PEARLS: Globular Clusters and Ultracompact Dwarfs in the El Gordo Galaxies at z = 0.87

Abstract

JWST/NIRCam 0.9 to 2.0 μ m 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 ultracompact 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 look-back 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 look-back 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.

Keywords

• Globular star clusters
• Brightest cluster galaxies
• Galaxy clusters