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

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 distance Rg 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 the 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 within GCs. Metal poor GCs contain a more massive and compact black hole subsystem, which drives more energetic few body encounters and injects greater kinetic energy into the stellar population. This extra energy, combined with strong tidal forces in central galactic regions, accelerates the dissolution of low metallicity GCs, producing the triangular depleted pattern in the Rg - Fe/H space. Stronger tidal fields in more massive galaxies confine surviving metal poor GCs to larger radii, broadening the depleted region. The morphology of this region may serve as a potential distance indicator for host galaxies. Our results also suggest that scenarios with substantial black hole natal kicks are unlikely, as too few retained black holes would erase the metallicity dependent cluster dissolution required to form the observed depletion region.

Figures (7)

• Figure 1: Minimum prestellar evolution mass ($M_0$) required for GCs to survive over Hubble time ($\tau\ge13.8\,\mathrm{Gyr}$), shown as a function of $R_{\rm g}^{\rm 3D}$. Results are presented for $Z=0.0006$ (blue) and $0.017$ (red) in host galaxies with circular velocities of $V_{\rm c} = 220$ (solid lines) and $440\,\mathrm{km\,s}^{-1}$ (dashed lines). Each curve represents the survival mass threshold; GCs with $M_0$ above the corresponding curve (shaded region) remain bound over Hubble time.
• Figure 2: Ratio of metal-poor ($Z=0.0002$) to metal-rich ($Z=0.02$) cluster mass-loss rates, $\dot{M}_{\rm MP}/\dot{M}_{\rm MR}$, as a function of the fractional mass loss, $f_{\rm Mloss}$. The modeled clusters follow the initial conditions of Rostami2024-metallicity, except for a half-mass radius of $r_{\mathrm{h},0}=1.2 \,\mathrm{pc}$, and are located at $R_{\rm g}^{\rm 3D}=2$ and $8\,\mathrm{kpc}$, covering BH retention from complete ejection ($\eta=0$) to full retention ($\eta=1$).
• Figure 3: CDFs and scaled PDFs for eight galaxies in the $R_\mathrm{g}$-[Fe/H] parameter space. Dashed isodensity contours represent boundaries enclosing 5-95$\%$ of all data in $10\%$ increments. White contours (dashed for $95\%$ and solid for $99\%$, respectively) mark outer boundaries. Gray dots denote the distribution of GCs in each host galaxy. The hatched area highlights the depleted region of metal-poor GCs ($\mathrm{A}_\mathrm{DEP}$), while the shaded area in the MW panel indicates the outer non-depleted region ($\mathrm{A}_\mathrm{OUT}$; see Section \ref{['sec:Statistic']}). Red solid lines show the least-squares fitted metallicity gradient (Harris2023; see Section \ref{['sec:Assembly']}). The CDFs are derived from the clustering results of the DBSCAN algorithm, with low-density regions excluded and classified as noise.
• Figure 4: CDFs for nine galaxies in the $R_\mathrm{g}$-[Fe/H] parameter space, plotted from clustering results obtained via the DBSCAN algorithm. Contours are plotted similarly to those in Figure \ref{['fig:8galaxies']}. The white contours mark boundaries that encompass 95 and 99$\%$ of the data, highlighting regions with very low probabilities of finding GCs, corresponding to approximate CDF values of 0.01 and 0.002. The hatched area highlights the depleted region of metal-poor GCs.
• Figure 5: Mean $R_\mathrm{DEP}$ versus $V$-band absolute magnitude of host galaxies ($M_V$). Error bars indicate the standard deviation of $R_\mathrm{DEP}$. The fitted dashed line reveals the trend that more luminous galaxies tend to have larger $R_\mathrm{DEP}$ values, with a Pearson correlation coefficient of $0.9$.