The “Fork” Geometry in the DESI Main Survey DR1: Implications of Target Selection for Galaxy Evolution Studies

Abstract The Dark Energy Spectroscopic Instrument (DESI) Main Survey Data Release 1 (DR1) provides an unprecedented spectroscopic view of galaxies and quasars across a large fraction of the observable Universe. However, the survey’s tracer-dependent targeting strategy introduces complex observational selection effects that strongly influence the apparent distribution of galaxies in the redshift–luminosity plane. In this work, we investigate the origin of the characteristic “fork” geometry observed in DESI DR1 and show that the observed distribution is consistent with an interpretation in which it arises from the combined action of intrinsic galaxy bimodality and the survey’s redshift-dependent selection function. We show that the observed underdensity separating the dominant galaxy populations is influenced not only by the physical green valley but also by survey selection effects, since it is further shaped by magnitude limits, surface brightness selection, and structural incompleteness. Using DESI DR1 spectroscopy together with Legacy Survey photometry, we analyze the roles of the Bright Galaxy Survey, luminous red galaxies, emission-line galaxies, and QSO target classes in producing the observed distribution and assess their impact on luminosity function measurements. Our results indicate that both target selection and observational biases significantly affect the observed galaxy distribution and the evolutionary trends inferred from it. We summarize practical observational considerations aimed at minimizing these effects and enabling more robust studies of galaxy evolution with DESI.