Target Selection and Sample Characterization for the DESI LOW-Z Secondary Target Program
Elise Darragh-Ford, John F. Wu, Yao-Yuan Mao, Risa H. Wechsler, Marla Geha, J. E. Forero-Romero, ChangHoon Hahn, Nitya Kallivayalil, John Moustakas, Ethan O. Nadler, Marta Nowotka, J. E. G. Peek, Erik Tollerud, Benjamin J. Weiner, J. Aguilar, S. P. Ahlen, D. Brooks, Andrew P. Cooper, Axel de la Macorra, Arjun Dey, K. Fanning, Andreu Font-Ribera, Satya Gontcho A Gontcho, K. Honscheid, Theodore Kisner, Anthony Kremin, Martin Landriau, M. E. Levi, P. Martini, Aaron Meisner, R. Miquel, Adam D. Myers, Jundan Nie, N. Palanque‐Delabrouille, Will J. Percival, Francisco Prada, David J. Schlegel, M. Schubnell, G. Tarlé, M. Vargas-Magaña, Zhimin Zhou, Hu Zou
Abstract
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 * < 10 9 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.