stars have been around since the earliest days of the Clouds’ existence, they offer an insight into the pair’s history. Studying the Large and Small Magellanic Clouds (LMC and SMC respectively) has always been difficult as they sprawl out over a large area. But with Gaia’s all-sky view, this has become a much easier task.

 

 

The clouds are the brightest, and largest, examples of dwarf companion galaxies to the Milky Way. Known to humanity since the dawn of history (and to Europeans since their first voyages to the Southern hemisphere) the Magellanic Clouds have been a constant fixture of the heavens, but only in the last century have astronomers had the chance to study them in any detail.

 

Whether the clouds fit the conventional theory of galaxy formation or not depends critically on their mass, and the time of their first approach to the Milky Way. The researchers found clues that could help answer both of these questions.

 

Firstly, the RR Lyrae stars detected by Gaia were used to trace the extent of the Large Magellanic Cloud (LMC), finding that it had a fuzzy low-luminosity ‘halo’ stretching as far as 20 degrees from its centre. The LMC would only be able to hold on to the stars at such large distances if it were substantially bigger than previously thought, totalling perhaps as much as a tenth of the mass of the entire Milky Way.

 thumb Magellanic clouds starbridgePale white veils and the narrow bridge between the Clouds represent the distribution of the RR Lyrae stars detected with the data from the Gaia satellite. Credit: V. Belokurov, D. Erkal (Cambridge, UK). Photo: Axel Mellinger (CMich, US). Click for a larger image

“Having marked the locations of the Gaia RR Lyrae stars on the sky, we were surprised to see a narrow bridge-like structure connecting the two clouds,” explains Dr Belokurov. “We believe that at least in part this ‘bridge’ is composed of stars stripped from the SMC by the LMC. The rest may actually be composed of stars pulled from the LMC by the Milky Way.”

 

The researchers believe the bridge will help to clarify the history of the interaction between the Magellanic Clouds and our galaxy.

 

“We have compared the shape and the exact position of the Gaia stellar bridge to the computer simulations of the Magellanic Clouds as they approach the Milky Way”, explains Dr Denis Erkal, a co-author of the study.

 

“Many of the stars in the bridge appear to have been removed from the SMC in the most recent interaction, some 200 million years ago, when the dwarf galaxies passed relatively close by each other. We believe that as a result of that flyby, not only stars, but also hydrogen gas was removed from the SMC. By measuring the offset between the RR Lyrae stars and hydrogen bridges, we can put constraints on the density of the gaseous Galactic corona.”

 

Enveloping the Milky Way, and composed of ionised gas at very low density, scientists believe the hot Galactic corona may contain most of the so-called ‘missing’ baryonic - or ordinary - matter. Dark matter is thought to make up roughly 85% of the total matter in the Universe, with baryonic matter comprising the remaining 15%, however a significant fraction of this normal matter still remains unobserved by astronomers.

 

Dr Erkal concludes, “Our estimate showed that the corona could make up a significant fraction of the missing baryons, in agreement with previous independent techniques. With the missing baryon problem seemingly alleviated, the current model of galaxy formation is holding up well to the increased scrutiny possible with Gaia.”

 


Media contact

 

Paul Seagrove

Research Communications Officer

Office of External Affairs and Communications

University of Cambridge

Tel: + 44 (0)1223 765542

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Science contact

 

Dr Vasily Belokurov

Institute of Astronomy

University of Cambridge

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Images and captions

 

Several tens of photographs were taken at the Paranal observatory in Chile and stitched together to produce this panorama. The Milky Way is arching across the entire picture. The LMC and the SMC can be seen just above the horizon. Credit: D. Erkal (Cambridge, UK)

 

Pale white veils and the narrow bridge between the Clouds represent the distribution of the RR Lyrae stars detected with the data from the Gaia satellite. Credit: V. Belokurov, D. Erkal (Cambridge, UK). Photo: Axel Mellinger (CMich, US)

 


Further information

 

The new work appears in, “Clouds, Streams and Bridges. Redrawing the blueprint of the Magellanic System with Gaia DR1”, Vasily Belokurov et al., Monthly Notices of the Royal Astronomical Society, 8th Feb. 2017: http://doi.org/10.1093/mnras/stw3357

 

Launched in 2013, scientists have been eagerly anticipating the results from Gaia. The first tranche of data was released three months ago and is freely accessible to everyone. Of unprecedented quality, this full dataset will be a catalogue of the positions and brightness of a billion stars in our Milky Way galaxy and its environs.

 

What Gaia has sent to Earth is unique. The satellite’s angular resolution is similar to that of the Hubble Space Telescope, but given its greater field of view, it can cover the entire sky rather than a small portion of it. In fact, Gaia uses the largest number of pixels of any space-borne instrument to take digital images of the sky.

 

Unlike typical telescopes, Gaia does not just point and stare: it constantly spins around its axis, sweeping the entire sky in less than a month. Therefore, it not only measures the instantaneous properties of the stars, but also tracks their changes over time. This provides a perfect opportunity for finding a variety of objects - for example stars that pulsate or explode - even if this is not what the satellite was primarily designed for.

 


Notes for editors

 

The Royal Astronomical Society (RAS, www.ras.org.uk), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes 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.

 

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