James Webb Space Telescope Discovery

JWST identifies fossil star cluster in distant galaxy

Through infrared data, NASA’s James Webb Telescope has identified an extraordinary set of globular star clusters in a distant galaxy not seen before in deep Hubble Space Telescope data. The discovery with JWST and its ability to detect near-infrared and mid-infrared wavelengths, the light beyond the red end of the visible spectrum, provides astronomers the opportunity to further their understanding of the age and distance of star clusters in the universe. The new data from JWST with infrared light reveals details previously not visible in the HST images. Researchers often refer to the term "redshift" to explain that, as the universe expands, light from distant objects is stretched or "shifted" to longer, redder wavelengths as it travels toward us. Globular clusters are among the galaxy's earliest and most pristine objects. They are tightly bound collections of stars that contain hundreds of thousands, and sometimes millions, of stars. These objects are intrinsically bright and with the JWST can be observed from very large distances. Nearly all galaxies contain globular clusters but they are particularly abundant around the oldest, elliptical galaxies, numbering in the thousands around the most massive ellipticals. Elliptical galaxies contain very little gas and dust, making it a relatively straightforward process to distinguish globular clusters against the background of the elliptical galaxy host. Currently, there are two categories of literature of models for forming and evolving globular clusters systems in the context of galaxy formation. In the first category, many globular clusters are thought to have formed shortly after the Big Bang, when the universe was only a billion years old, and before most of the other stars in the universe formed. In these models, globular clusters are thus akin to the oldest pre-Cambrian era fossils that an archeologist may find, except that their oldest stars are still alive and shining today. The second category of models considers globular cluster formation a natural byproduct of active star formation and places high importance on galaxy mergers in producing globular clusters. This hypothesis can explain the increase in globular clusters around ellipticals. If elliptical galaxies are the aftermath of spiral galaxy mergers, globular clusters may form throughout cosmic time in such mergers. A paper identifying the discovery of the distant galaxy containing fossil star clusters and analyzing the globular cluster population in the elliptical galaxy VV191a has been published in Astrophysical Journal Letters and was led by Graduate Associate Jessica Berkheimer of Arizona State University's School of Earth a Space Exploration. By using the galaxy’s redshift value of z=0.0513, Berkheimer and the team was able to determine the galaxy's distance is approximately 743,000,000 light-years from Earth. The data from the VV191a system was previously not seen before in deep HST data. Berkheimer used JWST NIRCam images in four broadband filters that cover the wavelength range 0.9–5 micron (i.e., up to seven times redder than the human eye can see). Berkheimer then finds 154 GC reliable candidates around the elliptical galaxy VV191a. “Many GC (globular cluster) systems around elliptical galaxies show bimodal color distributions. A galaxy with a bimodal GC population has a bluer, older, metal-poor population that formed very early on, and a redder, younger, metal-rich population that formed later,” Berkheimer said. The color of a globular cluster is thus directly related to its (heavy) metal content, which in turn tells researchers an approximate age of the globular cluster. Bimodal color and metal distributions are frequently interpreted as tracing these two distinct modes of cluster formation. “The color distribution of VV191’s GC population appears to be fairly uniform, with only a dozen or so outliers,” Berkheimer said. “With the use of models, we were able to predict that many of the detected GCs contain several million solar masses in faint old stars. With the new Webb data, we can now expand the search for these oldest fossils in galaxies to unprecedented distances.” Credit: Arizona State University Image of galaxy pair VV191a (elliptical galaxy on the bottom) and VV191b (spiral galaxy on the top), which includes near-infrared light from Webb and ultraviolet and visible light from Hubble. The white polygon indicates the region useable for globular cluster searches in VV191a. Image credit: NASA, ESA, CSA, Jessica Berkheimer (ASU), Rogier Windhorst (ASU), William Keel (University of Alabama), Stuart Wyithe (University of Melbourne), JWST PEARLS Team, Alyssa Pagan (STScI)