The star formation history of the first bulge fossil fragment candidate Terzan 5

¹ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
² Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2 40129 Bologna, Italy
³ INAF-Osservatorio di Astrofisica e Scienze dello Spazio di Bologna, Via Gobetti 93/3 40129 Bologna, Italy
⁴ Dipartimento di Fisica, Università di Pisa, Largo Pontecorvo 3, 56127 Pisa, Italy
⁵ INFN, Largo Pontecorvo 3, 56127 Pisa, Italy
⁶ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁷ Excellence Cluster ORIGINS, Boltzmann-Strasse 2, 85748 Garching bei München, Germany

^★ Corresponding author; ccrociat@ed.ac.uk

Received: 19 June 2024
Accepted: 18 October 2024

Abstract

Context. Terzan 5 and Liller 1 are the only bulge stellar clusters hosting multi-iron and multi-age stellar populations. They are therefore claimed to constitute a novel class of astrophysical objects: the fossils of massive star-forming clumps that possibly sank to the center of the Milky Way and contributed to the formation of the bulge. This is based on the hypothesis that the ancient clumps were able to retain iron-enriched supernova ejecta, later giving rise to younger and more metal-rich populations.

Aims. A way to investigate this scenario is reconstructing their star formation histories (SFHs) and proving a prolonged and multiepisode star formation activity.

Methods. Leveraging ground- and space-based high-resolution images, we derived the SFH of Terzan 5 by employing the colormagnitude diagram fitting routine SFERA.

Results. The best-fit solution predicts an old, main peak occurred between 12 and 13 Gyr ago that generated 70% of the current stellar mass, followed by a lower-rate star formation activity with two main additional bursts.

Conclusions. These results indicate that Terzan 5, similarly to Liller 1, experienced a prolonged, multiepisode star formation activity, fueled by metal-enriched gas deposited in its central regions, in agreement with the expectations of a self-enrichment scenario in a primordial massive clump.