The Spatial Distribution of Intra-Cluster Globular Clusters in the Fornax Cluster

TL;DR

The paper investigates the spatial distribution of intra-cluster globular clusters in the Fornax cluster core by decomposing the observed GC population into a homogeneous contaminant component, galaxy GC systems, and an extended ICGC component revealed through residual maps. The authors construct detailed simulations for the first two components using the Fornax Deep Survey data and known GCS models, then extract residuals to map the ICGC over-density, notably the elongated structure M and its sub-features. They find a predominantly blue ICGC population, with line-of-sight velocities of many GCs within these residuals consistent with the Fornax cluster potential and the NGC 1399 GC system, supporting a mix of stripping and tidal disruption as ICGC formation channels. The results imply distinct assembly paths on sub-cluster scales, with western regions linked to ancient mergers and eastern regions to ongoing tidal processes and pre-processing, highlighting a complex, multi-channel growth of ICGCs in cluster cores. The study demonstrates the value of wide-field, deep GC surveys and residual-mapping techniques for tracing ICGC distributions and linking them to ICL, dwarfs, and cluster assembly history.

Abstract

We investigate the spatial distribution of the Intra-Cluster Globular Clusters (ICGCs) detected in the core of the Fornax galaxy cluster. By separately modeling different components of the observed population of Globular Clusters (GCs), we confirm the existence of an abundant ICGCs over-density with a geometrically complex, elongated morphology roughly centered on the cluster dominant galaxy NGC 1399 and stretching along the E-W direction. We identify several areas in the ICGCs distribution that deviate from a simple elliptical model and feature large density enhancements. These regions are characterized based on their statistical significance, GCs excess number, position, size and location relative to the galaxies in their surroundings. The relations between the spatial distribution and features of the ICGCs structures, mostly populated by blue GCs, and properties of the intra-cluster light and dwarf galaxies detected in the core of the Fornax cluster are described and discussed. The line-of-sight velocity distribution of spectroscopically confirmed GCs within the ICGCs structures is compatible with the systemic velocities of nearby bright galaxies in the Fornax cluster, suggesting that the ICGCs population is at least partially composed of GCs stripped from their hosts. We argue that the findings here presented suggest that, on sub-cluster scales, different mechanisms contribute to the growth of the ICGC. The western region of Fornax is likely associated with old merging events that predate the assembly of the Fornax cluster. The eastern side instead points to a mix of tidal disruption of dwarf galaxies and stripping from the GCSs of massive hosts, more typical of relaxed, high-density cluster environments.

Figures (14)

• Figure 1: Left: Position of FDS GC candidates. The red diamonds show the positions of the galaxies from the Fornax Cluster Catalog ferguson1989; the brightest galaxies in the field are labeled and highlighted with a blue symbol. Right: Density map of FDS GC candidates obtained using the $K$-Nearest Neighbor (KNN) method dressler1980 with $K\!=\!3$, with low-density levels highlighted. In both plots, the four black boxes display the regions used to estimate the density of the homogeneous, component of the GC candidates population according to the first method described in Section \ref{['subsec:simulcomponents']}, while the gray circles are employed to determined the uncertainty of the density of the homogeneous component from candidate GCs located outside of the virial radius of the cluster (see Section \ref{['subsec:simulcomponents']} for details on both methods). The red dashed circles represent the virial radius of the Fornax cluster drinkwater2001.
• Figure 2: GCLFs for the MGS galaxies derived from ACSFCS observations villegas2010. The peak of the Gaussian functions are normalized to unity. The $u$-band completeness function of GCs detected in the FDS observations from mirabile2024 is also shown.
• Figure 3: Residual map of the spatial distribution of GC candidates obtained with KNN density maps with $K\!=\!10$. Orange and green pixels indicate over-dense and under-dense regions. Iso-residual contours (thin black lines) and green/orange shading indicate pixels with single-pixel statistical significance $-3\sigma,-2\sigma,-1\sigma,1\sigma,2\sigma,3\sigma$ (no residual pixels with single-pixel residual $<\!-1\sigma$ are present in the map). The thick black line shows the contours of the dominant over-density in the core region of the Fornax cluster (M), bounded by the continuous 1$\sigma$ single-pixel significance iso-residual contour. The red dashed circle represents the virial radius of the Fornax cluster drinkwater2001, and the black dashed rectangle shows the region of the core of the cluster displayed in Figure \ref{['fig:core_residual_k10']}.
• Figure 4: Residual map of the spatial distribution of all FDS GCs in the core of the Fornax cluster, obtained with with $K\!=\!10$. The color lines show the boundaries of the spatial features of the residual map described in the text, that have been selected by identifying spatial substructure that significantly deviate from the elliptical model of the main over-density M (see Section \ref{['sec:results']} for details). The red diamonds show the positions of the FCC galaxies in the SGS, while the annotated larger blue symbols indicate MGS galaxies. The gray-shaded circles represent five effective radii in the $g$ band for the main galaxies in the plot iodice2019b. Color coding of the residual maps and isoresidual contours are defined as in Figure \ref{['fig:residual_k10']}.
• Figure 5: Upper left: positions of FDS candidate GCs split in red and blue sub-classes using the threshold $g\!-\!i\!=\!0.97$. Upper right: histogram of the $g\!-\!i$ color distribution of all FDS GCs (upper panel), with two best-fit Gaussian models corresponding to the red and blue sub-classes and the color value $g\!-\!i\!=\!0.97$ used as threshold (green vertical line). The interval of average $g\!-\!i$ color spanned by the GCs included in the residual structures (Table \ref{['tab:residual_maps_features']}) are displayed by the yellow rectangle. The mid panel shows the color distribution for GC candidates within and outside the virial radius of the cluster (solid and dashed histograms, respectively), with corresponding best-fit blue and red gaussian subclasses, while the lower panel displays the color distribution within the virial radius after the color distribution of GC candidates located outside the virial radius has been statistically subtracted (see Section \ref{['subsec:colorsresidualstructures']} for details). Lower: smoothed cell-wise, background-subtracted average color map of GCs candidates in the core of the Fornax cluster, obtained on the same grid employed for the residual maps of the spatial distribution of observed GC candidates. The contours of the residuals structures derived from the spatial distribution of GC candidates and discussed in Section \ref{['sec:results']} (solid lines) are overplotted.