JWST observations of the stellar system Terzan 5 in the Galactic Bulge

Among the stellar systems in the Milky Way, Terzan 5 stands out as one of the most intriguing. Hidden in a heavily obscured bulge region, it preserves traces of the galaxy’s earliest formation. Despite its globular cluster–like morphology, extensive studies have revealed stellar populations with very different ages and chemical compositions (Ferraro et al. 2009; Ferraro et al. 2016). These findings show that Terzan 5 is in fact a "Bulge Fossil Fragment" (Ferraro et al. 2021), the relic of a primordial building block of the bulge. This interpretation links Terzan 5 to high-redshift stellar clumps observed in star-forming galaxies (Elmegreen et al. 2009), suggesting it may be the fossil remnant of an early massive structure formed through disk instabilities (Immeli et al. 2004). In this thesis I exploit the unprecedented resolution and NIR capabilities of JWST/NIRCam, combined with archival HST optical data, to further investigate Terzan 5’s sub-populations. This combination overcomes the severe extinction along the line of sight and provides the deepest, most accurate color–magnitude diagram to date. Proper motion selections and differential reddening corrections allowed us to isolate members and unveil the intrinsic features of the evolutionary sequences. Thanks to significantly increased stellar samples with respect to previous works, this study reveals with unparalleled accuracy the complex structure of the main-sequence turnoff, the most reliable stellar clock. Isochrone comparison identifies an old, metal-poor component (12.7 ± 0.5 Gyr) and a younger, metal-rich one (4.6 ± 0.5 Gyr). Moreover, we provide the first photometric evidence for an even younger population, extending recent star formation down to ~3.8 Gyr ago. Altogether, these results confirm the composite nature of Terzan 5 and strongly support its identification as a bulge fossil fragment, reinforcing the scenario of bulge assembly through the merger of massive clumps in the early universe.