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Globular Clusters in the Hubble Frontier Field cluster Abell 2744 at z ~ 0.31

Presentation #148.03 in the session “Galaxies and Galaxy Clusters”.

Published onJan 11, 2021
Globular Clusters in the Hubble Frontier Field cluster Abell 2744 at z ~ 0.31

We present a study of globular clusters (GCs) in the Hubble Frontier Field cluster Abell 2744, also known as Pandora’s cluster, at z~0.31. This cluster is rich in the number and variety of galaxies it contains. The X-ray distribution of hot gas is consistent with an ongoing collision of three clusters of galaxies. Our goal is to use compact stellar systems, GCs, as fossil records of the violent interactions that shaped the Pandora’s cluster and the galaxies in it to gain new insight into cluster formation processes. In our study, we cleaned the F814W Hubble Space Telescope (HST) image of the cluster by modeling the light of bright cluster galaxies and subtracting them off and then generated a single-band point-source catalog. Next, because GCs appear point-like at these distances, we selected for GC candidates that have a relatively bright central pixel and relatively faint apparent magnitude. To test the completeness of the Source Extractor detection algorithm, we added artificial stars to the image and measured the detection rate. Based on this, we found the magnitude cut-off for high completeness recovery, made corrections to account for the total area within a projected distance of 400kpc from the cluster center, and corrected the GC luminosity function (LF). This corrected LF is contaminated by point-like galaxies in the background of the cluster. To eliminate these, we subtracted the LF of the HST F814W parallel image, taken close to the cluster but in an area not expected to contain cluster galaxies, from the observed LF of the cluster, resulting in the final GC LF of Pandora’s cluster. In order to account for GCs that are fainter than our completeness limit, we fit a Gaussian to the LF and integrated it, which amounted to 118,084 GCs. We then found that the GC specific frequency - number of GCs per unit galaxy luminosity - is about 10, similar to elliptical galaxies. These results and further research on chemical properties and diversity of GCs can provide insight into cluster formation.

SS, NC, and SF conducted their research under the auspices of the Science Internship Program at the University of California Santa Cruz. This research was funded in part by NASA/STScI.

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