The most distant and oldest star clusters discovered

Thanks to observations from the James Webb Space Telescope (JWST - NASA/ESA/CSA), an international collaboration led by Angela Adamo of Stockholm University and the Oscar Klein Centre in Sweden has discovered a set of five gravitationally bound star clusters in a galaxy whose light was emitted when the Universe was only 460 million years old.

The work, published today in the journal Nature, involved members of the Consejo Superior de Investigaciones Científicas (CSIC), specifically the Instituto de Astrofísica de Andalucía (IAA-CSIC) and the Instituto de Física de Cantabria (IFCA-CSIC-UC), a joint centre also belonging to the University of Cantabria. 

"These structures are the oldest star clusters ever detected and could be precursors of the globular clusters that we currently observe in our galaxy," explains Yolanda Jiménez, postdoctoral researcher at the IAA-CSIC and co-author of the study. For José M. Diego, research scientist at IFCA-CSIC-UC and co-author of the paper, "this discovery demonstrates once again how, thanks to James Webb, we are unveiling the earliest stages of our universe".

THROUGH GRAVITATIONAL LENSING

Direct observation of these structures would not have been possible without the help of gravitational lenses, large accumulations of matter that interpose themselves in our line of sight of distant galaxies, acting as "magnifying glasses" that magnify the objects behind them and sometimes distort their image into an arc. 

In this case, the galaxy cluster SPT-CL J0615-5746 was responsible for magnifying the light from a galaxy called the Cosmic Gems arc from the earliest stages of the formation of the universe. This distant galaxy had previously been discovered by the RELICS collaboration using data from the Hubble Space Telescope, "but it is thanks to the James Webb telescope that we have been able to reveal its fascinating structure," says Yolanda Jiménez.

Thanks to its extraordinary resolution and sensitivity, the James Webb observations revealed the presence of five compact points perfectly distributed along the Cosmic Gems arc, like a string of pearls. These five 'Gems' appeared almost symmetrically duplicated at the other end of the arc, "a clear sign that they were points where the magnifying power of the lensing cluster was at its maximum," says José M. Diego.

COSMIC GEMS Arc magnified by gravitational lensing. It contains the five most distant and oldest star clusters ever observed. Credit: James Webb Space Telecope / Yolanda Jiménez (IAA-CSIC)

GLOBULAR CLUSTER PRECURSORS

A thorough and detailed analysis of these tiny structures revealed that they are star clusters, gravitationally bound systems of stars in which we cannot resolve their individual members. The clusters observed in the Cosmic Gems arc have significantly higher stellar densities (up to three orders of magnitude) and much smaller sizes (less than seven light-years) than typical young star clusters observed in nearby galaxies.

These characteristics suggest that these newly discovered star clusters may be the precursors of the globular clusters that we currently observe in our own Milky Way galaxy. Globular clusters are gravitationally bound groupings of thousands or tens of thousands of old stars, scattered across the Milky Way's halo and some with ages comparable to those of the galaxy itself. "This result is of great importance, as the origin of globular clusters is currently unknown. The discovery of the Gems provides for the first time a time scale for their formation and reveals their initial physical properties," explains Yolanda Jiménez.

Gems are also responsible for most of the ultraviolet emission in the galaxy in which they are found.They are therefore one of the main sources of reionisation in the early universe. The epoch of reionisation is a crucial period in the history of the universe between 150 million and 1 billion years after the Big Bang.During this stage, the first stars and galaxies began to shine, emitting radiation that ionised the existing neutral hydrogen gas.This facilitated the formation of the galaxies and cosmic structures we observe today.  "The search for these sources, as well as for the first stars, is one of the main goals for which the James Webb Space Telescope was built," says José M. Diego.

SPANISH PARTICIPATION

The participation of the CSIC in this discovery has been crucial. The Instituto de Astrofísica de Andalucía (IAA) has developed an algorithm needed to analyse the light coming from the Gems. "Measuring this light is very complex, as we have to separate it from other sources of contaminating light, such as the intracumulus light from the lensing cluster, from nearby galaxies, or the diffuse light emitted by the disc of the galaxy where the Gems are located," says Yolanda Jimenez, postdoctoral researcher at IAA-CSIC, "This sophisticated and powerful in-house algorithm allows us to accurately measure and extract this contaminating light.

"Once the flux of light from the Gems has been measured, it is necessary to have a model of the gravitational lensing effect to understand the magnification of the arc," says José M. Diego, CSIC research scientist at the Institute of Physics of Cantabria (IFCA) where one of these models has been developed. "In addition, one of the questions still to be resolved is why we only see five double images when, in reality, we expect six. One possible explanation is the presence of a dwarf galaxy, not detected by the James Webb, with a lower magnification power that does not allow us to see this image," concludes the IFCA-CSIC-UC researcher.

New observations with the James Webb Space Telescope are scheduled for 2025. They will study this interesting arc and its globular clusters in great detail. These studies promise to reveal some of the best-kept secrets about the formation of the Universe.