The multi-age stellar populations of Terzan 5 as revealed by JWST^★

¹ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Piero Gobetti 93/2, 40129 Bologna, Italy
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna (OAS), Via Piero Gobetti 93/3, 40129 Bologna, Italy
³ Dept. of Astronomy, Indiana University, Bloomington, IN 47401, USA
⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH93HJ, UK
⁵ Department of Physics and Astronomy, UCLA, 430 Portola Plaza, Box 951547, Los Angeles, CA 90095-1547, USA
⁶ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei Munchen, Germany
⁷ Excellence Cluster ORIGINS, Boltzmann-Strasse 2, 85748 Garching Bei Munchen, Germany

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 6 February 2026
Accepted: 30 March 2026

Abstract

The James Webb Space Telescope (JWST) provides an exciting opportunity to investigate stellar systems located in heavily obscured regions such as the Galactic bulge. Possibly the most enigmatic among them is Terzan 5; long classified as a globular cluster, it is now known to host distinct stellar populations with different iron abundances (ranging from approximately [Fe/H]=-0.8 to [Fe/H]=+0.3 dex). Indeed, the chemical and structural properties collected so far suggest that it is the remnant of one of the primordial clumps that contributed to the early assembly of the bulge, a so-called “Bulge Fossil Fragment.” Here we present a new photometric analysis of Terzan 5 based on JWST/NIRCam observations in the F115W and F200W filters, as well as archival HST/ACS optical (F606W and F814W) data. The dataset overcomes the severe and spatially variable extinction along the line of sight and yields the deepest color-magnitude diagram ever obtained for Terzan 5. Proper motion selections and high-resolution differential reddening corrections allowed us to isolate bona fide cluster members and to provide an unprecedented view of the main-sequence turn-off region. We clearly identify two main components and determine their respective ages: the old, sub-solar component has an age of 12.5 ± 0.5 Gyr, while the super-solar component is significantly younger with an age of 4.7 ± 0.5 Gyr. Interestingly, we also find hints of an even younger main sequence turn-off and sub-giant branch, consistent with the presence of a further stellar component with an age of 3.8 ± 0.5 Gyr. There is also evidence of a blue plume populated by stars as bright as m_F115W ~ 17.4, suggesting a prolonged period of star formation extending up to 2.5 Gyr ago.