Redshifts and fluxes for emission-line galaxies

The forthcoming Dark Energy Spectroscopic Instrument (DESI) experiment plans to measure the effects of dark energy on the expansion of the Universe and create a 3D map of the Universe using galaxies up to z~1.6 and QSOs up to z~3.5.In order to create this map, DESI will obtain spectroscopic redshifts of over 30 million objects; among them, a majority are [OII] emitting star-forming galaxies known as emission-line galaxies (ELGs).

These ELG targets will be pre-selected by drawing a selection region on the g-r versus r-z colour-colour plot, where high-redshift ELGs form a separate locus from the lower redshift ELGs and interlopers. In this paper, we study the efficiency of three ELG target selection algorithms - the Final Design Report (FDR) cut based on the DEEP2 photometry, Number Density Modelling (NDM) and Random Forest - to determine how the combination of these three algorithms can be best used to yield a simple selection boundary that will be best suited to meet DESI's science goals.