Speaker
Description
Understanding galaxy assembly bias is crucial for deriving unbiased cosmological constraints from Stage
IV surveys such as the Dark Energy Spectroscopic Instrument (DESI). Taking advantage of the state-of-the-art
DESI group catalog, we develop a robust method to directly measure galaxy assembly bias, parametrized by
$Q_{\rm env}$, for a volume-limited galaxy sample at redshift $z\in[0.05,0.2]$ from the DESI Bright Galaxy Survey. This
is achieved by examining how the halo occupation distribution (HOD) of galaxies depends on the large-scale
overdensity environment $\delta_g$. For each subsample of groups binned by total luminosity at fixed $\delta_g$, we develop a
novel method to measure the occupation number $N_{\rm gal}$ using the group-galaxy cross-correlation function, while
weak gravitational lensing constrains the average halo mass Mh. By comparing the HOD $N_{\rm gal}(Mh)$ between
high- and low-δg environments, we obtain a stringent constraint of $Q_{\rm env}=−0.06\pm0.13$, statistically consistent
with no galaxy assembly bias ($Q_{\rm env}=0$). Our work provides a direct method of measuring the HOD of any tracer
population without assuming an arbitrary cosmology, offering a straightforward approach to characterizing
galaxy assembly bias for precision cosmology in DESI and future surveys.