Euclid: Early Release Observations -- Globular clusters in the Fornax galaxy cluster, from dwarf galaxies to the intracluster field

Abstract: We present an analysis of Euclid observations of a 0.5 deg$^2$ field in the central region of the Fornax galaxy cluster that were acquired during the performance verification phase. With these data, we investigate the potential of Euclid for identifying GCs at 20 Mpc, and validate the search methods using artificial GCs and known GCs within the field from the literature. Our analysis of artificial GCs injected into the data shows that Euclid's data in $I_{\rm E}$ band is 80% complete at about $I_{\rm E} \sim 26.0$ mag ($M_{V\rm } \sim -5.0$ mag), and resolves GCs as small as $r_{\rm h} = 2.5$ pc. In the $I_{\rm E}$ band, we detect more than 95% of the known GCs from previous spectroscopic surveys and GC candidates of the ACS Fornax Cluster Survey, of which more than 80% are resolved. We identify more than 5000 new GC candidates within the field of view down to $I_{\rm E}$ mag, about 1.5 mag fainter than the typical GC luminosity function turn-over magnitude, and investigate their spatial distribution within the intracluster field. We then focus on the GC candidates around dwarf galaxies and investigate their numbers, stacked luminosity distribution and stacked radial distribution. While the overall GC properties are consistent with those in the literature, an interesting over-representation of relatively bright candidates is found within a small number of relatively GC-rich dwarf galaxies. Our work confirms the capabilities of Euclid data in detecting GCs and separating them from foreground and background contaminants at a distance of 20 Mpc, particularly for low-GC count systems such as dwarf galaxies.

• The paper demonstrates Euclid’s capability by detecting over 5000 globular clusters with greater than 80% completeness.
• The paper employs King profile modeling and PSF photometry to accurately separate globular clusters from stars and galaxies.
• The analysis reveals a strong correlation between cluster distribution and intracluster light, offering new insights into galaxy assembly.

Analysis of the Euclid Early Release Observations of Globular Clusters in the Fornax Galaxy Cluster

The paper under consideration provides a thorough analysis of early observations from the Euclid mission, specifically targeting a field within the Fornax galaxy cluster. This study aims to assess Euclid's capability for detecting globular clusters (GCs) at a distance of 20 Mpc, and to evaluate the methodology for identifying these clusters using Euclid's data. The analysis highlights the potential for Euclid to observe GCs and investigates the spatial distribution of these clusters within the intracluster field, as well as around dwarf galaxies.

Methodology and Data Processing

The field of view covered by the Euclid observations is 0.5 square degrees, centered on the Fornax cluster, with images captured in the optical and near-infrared bands. The main thrust of the methodology involves using Euclid's VIS and NISP instruments to capture data with sufficient depth to resolve GC structures. The study employs techniques such as King profile modeling to simulate artificial GCs and assess the detection efficiency. Through PSF modeling, source detection, and photometry, the completeness of source extraction is evaluated, with completeness rates exceeding 80% for GCs down to specific magnitude limits in the I_E band.

Detection and Characterization of Globular Clusters

The study successfully identifies over 5000 new GC candidates brighter than I_E = 25.0 mag in the Fornax cluster region. Notably, these detections extend significantly fainter than the typical turnover magnitude of the GC luminosity function (GCLF), illustrating Euclid’s capability to probe GC populations to unprecedented depths. By applying stringent selection criteria based on compactness and color indices, the analysis demonstrates excellent separation of GCs from foreground stars and background galaxies.

The study addresses the issue of contamination within the GC sample by employing multi-color diagrams, comparing the Euclid-based samples with previously cataloged GCs. These comparisons show a high concordance, indicating both reliability and purity of the Euclid-detected sample.

Spatial Distribution and Intracluster Light Correlation

The results on the spatial distribution of GCs reveal intriguing correlations with the intracluster light within the Fornax cluster. The GC density maps, akin to the diffuse light structure, suggest that these clusters might be tracing the underlying distribution of dark matter or the tidal features caused by galaxy interactions. This correlation provides an essential insight into the assembly history of the Fornax cluster and demonstrates Euclid's efficacy in studying such extended faint structures.

GC Systems Around Dwarf Galaxies

A significant part of the study is dedicated to examining GC systems around dwarf galaxies in the Fornax cluster. The study finds that these galaxies typically harbor few GCs, especially those with stellar masses below 10^7 solar masses. However, a subset of dwarf galaxies, analogous to ultra-diffuse galaxies, show an unexpected richness in GC numbers. The analyses lead to discussions about the formation mechanisms of such systems and their GC content relative to their stellar and dark matter mass.

Implications and Future Directions

The findings highlight Euclid’s potential to drastically improve our understanding of GC distributions in different environments, effectively bridging observations of nearby galaxies with high-redshift systems. The ability to map GCs and related faint structures across more extensive fields allows for a comprehensive investigation of the correlation between GCs and other galactic parameters. Additionally, the insights gained from analyzing the GC content in various galaxy types bolster our theoretical understanding of galaxy formation and evolution.

Future observations and analyses using Euclid's forthcoming data from its wide survey will enable a broader application of these methodologies to other galaxy clusters and environments, promising a transformative impact on extragalactic astronomy, particularly in the field of dark matter halo mapping and galaxy assembly history. This work sets the stage for extensive future studies and follow-up observations, potentially with complementary ground-based telescopes, to further elucidate the nature of GCs as relics of the early universe.