DESI DR2 reference mocks: clustering results from Uchuu-BGS and LRG

Abstract

The aim of this work is to construct mock galaxy catalogues that accurately reproduce the redshift evolution of galaxy number density, clustering statistics, and baryonic properties, such as stellar mass for luminous red galaxies (LRGs) and absolute magnitude in the $r$-band for the bright galaxy sample (BGS), based on the first three years of observations from the Dark Energy Spectroscopic Instrument (DESI). To achieve this, we applied the subhalo abundance matching (SHAM) technique to the Uchuu $N$-body simulation, which follows the evolution of 2.1 trillion particles within a volume of $8\,h^{-3}\,\mathrm{Gpc}^{3}$, assuming a Planck base-$Λ$CDM cosmology. Using SHAM, we populated Uchuu subhalos with LRGs and BGS-BRIGHT ($r<19.5$) galaxies up to redshift $z=1.1$, assigning stellar masses to LRGs and luminosities to BGS galaxies (up to $M_{\rm r}\leq 20$). Furthermore, we analyzed the clustering dependence on stellar mass and luminosity for each tracer. Our results show that the Uchuu BGS-BRIGHT and LRG mocks accurately reproduce the observed redshift evolution of clustering, with better than 5\% agreement for separations of $1<r<20\,h^{-1}\,\mathrm{Mpc}$ and below 10\% for $0.1<r<1\,h^{-1}\,\mathrm{Mpc}$. For the Uchuu-LRG mock, we successfully captured the stellar mass dependence of clustering, while for the Uchuu-BGS mock, we replicated the clustering for various volume-limited subsamples. We also find good agreement between the data and mocks in the dependence of large-scale bias on luminosity for BGS-BRIGHT galaxies and on stellar mass for LRGs. Altogether, these results equip DESI with robust tools for generating high-fidelity lightcones for the remainder of the survey, thereby enhancing our understanding of the galaxy--halo connection.

Figures (14)

• Figure 1: The comoving number density corrected by completeness of Y3 LRG and BGS-BRIGHT DESI (points) and Y3 Uchuu mock lightcones (solid lines) is shown in the top panel. The ratio between data and the prediction of the mock for each target is shown in the bottom panel. The shaded area represents the limits of the 10$\%$.
• Figure 2: Number density as a function of redshift for different volume-limited samples (detailed in table \ref{['bgs_volsam_tab']} calculated from the Uchuu-BGS lightcone (solid lines) and DESI BGS-BRIGHT Y3 (points).
• Figure 3: Left: Luminosity function from Y3 NGC BGS-BRIGHT (green points) and Uchuu-BGS (green line). The shaded area and the error bars indicates the 1$\sigma$ uncertainty. Right: stellar mass function of Y1 LRG DESI (points) and Y1 and Y3 Uchuu-LRG lightcones (dotted and solid green lines) are shown for several redshift bins within the range $0.4<z<1.1$. The dashed and dotted black lines represent the complete SMF adopted in each redshift range, indicated in the legend.
• Figure 4: Mean halo occupancy of BGS volume-limited samples (left panel) and LRG samples with stellar mass cuts (right panel) as determined from our SHAM Uchuu boxes. The redshift of the Uchuu-BGS box is $z=0.19$, while the redshift of the Uchuu-LRG box is $z=0.78$. The mean number of galaxies of a halo with a given mass $M_{\rm halo}$ is denoted by $<N_{\rm gal}>$. The solid lines represent the combined centrals and satellite occupation. The shaded area indicates the 1$\sigma$ uncertainty of the occupation measured from the Uchuu lightcone.
• Figure 5: A slice of the universe mapped by Uchuu DR2 drawn from a small wedge of the DESI footprint between $\pm$5 degrees in declination out to $z\approx4$. We render the four major extragalactic samples—bright galaxy survey (BGS) galaxies, luminous red galaxies (LRG), emission– line galaxies (ELG), and QSOs—using yellow, orange, blue, and green points, respectively. Within each target class, the shade of the colour maps to declination (lighter colours correspond to higher declination). The inset shows a subset of the BGS survey extending out to redshift $z = 0.2$, highlighting the large-scale structure traced by galaxies in the densest survey region. For reference, this small wedge of the BGS survey represents less than 0.1$\%$ of the comoving cosmological volume in DR2.