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 $N_{\rm gal}$ 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, $N_{\rm gal}(M_h)$, on large-scale overdensity $δ_{g}$. Focusing on the satellite galaxies, we find an assembly bias parameter of $Q_{\rm sat}{=}0.05{\pm}0.14$, 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.

Figures (4)

• Figure 1: Distribution of DESI groups (circles) in a redshift wedge ($|\rm Dec| {<} 1^\circ$) within the DESI DR1 volume. Each group is color-coded by its rank-order in galaxy overdensity $\tilde{\delta}_{g}$, according to the curved colorbar in the bottom left. Gray dots show the distribution of DESI BGS galaxies. The upper right panel shows the PDF of $\delta_{g}$, with the vertical dotted line indicating the median used for splitting the groups into low- and high-$\delta_{g}$ subsamples.
• Figure 2: Top: Weak lensing profiles $\Delta\Sigma$ of the high-$\delta_{g}$ (red) and low-$\delta_{g}$ (blue) groups in five different $M_h^{\mathrm{AM}}$ bins. In each panel, symbols with errorbars and solid curves of corresponding colors are the measurements and best-fitting model predictions, respectively. Dashed curves indicate the 1-halo contribution to the model, with the corresponding halo mass constraints listed in the bottom left corner. Fractional differences between the measured and predicted profiles are shown in the bottom subpanel. Bottom: Similar as the top but for the projected halo-galaxy cross-correlation functions $w_p^{hg}$. In each panel, constraints on the occupation numbers are listed in the bottom left corner, while the inset panel shows the isotropic halo-galaxy cross-correlation function $\xi_{hg}$ predicted by the best-fitting model. The vertical colored lines mark the average halo radii of the two subsamples inferred from the weak lensing masses.
• Figure 3: Satellite occupation number as a function of halo mass in high-$\delta_{g}$ (red) and low-$\delta_{g}$ (blue) environment. Each ellipse represents the $68\%$ confidence region of the joint constraint on the $N_{\mathrm{sat}}$ vs. $M_h$ plane for one group subsample. Red and blue lines with shaded bands are the respective predictions from our best-fitting satellite occupation model with galaxy assembly bias ($Q_{\mathrm{sat}}{=}0.05{\pm}0.14$).
• Figure 4: Mock experiment testing the sensitivity of "galaxy occupation bias" $q$ (fractional difference between the inferred and true occupation number; see Equation \ref{['eqn:q']}) to large-scale overdensity $\delta_{g}$. Left: Projected halo-galaxy cross-correlations $w_p^{hg}$ measured from the mock data (circles with errorbars) and predicted by the best-fitting model (curves) for halos in four quartiles of $\delta_{g}$, color-coded by the colorbar in the bottom left. Bottom subpanel shows the fractional differences between the mock measurements and predictions. Middle: Comparison between the isotropic cross-correlation functions $\xi_{hg}$ measured from the mock (circles) and inferred from the fits to $w_p^{hg}$ (curves). Vertical dotted line marks the average halo radius. Right: Occupation bias $q$ as a function of overdensity rank-order $\tilde{\delta}_{g}$. Thick horizontal lines with shaded bands indicate the $1\sigma$ measurements of $q$ in the four quartiles of $\delta_{g}$. Dotted horizontal line denotes the mean value of $q$, while solid horizontal line marks the $q{=}0$ case when there is no miscentering or scatter in the mass-observable relation.