Bulge fossil fragments as a new population of factories of gravitational wave sources in the Galaxy

¹ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
² INAF, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
³ Dept. of Astronomy, Indiana University, Bloomington, IN 47401, USA

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

Received: 26 August 2025
Accepted: 24 March 2026

Abstract

The discovery of the complex stellar populations hosted in two massive stellar systems in the Galactic bulge, namely Terzan 5 and Liller 1, posed intriguing questions about their origin and their possible connection with the formation and early evolution of the bulge itself. Indeed, despite their globular cluster appearance, they host subpopulations with significantly different ages (by several gigayears) and metallicities (by about 1 dex) tracing a chemical abundance pattern that is consistent only with that observed in the bulge. These surprising properties can be naturally explained in the context of a self-enrichment scenario, opening the fascinating possibility that they could be the remnants of primordial massive structures that contributed to the bulge formation (so-called bulge fossil fragments, BFFs) capable of retaining supernova ejecta within their potential well. In this paper we present a first attempt to quantify the expected contribution of BFF s to the gravitational wave emission. In particular, by adopting Terzan 5 as a prototype of BFF, using its chemical evolutionary model, and following a scaling relation derived for globular clusters, we present a first-guess estimate of the number of binary black hole mergers expected in this stellar system. Within the adopted simplifying assumptions and the uncertainties about the initial conditions of the proto-Terzan 5 system, we find that several hundreds of binary black hole mergers are expected, a number that is between ~15 and ~250 times larger than that produced by a typical globular cluster. Hence, this study identifies in the BFF family a new population of stellar systems potentially able to produce a significant number of gravitational wave emitters, which has not been considered in any previous investigation of gravitational wave sources. Moreover, considering the deep potential well and the high collisional rate of these systems, we speculate that they could also be the natural place where black holes with masses above 60 M_⊙ and even intermediate-mass black holes can form via repeated dynamical interactions.