z~7-10 Galaxies behind Lensing Clusters: Contrast with Field Search Results

TL;DR

This study searches for z>~7 galaxies behind lensing clusters using HST data, finding fewer candidates than previous reports and suggesting lensing is more effective for brighter galaxies, with limited utility for faint-end LF constraints.

Contribution

It provides a systematic search for high-redshift galaxies behind lensing clusters and compares results with field expectations, highlighting the limited effectiveness of lensing for faint galaxy detection.

Findings

Only one robust z~7 candidate found, consistent with expectations.

Number of candidates is lower than previous studies, indicating possible overestimations.

Lensing is more effective for detecting brighter galaxies than faint ones.

Abstract

We conduct a search for z>~7 dropout galaxies behind 11 massive lensing clusters using 21 arcmin**2 of deep HST NICMOS, ACS, and WFPC2 image data. In total, over this entire area, we find only one robust z~7 z-dropout candidate (previously reported around Abell 1689). Four less robust z-dropout and J-dropout candidates are also found. The nature of the four weaker candidates could not be precisely determined due to the limited depth of the available optical data, but detailed simulations suggest that all four could be low-redshift interlopers. We compare these numbers with what we might expect using the z~7 UV luminosity function (LF) determined from field searches. We predict 2.7 z~7 z-dropouts and 0.3 z~9 J-dropouts over our cluster search area, in reasonable agreement with our observational results, given the small numbers. The number of z>~7 candidates we find in the present search are much lower than has been reported in several previous studies of the prevalence of z>~7 galaxies behind lensing clusters. To understand these differences, we examined z>~7 candidates in other studies and conclude that only a small fraction are likely to be z>~7 galaxies. Our findings support models that show that gravitational lensing from clusters is of the most value for detecting galaxies at magnitudes brighter than L* (H<~ 27) where the LF is expected to be very steep. Use of these clusters to constrain the faint-end slope or determine the full LF is likely of less value due to the shallower effective slope measured for the LF at fainter magnitudes, as well as significant uncertainties introduced from modelling both the gravitational lensing and incompleteness.