PEARLS: Globular Clusters and Ultra-Compact Dwarfs in the El Gordo Galaxies at z=0.87

William E. Harris; Marta Reina-Campos; Anton M. Koekemoer; Jessica M. Berkheimer; Timothy Carleton; Seth H. Cohen; Brenda L. Frye; Tyler R. Hinrichs; Benne W. Holwerda; Rachel Honor; Massimo Ricotti; S. P. Willner; Rogier A. Windhorst; Haojin Yan

arXiv · 2508.12862

PEARLS: Globular Clusters and Ultra-Compact Dwarfs in the El Gordo Galaxies at z=0.87

William E. Harris·Marta Reina-Campos·Anton M. Koekemoer·Jessica M. Berkheimer·Timothy Carleton·Seth H. Cohen·Brenda L. Frye·Tyler R. Hinrichs·Benne W. Holwerda·Rachel Honor·Massimo Ricotti·S. P. Willner·Rogier A. Windhorst·Haojin Yan

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Abstract

JWST/NIRCam 0.9 to 2.0 micron images reveal a population of point sources around the major galaxies in the El Gordo cluster at redshift z=0.87. Their distribution in the color--magnitude diagrams shows a narrow sequence well separated from field-galaxy contamination and consistent with their identification as ultra-compact dwarf galaxies (UCDs) or luminous globular clusters (GCs). The point-source sequence is more luminous by almost a magnitude than the corresponding sequence in Abell 2744 at z=0.31, matching the predicted evolutionary change for GC/UCDs over the 4-Gyr difference in lookback time between these two clusters. Deeper observations should allow direct JWST imaging of GC/UCD populations, even without the help of lensing, up to z ~ 1.4, a lookback time of more than 9 Gyr. Such observations would directly reveal the evolution of these compact stellar systems two-thirds of the way back to the Big Bang.

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William E. Harris, Marta Reina-Campos, Anton M. Koekemoer, Jessica M. Berkheimer, Timothy Carleton, Seth H. Cohen, Brenda L. Frye, Tyler R. Hinrichs, Benne W. Holwerda, Rachel Honor, Massimo Ricotti, S. P. Willner, Rogier A. Windhorst, Haojin Yan. 2025-08-18. PEARLS: Globular Clusters and Ultra-Compact Dwarfs in the El Gordo Galaxies at z=0.87. https://arxiv.org/abs/2508.12862

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