Intracluster globular clusters as tracers of the mass assembly of the Hydra I galaxy cluster

¹ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
² Universitätssternwarte der Ludwig-Maximilians Universität, Scheinerstraße 1, 81679 München, Germany
³ INAF – Astronomical Observatory of Capodimonte, Salita Moiariello 16, I-80131 Naples, Italy
⁴ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
⁵ INAF – Astronomical Observatory of Abruzzo, Via Maggini, 64100 Teramo, Italy
⁶ Gran Sasso Science Institute, Viale Francesco Crispi 7, I-67100 L’Aquila, Italy

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

Received: 20 July 2025
Accepted: 13 February 2026

Abstract

In galaxy clusters, the hierarchical model of galaxy assembly predicts the formation of stellar substructures and intracluster light (ICL), a diffuse component consisting of stars that are not gravitationally bound to any single galaxy but instead follow the global gravitational potential of the cluster. These features encode the details of the cluster’s assembly history. However, observations are challenging due to their faint surface brightness, so independent tracers such as intracluster planetary nebulae and globular clusters (GCs) can provide valuable insight. For this work, we used deep VLT/FORS V- and I-band imaging to study the GC population in the Hydra I galaxy cluster, a rich environment of galaxies that is located at a distance of 45.7 Mpc. Our photometric sample of GC candidates was constructed from the VI colour-magnitude diagram, where point sources with a similar colour as confirmed GCs were selected. Dividing our GC sample in two colour groups, we show a striking difference between the two populations: while red GCs tend to be clustered around Hydra’s massive galaxies (mainly NGC 3311 and NGC 3309), blue GCs are more extended and spatially coincide with the peak of the cluster’s X-ray emitting gas. The GCs around the central galaxies also have different spatial distributions according to their stellar population properties. Young metal-rich GCs are more extended and may be associated with ram-pressure tails, whereas old metal-poor GCs are more concentrated and could be related to disrupted dwarfs. The red, old, and metal-rich GCs are likely associated with the central massive galaxies. Comparing the GC number density profiles to the surface brightness profile of NGC 3311, we find that the red GCs closely follow the galaxy’s light, while the blue population significantly deviates from it and traces the global gravitational potential of the cluster. This result is also evidenced by the specific frequency of blue GCs, which is ∼5 times larger in the ICL-dominated outskirts when compared to the inner parts of the cluster and to the red population. Finally, we introduce a novel method to constrain the evolution of the galaxy luminosity function of the cluster from GC specific frequencies and colour distributions. This method results in a past Schechter slope of α = −1.81^+0.16_−0.16 for the faint end compared to α = −1.41^+0.08_−0.05 in the present day, which is consistent with measurements at high redshift and with cosmological simulations.