Astronomers used the James Webb Space Telescope to find 104 magnified stellar events in the Dragon galaxy. Published in Astronomy & Astrophysics on Sept. 30, the study used images from 2022 to 2024. These events show how gravity bends light, helping experts learn more about stars and dark matter today.
Using the James Webb Space Telescope (JWST), a team of astronomers identified 104 magnified stellar events in a galaxy known as the Dragon, located around 6.5 billion light-years away. This discovery more than doubles the previous record of 44 such detections in the same galaxy. Because these kinds of events reveal some of the most luminous stars and likely the presence of dark matter, this study offers a rare look at the galaxy's stellar population along with a possible hint at the nature of dark matter.
The paper, describing the record-breaking discovery, was published in Astronomy & Astrophysics on Sept. 30.
Sparkling Dragon
The light from the Dragon galaxy, at redshift 0.725, is stretched and magnified by a massive galaxy cluster called Abell 370. A portion of the galaxy is stretched into an arc, famously called the Dragon arc. This phenomenon is called gravitational lensing. But the lensing effect is not the same everywhere. Along certain lines called caustics, the light from the Dragon can get focused so strongly that the light's brightness can increase dramatically. This is because a galaxy cluster is not a uniform blob and its gravity bends light by different amounts in different places.
The cluster's own stars, and possibly dark matter, create small distortions too. They can break a single caustic line into "microcaustics." As the background galaxy moves slowly relative to the lensing cluster, individual stars in it drift across these microcaustics, creating a caustic crossing event.
At the crossing, two of the star's images merge and the star reaches maximum brightness, up to hundreds or thousands of times its normal level. Once it is done crossing, those two images vanish, and the star fades back within days to weeks. Because the events are temporary, astronomers find them by comparing images of the same distorted region or arc taken at different times.
Record-breaking
In this new study, led by José María Palencia of the Institute of Physics of Cantabria, researchers compared JWST images collected from 2022 to 2024 to look for changes in brightness.
Since the images were taken at slightly different angles, the team made corrections to prevent any false residues or leftovers when subtracting images to find the brightness changes. They also removed bright unrelated objects, the galaxy's bright core and other contaminants that could be mistaken for flares. Then, they measured the normal noise level using another image of the same galaxy that lies far from the critical curve, the line where magnification is strongest, and therefore has no microlensing.
After merging repeated detections across filters and observations, the team detected 104 unique stellar events. "... we obtain a catalog of 104 unique CCEs in the Dragon arc, which currently represents the largest sample of highly magnified star candidates identified in a single galaxy," the team writes in the paper.
Blues and reds
Only the most luminous stars can be detected through these events, which means the stars astronomers catch are almost always supergiants. They come in two main types: blue supergiants (BSGs) and red supergiants (RSGs). Blue ones are hot and shine mostly at short wavelengths, while red supergiants are cool and emit mostly in the near-infrared. The ratio between the two types reveals the nature of the stellar population in the galaxy. Younger and more metal-poor populations produce more luminous blue supergiants, while higher metallicity and older populations evolve toward the red category.
A key question for the Dragon arc is what the blue-to-red ratio looks like and whether it points to a young, star-forming system or an older one.
The team carried out a color analysis, which showed roughly equal numbers of blue and red supergiants, giving a ratio of 1. "This suggests that both BSGs and RSGs contribute to the observed CCEs, in agreement with expectations for a relatively evolved, still star-forming system," the team says.
A fuzzy type
Besides stars, dark matter in the cluster may also create microcaustics, so the pattern of events could help reveal its nature.
To verify this, the team counted how many events appear on each side of the critical curve. They found that far from the curve, both sides appeared the same. But close to it, the count dropped on one side but stayed the same on the other side. This pattern fit the 'fuzzy' dark matter model. It is a proposed form of dark matter made of very light and wavelike particles. "This could provide a hint of a fuzzy nature of DM, although it may also arise from an incorrect location of the CC in the lens model," they say.
Applying this methodology to other galaxies that are also strongly lensed, along with repeated, frequent observations and better lens models, can turn out to be a tool for studying both ordinary stars and dark matter in and around massive galaxy clusters.