Direct Measurement of Galaxy Assembly Bias using DESI DR1 Data

Abstract

We report the first direct measurement of galaxy assembly bias, a critical systematic in cosmology, from the Dark Energy Spectroscopic Instrument (DESI) Bright Galaxy Survey. We introduce a novel, cosmology-independent method to measure the halo occupation distribution (HOD) by combining a state-of-the-art group catalog with weak gravitational lensing. For groups binned by total luminosity, we determine the galaxy occupation number $N_{\mathrm{gal}}$ from group-galaxy cross-correlations, while weak lensing constrains the average halo mass $M_h$. Applying this to a volume-limited sample at $z\in [0.05,0.2]$, we measure the dependence of HOD, $N_{\mathrm{gal}}(M_h)$, on large-scale overdensity $\hat{I}{´}_g$. Focusing on the satellite galaxies, we find an assembly bias parameter of $Q_{\mathrm{sat}}=0.05\pm 0.14$, a result consistent with zero and in tension with many empirical galaxy formation models. Our method provides a robust approach for characterizing galaxy assembly bias to achieve precision cosmology with DESI and future Stage-V surveys.

5 pages + appendix, 4 figures