More than 40 different stars observed at an incredible distance using the James Webb Space Telescope
One of the predictions of Albert Einstein's theory of general relativity is that massive objects bend the fabric of space and time around them. As a result, the light from distant sources lying behind massive objects gets deflected, and the sources appear magnified and distorted. This effect is called "Gravitational lensing", because those massive bodies act as lenses. Astronomers, and Prof. Adi Zitrin's group in BGU's Physics Department among them, have been routinely using this phenomenon to study distant objects in more detail, and observe more distant sources than would otherwise be possible. Massive clusters of galaxies, for example, are known to cause significant lensing effects due to their very high mass.
A few years ago, a new and exciting application emerged: By monitoring the magnified images of certain background galaxies, it was possible to sometimes capture individual stars in those galaxies that cross lines of extremely high magnification, dubbed caustics.
"A caustic is a region in or from which many light rays are focused, thus producing high magnification," says Prof. Zitrin. "We are also used to seeing caustics in everyday life, such as at the bottom of the sea or a swimming pool on a sunny day, where rays of sun concentrate. This web of dancing caustics similarly exists when dealing with gravitational lensing."
When a background star happens to temporarily lie on a caustic, it gets extremely magnified, by factors of many thousands, so that it can be observed in distant galaxies.
"Normally we can only see individual stars in our own Milky Way galaxy or its nearest neighborhood," says Zitrin. "When looking at distant galaxies, we only see the aggregated light from the many billions of stars in that galaxy. Each single star in that galaxy is too distant and dim to be seen individually. But if it happens to be crossing a caustic due to a foreground lens, we would - for a brief moment in time, typically a few days - see it very brightly", he adds.
Before the launch of the James Webb Space Telescope (JWST), several individual stars had been detected in distant lensed galaxies with the Hubble Space Telescope. Since the launch of JWST, the numbers have been increasing and a couple of dozen stars have already been detected in its nearly three years of operation. A few months ago, in an international collaboration led by Dr. Fengwe Sun from the University of Arizona, in which Prof. Zitrin's group took part, a new leap occurred: More than 40 stars were seen in a single observation in just one single background galaxy, lying several billion light years away. Prof. Zitrin's group helped in the interpretation and analysis.
"This was an amazing surprise, which now opens the door to significantly studying stars not only in our galaxy but also in distant galaxies across cosmic history ", says Prof. Zitrin. "The paper has now been published in Nature Astronomy. It was led by Dr. Yoshi Fudamoto from the University of Chiba in Japan."
Caustic crossing events of lensed stars may also be beneficial for studying another astronomical enigma. It has long been believed that most of the matter in the universe is invisible, which has been dubbed dark matter. Because the frequency of caustic crossing events depends on the underlying populations of lenses, such as stars that float around in the galaxy cluster that is responsible for the lensing, their rate can be used to constrain the composition of the dark matter component. For example, if dark matter consisted of small bodies with masses akin to stars, that would significantly enhance the number of expected events. Prof. Zitrin's group, using funds from the Ministry of Science and Technology and the Israel Science Foundation, has been developing tools to simulate and study those events. The group also has some time scheduled soon on the JWST for more observations in which they hope to detect such stars at larger distances.
