The dynamical evolution of the stellar clumps in the Sparkler galaxy

¹ Dipartimento di Fisica e Astronomia “Augusto Righi”, Università di Bologna, Via Piero Gobetti 93/2, I-40129 Bologna, Italy
² The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, SE-10691 Stockholm, Sweden
³ Univ Lyon, Univ Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, Saint-Genis-Laval, France
⁴ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁵ INAF – Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, I-34143 Trieste, Italy

^⋆ Corresponding author: eric.giunchi2@unibo.it

Received: 20 March 2025
Accepted: 19 July 2025

Abstract

Context. Recent James Webb Space Telescope observations detected a system of stellar clumps around the z ≃ 1.4 gravitationally lensed Sparkler galaxy (of stellar mass M_* ∼ 10⁹ M_⊙) with ages and metallicities compatible with globular cluster (GC) progenitors. However, most of their masses (> 10⁶ M_⊙) and sizes (> 30 pc) are about ten times those of GCs in the local Universe.

Aims. To assess whether these clumps can evolve into GC-like objects, we performed N-body simulations of their dynamical evolution from z ≃ 1.4 to z = 0 (∼9.23 Gyr) under the effect of dynamical friction and tidal stripping.

Methods. We studied dynamical friction by performing multiple runs of a system of clumps in a Sparkler-like spherical halo of mass M₂₀₀ ≃ 5 × 10¹¹ M_⊙, that was inferred from the stellar-to-halo mass relation. For the tidal stripping, we simulated resolved clumps orbiting in an external static gravitational potential including the same halo as in the dynamical friction simulations and a Sparkler-like stellar disc.

Results. Dynamical friction causes the clumps with a mass greater than 10⁷ M_⊙ to sink into the central galaxy regions, possibly contributing to the bulge growth. In absence of tidal stripping, the mass distribution of the surviving clumps (≈40%) peaks at ≈5 × 10⁶ M_⊙, implying the presence of uncommonly over-massive clumps at z = 0. Tidal shocks from the stellar disc strip considerable mass from low-mass clumps, but their sizes remain larger than those of present-day GCs. When the surviving clump masses are corrected for tidal stripping, their distribution peak shifts to ∼2 × 10⁶ M_⊙, that is compatible with very massive GCs.

Conclusions. Our simulations suggest that a fraction of the Sparkler clumps is expected to fall into the central regions, where they might become bulge fossil fragments or contribute to the formation of a nuclear star cluster. The remaining clumps are too large in size to be progenitors of GCs.