R. Ibarra-Nuño | Searcharxiv

arXiv2026

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

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 BGS, LRG, ELG, 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.

astro-ph.GA↗

arXiv2026

Tracing Galaxy Evolution in the Nearby Universe: The Role of Dark Matter

Using a sample of 126000 late-type galaxies from the SDSS, we analyzed stellar mass as a function of the dynamical mass. Stellar masses are estimated using eight stellar population synthesis (SPS) models with constant IMF, while dynamical masses are derived from seven formulations based on Newtonian dynamics and virial equilibrium, incorporating both stellar and gas velocity dispersions. We account for key factors affecting mass estimation, including inclination, color, concentration, and Sérsic index. The difference between dynamical and stellar mass (Delta log(M)) ranges from nearly zero to 95% of the dynamical mass, depending on mass and redshift, decreasing with increasing redshift, exhibiting a saddlelike trend at low mass and low redshift -- especially in disk-dominated LTGs, and transitioning into a steep, linear trend at higher masses and redshifts. This trend is not discrete but follows a continuous transition between morphological regimes. In the high-mass regime, the behavior resembles that of early-type galaxies. More massive or higher-redshift LTGs seem to be increasingly baryon-dominated, likely reflecting efficient baryon aggregation in compact, gas-rich early-Universe galaxies, less concentrated dark matter halos at high redshift, and dynamic gas inflows that concentrate baryons in galaxy centers, diminishing the dynamical role of dark matter. Dark matter within LTGs is at most equal to Delta log(M), depending on the impact of the IMF and SPS on stellar mass estimation. Although SPS-based stellar masses do not include the gas component, previous studies have shown that galaxies with log(M_Stellar/M_Solar) > 10 at z <= 0.3 are predominantly stellar-mass dominated, indicating that our sample mass estimates have minimal impact from gas exclusion. The results provide insight into the role of dark matter in determining galaxies' structure and evolution in the nearby Universe.

astro-ph.GA↗