Physicists use telescopic magnification trick to discover over 40 new stars

Physicists studying a distant galaxy using a telescopic technique called gravitational lensing, or telescopic magnification, have discovered over 40 previously unknown stars. The discovery, published in Nature Astronomy, shows how these stars were behaving eight billion years ago, giving a glimpse into the population of stars at ‘cosmic noon’ – the Middle Ages of the Universe.

The research was led by the Centre for Frontier Science at Chiba University, in Japan, and involved over 45 international partners. In the UK this was led by Durham University’s Centre for Extragalactic Astronomy and involved the Jodrell Bank Centre for Astrophysics, Manchester University.

New telescopic magnification trick to discover over 40 new stars

The international team used observations from the James Webb Space Telescope (JWST) and gravitational lensing to study a galaxy known as the Dragon Arc, located behind a massive cluster of galaxies called Abell 370. In gravitational lensing a foreground galaxy cluster bends the light from a more distant object and magnifies it, allowing scientists to study the distant object (here the Dragon Arc galaxy). Due to its gravitational lensing effect, Abell 370 stretches the Dragon Arc’s signature spiral into an elongated shape – like a hall of mirrors of cosmic proportions.

Using this technique, and high-resolution images from the JWST, taken across a full year, the team was able to identify 44 previously unknown stars in the Dragon Arc. They observed that the brightness of these individual stars changed over the course of the study due to variations in the gravitational lensing landscape. The findings show what this galaxy is made of in a way not previously achieved. They also tell us more about dark matter – a mysterious substance that binds together galaxies, creating the environment for stars, planets and life to exist.

Dr David Lagattuta from the Centre for Extragalactic Astronomy at Durham University said: “When the team made this discovery, we knew that, given the size of the dots seen in the JWST images, the most logical explanation was that these were individual stars, seen for the first time, which is a hugely exciting discovery.

“We know these are stars that have not been seen before by comparing them to previous image of the Dragon Arc which do not show these bright dots. “Other possibilities such as these findings being a cluster of stars or exploding supernovae simply did not fit the data.

“It would be a huge coincidence to find so many supernovae all in the same galaxy and all exploding at the same time. Supernovae also tend to suppress star formation, but spectroscopy tells us the Dragon Arc is still actively forming stars. “We also reasoned that these objects had to be individual stars, rather than star clusters, since the size of what we’re seeing (after accounting for the extreme lensing magnification) is much too small to fit in the tens of hundreds of bright stars in a star cluster at once.

Many of the stars identified through this study are ‘red supergiants’, a type of star that has typically been very difficult to identify outside of the Milky Way. This is because they are covered in a layer of cosmic ‘dust’ making them almost invisible to telescopes. The JWST enabled the research team to peer through this dust more easily, revealing the hidden stars inside.

Professor Mathilde Jauzac from the Centre for Extragalactic Astronomy and the Institute for Computational Cosmology at Durham University said: “This is the first time, that we are aware of, that so many stars have been discovered in one cluster. This finding enables us to see what the galaxy is made of in ways not possible before.

“This provides a fascinating and unique view into the behaviour of stars at the critical ‘cosmic noon’, the Middle Ages of the Universe. “We know that in the early stages of the Universe there is lots of gas and early ‘protostars’ and then by nine to 10 billion years ago star formation peaks and everything is turning on.

“We also know that stars in the early Universe were born and behaved in a different way to today. It is roughly in the cosmic noon, that these behaviours start to change. So, this new study provides an insight into stars at this point in time.” The ability to see these hidden stars helps physicists develop a better understanding of how common different types of stars were in the cosmic Middle Ages.

The research team hopes to continue its study by taking further images of the Dragon Arc over the next few years, to help differentiate between different stellar populations being magnified by the cluster. This would give physicists a wealth of extra information about how stars formed and lived eight billion years ago.

Acknowledgements:
Mathilde Jauzac and David J Lagattuta are supported by the United Kingdom Research and Innovation (UKRI) Future Leaders Fellowship `Using Cosmic Beasts to uncover the Nature of Dark Matter’ (grant number MR/S017216/1).

David J Lagattuta is also supported by STFC grants ST/T000244/1 and ST/W002612/1.

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