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

Technical Deep Dive: Intracluster Globular Clusters in the Hydra I Galaxy Cluster

Overview

This technical deep dive examines the properties and distribution of globular clusters (GCs) in the Hydra I galaxy cluster, using deep optical imaging from the Very Large Telescope. The GC population is used as a tracer to study the hierarchical assembly and mass build-up of Hydra I, one of the nearest rich galaxy clusters.

Problem & Context

• Galaxy clusters form through the continuous merging of smaller structures like galaxies and groups, resulting in a diffuse stellar component called intracluster light (ICL).
• GCs and planetary nebulae can serve as luminous tracers of the ICL and provide insights into the cluster's assembly history.
• Hydra I is a nearby, dynamically active cluster that provides an excellent testbed to study these processes.

Methodology

• Photometric selection of 4,886 GC candidates from deep $V$ and $I$ band imaging, with additional $U$-band data for the central region.
• Separation of GCs into red and blue subpopulations based on their $(V-I)_0$ colors.
• Analysis of the spatial distributions, number density profiles, and specific frequencies (ratio of GCs to host galaxy light) of the GC populations.
• Constraining the evolution of the galaxy luminosity function using the GC specific frequencies and color distributions.

Data & Experimental Setup

• VLT/FORS1 imaging in $V$, $I$, and $U$ bands, covering the central $\sim$265 kpc of the Hydra I cluster.
• Photometric catalog of 4,886 GC candidates, with $\sim$34% completeness of the GC luminosity function.
• Background field $\sim$1.2 Mpc from the cluster center used for statistical subtraction of contaminants.

Results

Spatial Distributions

• Red GCs are more concentrated around the central galaxies NGC 3311 and NGC 3309.
• Blue GCs have a more extended distribution, with a significant offset from the central galaxies, coinciding with a displaced X-ray halo.
• Extreme blue and red GC subpopulations show opposite displacements from NGC 3311, potentially tracing the sloshing of the central galaxy.

Number Density Profiles

• Red GCs closely follow the surface brightness profile of NGC 3311 out to large distances.
• Blue GCs have a shallower profile, flattening at large radii and tracing the global gravitational potential of the cluster.

Specific Frequencies

• Red GCs maintain a constant specific frequency ($S_N \sim 2.5$) at all radii.
• Blue GCs show a sharp increase in $S_N$ to $\sim$15 at large distances, indicating they are associated with disrupted dwarf galaxies.
• Blue GCs are better tracers of the cluster's total mass profile than red GCs.

Luminosity Function Evolution

• Assuming blue GCs trace disrupted dwarf galaxies, the measured $S_N$ profile implies a steep past faint-end slope of the galaxy luminosity function ($\alpha = -1.81 \pm 0.16$), consistent with high-redshift observations.
• Fitting the GC color distributions to Virgo cluster templates suggests a shallower past bright-end slope ($\alpha = -1.29 \pm 0.34$), in agreement with simulations.

Interpretation

• The spatial and kinematic offsets of the GC populations indicate that Hydra I is in an active assembly stage, with signatures of recent galaxy interactions and mergers.
• The high specific frequencies of blue GCs at large radii suggest they trace the disruption of dwarf galaxies that contributed to the build-up of the ICL.
• Constraining the past galaxy luminosity function using the GC properties provides insights into the hierarchical assembly of galaxy clusters.

Limitations & Uncertainties

• Photometric completeness corrections and separation of young/old GC subpopulations have some uncertainties.
• Comparison of Hydra I GC color distributions to Virgo cluster templates may be limited by differences in the dynamical states of the two clusters.
• Assumptions in the luminosity function constraint method, such as a one-to-one mapping between $S_N$ and progenitor galaxy magnitude, may oversimplify the relationship.

What Comes Next

• Further investigation of the sloshing motion of the central galaxy NGC 3311 using GC kinematics and N-body simulations.
• Applying the luminosity function constraint method to other well-studied galaxy clusters with deep GC data.
• Validating the method using cosmological simulations that include GC formation physics.
• Upcoming wide-field surveys like Euclid will provide unprecedented deep imaging of local galaxy clusters, enabling more comprehensive studies of intracluster GC populations.