Universal depletion of metal-poor globular clusters in inner galaxy regions: Fossil record of black hole retention

¹ School of Astronomy, Institute for Research in Fundamental Sciences (IPM), PO Box 19395-5531, Tehran, Iran
² Department of Physics, Faculty of Physics, Alzahra University, Vanak, 1993891176 Tehran, Iran
³ Department of Physics, Institute for Advanced Studies in Basic Sciences (IASBS), 444 Prof. Sobouti Blvd., Zanjan 45137-66731, Iran
⁴ Helmholtz-Institut für Strahlen-und Kernphysik (HISKP), Universität Bonn, Nussallee 14–16, 53115 Bonn, Germany

^★ Corresponding author: ali.rostami.shirazi@ipm.ir

Received: 31 August 2025
Accepted: 27 October 2025

Abstract

We analyzed the spatial distribution of globular cluster (GC) systems across 37 host galaxies in a two-dimensional parameter space defined by projected galactocentric distances (R_g) and metallicity ([Fe/H]). We identified a universal triangular depleted region, characterized by a lack of metal-poor GCs in the inner parts of host galaxies. The morphology of this depleted region correlates with the luminous mass of host galaxies; more massive galaxies consistently exhibit more extended depleted regions. We attribute this phenomenon to the combined influence of large-scale galactic assembly and internal GC dynamics, particularly the initial retention of black holes (BHs) within GCs. Metal-poor GCs harbor a more massive and compact BH subsystem, which fosters more energetic few-body encounters, injecting greater kinetic energy into the stellar population. This extra energy, combined with the strong tidal forces in the galactic central regions, accelerates the dissolution of lower-metallicity GCs on timescales shorter than the host galaxy’s age, leading to the emergence of a triangular depleted pattern in the R_g–[Fe/H] parameter space. Stronger tidal fields in more massive galaxies confine surviving metal-poor GCs to larger radii, broadening the depleted region. The depleted region’s morphology may serve as a potential distance indicator for host galaxies. Our results also suggest that scenarios involving substantial BH natal kicks are unlikely, as too few retained BHs would erase the metallicity-dependent cluster dissolution required to form the observed depletion region.