Target Selection and Sample Characterization for the DESI LOW-Z Secondary Target Program

Abstract

We introduce the DESI LOW-Z Secondary Target Survey, which combines the wide-area capabilities of the Dark Energy Spectroscopic Instrument (DESI) with an efficient, low-redshift target selection method. Our selection consists of a set of color and surface brightness cuts, combined with modern machine-learning methods, to target low-redshift dwarf galaxies (z < 0.03) between 19 < r < 21 with high completeness. We employ a convolutional neural network (CNN) to select high-priority targets. The LOW-Z survey has already obtained over 22,000 redshifts of dwarf galaxies (M * < 109 M ⊙), comparable to the number of dwarf galaxies discovered in the Sloan Digital Sky Survey DR8 and GAMA. As a spare fiber survey, LOW-Z currently receives fiber allocation for just ∼50% of its targets. However, we estimate that our selection is highly complete: for galaxies at z < 0.03 within our magnitude limits, we achieve better than 95% completeness with ∼1% efficiency using catalog-level photometric cuts. We also demonstrate that our CNN selections z < 0.03 galaxies from the photometric cuts subsample at least 10 times more efficiently while maintaining high completeness. The full 5 yr DESI program will expand the LOW-Z sample, densely mapping the low-redshift Universe, providing an unprecedented sample of dwarf galaxies, and providing critical information about how to pursue effective and efficient low-redshift surveys.

Dwarf–Dwarf Interactions Can Both Trigger and Quench Star Formation

“…Characterizing just one of these rare quiescent field dwarfs has shed new light on the role of low-mass interactions in regulating dwarf star formationassembling a larger sample of quenched field dwarfs would provide even more novel constraints on the duty cycle and efficiency of such quenching events. TiNy Titans has established a sample of dwarf multiples for galaxies with SDSS spectroscopy; the upcoming generation of deeper surveys will, on timescales within a few years, push this pair finding out to larger volumes and lower masses (Darragh-Ford et al 2023;Luo et al 2023) and shed new light into the regulation and evolution of star formation in low-mass galaxies.…”

PROVABGS: The Probabilistic Stellar Mass Function of the BGS One-percent Survey

“…With 3 out of the 5 yr of operation remaining, BGS has completed ∼60% of its observations and is ahead of schedule. BGS will also be further extended by additional low-redshift dwarf galaxies observed with the DESI low-z secondary program (Darragh-Ford et al 2023). DESI observations will be publicly released periodically, starting with the Early Data Release (EDR) later this year.…”

A Large Sample of Extremely Metal-poor Galaxies at z < 1 Identified from the DESI Early Data

“…During the bright time, DESI will carry the BGS of galaxies at z < 0.6 and the Milky Way survey of stars (Allende Prieto et al 2020;Ruiz-Macias et al 2020;Cooper et al 2023;Hahn et al 2023). There are also some secondary target programs to utilize the spare fibers and DESI capabilities, such as the LOW-Z Secondary Target Survey targeting low-redshift (z < 0.03) dwarf galaxies (Darragh-Ford et al 2023).…”