# High-redshift LBG selection from broadband and wide photometric surveys using a Random Forest algorithm

**Authors:** C. Payerne, W. d'Assignies Doumerg, C. Yèche, V. Ruhlmann-Kleider, A. Raichoor, D. Lang, J. N. Aguilar, S. Ahlen, D. Bianchi, D. Brooks, T. Claybaugh, S. Cole, A. de la Macorra, B. Dey, P. Doel, A. Font-Ribera, J. E. Forero-Romero, S. Gontcho A Gontcho, G. Gutierrez, K. Honscheid  
**arXiv:** [2410.08062](https://arxiv.org/abs/2410.08062)  
**Date:** 2024-10-10  
**Summary page:** https://www.summarizepaper.com/fr/paper/2410.08062v1/

## Abstract

In this paper, we investigate the possibility of selecting high-redshift Lyman-Break Galaxies (LBG) using current and future broadband wide photometric surveys, such as UNIONS or the Vera C. Rubin LSST, using a Random Forest algorithm. This work is conducted in the context of future large-scale structure spectroscopic surveys like DESI-II, the next phase of the Dark Energy Spectroscopic Instrument (DESI), which will start around 2029. We use deep imaging data from HSC and CLAUDS on the COSMOS and XMM-LSS fields. To predict the selection performance of LBGs with image quality similar to UNIONS, we degrade the $u, g, r, i$ and $z$ bands to UNIONS depth. The Random Forest algorithm is trained with the $u,g,r,i$ and $z$ bands to classify LBGs in the $2.5 < z < 3.5$ range. We find that fixing a target density budget of $1,100$ deg$^{-2}$, the Random Forest approach gives a density of $z>2$ targets of $873$ deg$^{-2}$, and a density of $493$ deg$^{-2}$ of confirmed LBGs after spectroscopic confirmation with DESI. This UNIONS-like selection was tested in a dedicated spectroscopic observation campaign of 1,000 targets with DESI on the COSMOS field, providing a safe spectroscopic sample with a mean redshift of 3. This sample is used to derive forecasts for DESI-II, assuming a sky coverage of 5,000 deg$^2$. We predict uncertainties on Alcock-Paczynski parameters $\alpha_\perp$ and $\alpha_{\parallel}$ to be 0.7$\%$ and 1$\%$ for $2.6<z<3.2$, resulting in a 2$\%$ measurement of the dark energy fraction. Additionally, we estimate the uncertainty in local non-Gaussianity and predict $\sigma_{f_{\rm NL}}\approx 7$, which is comparable to the current best precision achieved by Planck.