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

Abstract

In galaxy clusters, hierarchical assembly predicts the formation of stellar substructures and intracluster light (ICL), a diffuse stellar component tracing the global cluster potential. Because these features are extremely faint, alternative tracers such as globular clusters (GCs) provide a powerful tool to study cluster assembly. We use deep VLT/FORS $V$- and $I$-band imaging to investigate the GC population in the nearby Hydra I galaxy cluster ($\sim 45.7$ Mpc). GC candidates were selected from the $VI$ colour-magnitude diagram and divided into blue and red subpopulations. We find a clear spatial dichotomy: red GCs are concentrated around the massive central galaxies NGC 3311 and NGC 3309, while blue GCs are more extended and offset from the centre, coinciding with a secondary peak of X-ray-emitting gas. In the central regions, GC spatial distributions further depend on stellar population properties: young metal-rich GCs are more extended and may be linked to ram-pressure stripping, whereas old metal-poor GCs are more centrally concentrated, possibly originating from disrupted dwarf 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 light, while the blue population significantly deviates from it and traces the global gravitational potential of the cluster. This is also reflected in the specific frequency of blue GCs, which is approximately $\sim 5\times$ higher in the ICL-dominated outskirts than in the inner regions dominated by red GCs. Finally, we present a novel method to constrain the evolution of the galaxy luminosity function of the cluster using GC specific frequencies and colour distributions, yielding a past faint-end slope of $α=-1.81^{+0.16}_{-0.16}$ compared to $α=-1.41^{+0.08}_{-0.05}$ today, consistent with high-redshift observations and cosmological simulations.

Figures (20)

• Figure 1: Optical image of the centre of the Hydra I cluster obtained from the Digitized Sky Survey. The green squares show the region covered by the seven $6.8' \times 6.8'$ FORS1 pointings from which we obtain the $V$ and $I$ photometry for our GC candidates, while the blue squares show the regions with $U$-band coverage. The dark blue inset shows the background field located outside the cluster in the direction indicated by the arrow. The dashed gray circle corresponds to a radius of 0.15R_vir = 240kpc.
• Figure 2: Globular cluster candidates selection using the $VI$ colour-magnitude diagram. The red diamonds are point sources from Misgeld2011 that are spectroscopically confirmed to be associated with Hydra I. These objects reveal the locus of GCs in the colour-magnitude space, and we use them to select our sample of GC candidates, shown as the black points delimited by the dashed polygon. The right panel shows the $V$-band luminosity function of our selected candidates and of the objects in the background field, normalised to the area covered by the seven FORS fields.
• Figure 3: Left: Two-component Gaussian mixture model that best describes the $(V-I)_0$ colour distribution of our GC candidates (grey histogram). The individual components are shown as the blue and red dashed lines. Right: Red and blue GC subsamples obtained by assigning each GC to one Gaussian component. The expected numbers of background contaminants are shown as the step histograms.
• Figure 4: Spatial distribution of all (top panel), red (middle panel) and blue (bottom panel) GCs around Hydra I. The insets show the distributions around the central galaxies, NGC 3311 and NGC 3309. The spatial distribution of background objects for each population is shown in the top left inset in each panel. The dashed green contours on the bottom panel shows the X-ray emission from Hayakawa06.
• Figure 5: Central ($d \leq$ 6arcmin, grey dashed circle) spatial distribution of very blue (left) and very red (right) GC candidates, with colour ranges indicated in the insets.