CLASH: A Census of Magnified Star-Forming Galaxies at z ~ 6-8

TL;DR

This study uses extensive HST observations of 18 lensing clusters to identify and analyze a large sample of magnified star-forming galaxies at redshifts 6 to 8, providing new insights into early galaxy populations.

Contribution

It presents the largest sample to date of magnified galaxies at z~6-8, with detailed lensing models and photometric redshift analysis, demonstrating the efficiency of lensing clusters in detecting high-redshift galaxies.

Findings

Large sample of 262 high-redshift galaxy candidates identified.

Lensing clusters increase detection efficiency over field surveys.

Number counts agree with UV luminosity function predictions at bright magnitudes.

Abstract

We utilize 16 band Hubble Space Telescope (HST) observations of 18 lensing clusters obtained as part of the Cluster Lensing And Supernova survey with Hubble (CLASH) Multi-Cycle Treasury program to search for $z\sim6-8$ galaxies. We report the discovery of 204, 45, and 13 Lyman-break galaxy candidates at $z\sim6$, $z\sim7$, and $z\sim8$, respectively, identified from purely photometric redshift selections. This large sample, representing nearly an order of magnitude increase in the number of magnified star-forming galaxies at $z\sim 6-8$ presented to date, is unique in that we have observations in four WFC3/UVIS UV, seven ACS/WFC optical, and all five WFC3/IR broadband filters, which enable very accurate photometric redshift selections. We construct detailed lensing models for 17 of the 18 clusters to estimate object magnifications and to identify two new multiply lensed $z \gtrsim 6$ candidates. The median magnifications over the 17 clusters are 4, 4, and 5 for the $z\sim6$, $z\sim7$, and $z\sim8$ samples, respectively, over an average area of 4.5 arcmin$^2$ per cluster. We compare our observed number counts with expectations based on convolving "blank" field UV luminosity functions through our cluster lens models and find rough agreement down to $\sim27$ mag, where we begin to suffer significant incompleteness. In all three redshift bins, we find a higher number density at brighter observed magnitudes than the field predictions, empirically demonstrating for the first time the enhanced efficiency of lensing clusters over field surveys. Our number counts also are in general agreement with the lensed expectations from the cluster models, especially at $z\sim6$, where we have the best statistics.