Ahen the first images from the James Webb Space Telescope (JWST) caused astonishment and enthusiasm around the world in July, the question kept coming up: are these just beautiful images for the public? Or will we be able to learn something new about the cosmos from these postcard-worthy shots? The fact that the visual wow effect naturally played an important role for NASA in the selection of motifs was not even denied at the time – after all, the public should see that the financing of the ten billion dollar project was worthwhile. However, the fact that the observations also have some scientific potential can now be followed in the first publications based on JWST data.
In the Astrophysical Journal Letters, for example, an international group of astronomers took a closer look at the 12.5-hour long-term image in the direction of the galaxy cluster SMACS 0723, dubbed “Webb’s First Deep Field”. In the image, which is now likely to adorn numerous walls as a poster, they focused on a background galaxy nine billion light-years away, which they call “The Sparkler Galaxy.” The name is motivated by the fact that the galaxy is surrounded by compact yellow-red spots. The Sparkler can be seen multiple times in the image, which is because the galaxy cluster in the foreground of the image acts as a gravitational lens: its gigantic mass warps space in such a way that light is deflected from more distant objects, as if through an optical lens, and the viewer achieved in several ways – hence the multiple images. In addition, the radiation from distant sources is amplified. Gravitational lenses therefore make it possible to see even further into the universe than would otherwise be possible even with the extremely sensitive James Webb telescope.
Gravitational lensing also encourages astronomers to believe that the yellow-red spots are in fact part of the Sparkler Galaxy, as they can be seen along with the galaxy in the multiple images. The researchers were now able to find out what these spots physically are based on their analysis, whereby they supplemented the JWST data with data from the Hubble Space Telescope. The absence of spectral lines of highly ionized oxygen in the spots, which are classically considered signatures of active star formation, and the general spectral distribution of the radiation received, lead astronomers to conclude that the spots are old globular clusters, groups of hundreds of thousands of stars like ours also know from our Milky Way. The age of the star clusters could also be derived from the spectrum of sources with the help of models. Accordingly, these formed only around 500 million years after the Big Bang and are therefore among the youngest objects in the universe that have already left the stage of star formation behind.
From the observations one will also be able to learn fundamentals about globular star clusters. Because until now it has remained unclear when and how these objects were created in the first place. This is because the approximately 150 members of this class in the Milky Way are already in a state where their age is difficult to determine. Younger clusters are easier to do, and the new data provide valuable information for understanding how they formed. This requires further modeling. In addition, the JWST is still so new that a certain amount of caution is advisable when dealing with the first data. As so often, future observations should help here.