Looking for the first galaxies: Lensing or blank fields?

TL;DR

This study compares the efficiency of lensing versus blank fields in detecting high-redshift galaxies, finding lensing clusters significantly enhance detection rates, especially at fainter magnitudes and higher redshifts.

Contribution

It provides a detailed simulation-based analysis of the relative effectiveness of lensing and blank fields for high-z galaxy surveys, guiding optimal observing strategies.

Findings

Lensing clusters increase detection of high-z galaxies, especially at faint magnitudes.

Positive magnification bias is stronger at higher redshifts and shallower surveys.

Lensing is more efficient for exploring the faint end of the luminosity function at z>6.

Abstract

The identification and study of the first galaxies remains one of the most exciting topics in observational cosmology. The determination of the best possible observing strategies is a very important choice in order to build up a representative sample of spectroscopically confirmed sources at high-z (z>7), beyond the limits of present-day observations. This paper is intended to precisely adress the relative efficiency of lensing and blank fields in the identification and study of galaxies at 6<z<12. The detection efficiency and field-to-field variance are estimated from direct simulations of both blank and lensing fields observations. The presence of a strong-lensing cluster along the line of sight has a dramatic effect on the number of observed sources, with a positive magnification bias in typical ground-based ``shallow'' surveys (AB<~25.5). The positive magnification bias increases with the redshift of sources and decreases with both depth of the survey and the size of the surveyed area. The maximum efficiency is reached for lensing clusters at z~0.1-0.3. Observing blank fields in shallow surveys is particularly inefficient as compared to lensing fields if the UV LF for LBGs is strongly evolving at z>~7. Also in this case, the number of z>8 sources expected at the typical depth of JWST (AB~28-29) is much higher in lensing than in blank fields (e.g. a factor of ~10 for AB<~28). Blank field surveys with a large field of view are needed to prove the bright end of the LF at z>6-7, whereas lensing clusters are particularly useful for exploring the mid to faint end of the LF.