Halo Occupation Distribution estimation performance for LSST data

Abstract

Upcoming imaging surveys, such as the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST), will enable high signal-to-noise measurements of galaxy clustering. The halo occupation distribution (HOD) is a widely used framework to describe the connection between galaxies and dark matter haloes, playing a key role in evaluating models of galaxy formation and constraining cosmological parameters. Consequently, developing robust methods for estimating this statistic is crucial to fully exploit data from current and future galaxy surveys. The main goal of this project is to extend a background subtraction method to estimate the HOD with more photometry-based information in preparation for the clustering analysis of the upcoming LSST data and to enable the study of the HOD with significantly improved statistical power. We evaluate the performance of the method using a mock galaxy redshift survey constructed from the cosmoDC2 catalogue. We implement an extension of the background subtraction technique to utilize information from photometric galaxy surveys. To identify the centres of galaxy groups, we implement an iterative centroiding approach (Central Galaxy Finder). We evaluate the impact of each step in our pipeline, including group size estimation from luminosity and purity, and completeness on group identification, along with the influence of observational systematics such as the use of photometric redshifts and halo mass uncertainties. We demonstrate the validity of the proposed method using a mock galaxy catalogue, recovering the HOD from cosmoDC2 over the absolute magnitude range $M_r = -20.0$ to $-17.0$ and halo masses up to $10^{15}\, \mathrm{M_\odot}$. We present key performance metrics to quantify the precision and reliability of the group finder and the resulting HOD measurements.

Figures (16)

• Figure 1: Construction of volume-limited galaxy samples with varying luminosity thresholds. The colour map shows the distribution of cosmoDC2 galaxies as a function of true redshift $z$ and $r$-band absolute magnitude $M_{r}$. The different coloured lines delineate the selection boundaries for the volume-limited samples. The inset panel shows their mean galaxy number density $n_{\mathrm{g}}$ (colour lines) and total sample density (black line). The solid horizontal lines show the average number density for each sample (the values are given in Tab. \ref{['tab:tab1']}).
• Figure 2: Normalized distributions for our galaxy catalogue, shown separately for centrals (black), group centres (red), and satellites (orange) galaxies. Panels show the distributions of halo mass $M_{\mathrm{200}}$ (top left), stellar mass $M_{\star}$ (top right), galaxy redshift $z$ (bottom left); and halo radius $r_{\mathrm{200}}$, derived from the halo mass (bottom right). The halo radius distribution includes only the central and group centre samples. All distributions are shown for the combined sample used in our analysis, which includes galaxies across the four absolute magnitude cuts.
• Figure 3: (Left) Comparison between central galaxies’ true redshifts ($z_{\mathrm{true}}$) and photometric redshifts using FlexZBoost ($z_{\mathrm{photo}}$, mean value). (Right) same as left but for BPZ redshifts. The dashed line represents the one-to-one relation ($z_{\mathrm{photo}} = z_{\mathrm{true}}$). The inset panels show the cumulative distribution function (CDF) of Odds parameters.
• Figure 4: Illustration of the BST for a single group. The red-shaded area and blue dots indicate the region and galaxies located within the characteristic radius of the host halo ($r < r_{\mathrm{c}}$). The green-shaded area and yellow dots represent the region and galaxies located in the outer annulus ($r_{\mathrm{in}} < r < r_{\mathrm{out}}$). Gray dots correspond to galaxies that lie outside the region considered by the method.
• Figure 5: Scheme of the CGF method applied to a galaxy sample. Top left: selection of candidate group centres, ranked by brightness (orange dots). Top right: definition of a discard region centred on the brightest galaxy (I), determined by its luminosity. Bottom left: removal of all galaxies within the discard region from the candidate list, except for galaxy (I). Bottom right: continue with the next remaining brightest galaxy. In all panels, the line of sight is along the direction perpendicular to the plane of the plot (i.e. viewed from the side of the plot).