[Paper Review] Clustering of red-sequence galaxies in the fourth data release ofthe Kilo-Degree Survey

Mohammadjavad Vakili, Henk Hoekstra+14 more|arXiv (Cornell University)|Aug 30, 2020

Galaxies: Formation, Evolution, Phenomena7 references4 citations

TL;DR

This study presents a data-driven selection of luminous red-sequence galaxies in the KiDS-4 survey using multi-band photometry (optical + near-infrared) and spectroscopic redshifts from SDSS, GAMA, 2dFLenS, and G10-COSMOS. It measures angular two-point correlation functions and finds consistent linear bias values across two luminosity-threshold samples, validating the passive evolution model and enabling future cosmological 3×2pt analyses.

ABSTRACT

We present a sample of luminous red-sequence galaxies to study the large-scale structure in the fourth data release of the Kilo-Degree Survey. The selected galaxies are defined by a red-sequence template, in the form of a data-driven model of the colour-magnitude relation conditioned on redshift. In this work, the red-sequence template is built using the broad-band optical+near infrared photometry of KiDS-VIKING and the overlapping spectroscopic data sets. The selection process involves estimating the red-sequence redshifts, assessing the purity of the sample, and estimating the underlying redshift distributions of redshift bins. After performing the selection, we mitigate the impact of survey properties on the observed number density of galaxies by assigning photometric weights to the galaxies. We measure the angular two-point correlation function of the red galaxies in four redshift bins, and constrain the large scale bias of our red-sequence sample assuming a fixed $Λ$CDM cosmology. We find consistent linear biases for two luminosity-threshold samples (dense and luminous). We find that our constraints are well characterized by the passive evolution model.

Motivation & Objective

• To construct a robust, data-driven sample of red-sequence galaxies in KiDS-DR4 using photometric and spectroscopic data.
• To measure the angular two-point correlation function of red-sequence galaxies in four redshift bins.
• To constrain the large-scale linear bias of the sample under a fixed ΛCDM cosmology.
• To ensure sample purity and mitigate survey systematics through photometric weighting and redshift distribution estimation.
• To enable future joint analyses with cosmic shear for tighter cosmological constraints.

Proposed method

• A data-driven red-sequence template is built using the colour-magnitude relation conditioned on redshift, derived from KiDS-DR4 and VIKING photometry.
• Spectroscopic redshifts from SDSS DR13, GAMA, 2dFLenS, and G10-COSMOS are used to calibrate the red-sequence model.
• Galaxies are selected using a multivariate Gaussian model of the colour-magnitude relation, with luminosity ratio cuts and constant comoving density thresholds.
• Photometric weights are assigned to correct for survey geometry and depth effects on galaxy number density.
• The angular two-point correlation function is measured in four redshift bins to assess clustering strength.
• Linear bias is constrained under a fixed ΛCDM cosmology, with consistency tested across 'dense' and 'luminous' sample variants.

Experimental results

Research questions

• RQ1 What is the clustering amplitude of red-sequence galaxies in the KiDS-DR4 survey across different redshift bins?
• RQ2 How consistent are the linear bias measurements between two luminosity-threshold samples (dense and luminous)?
• RQ3 To what extent does the observed clustering support the passive evolution model of red-sequence galaxies?
• RQ4 How effective is the photometric weighting scheme in mitigating survey systematics on galaxy number density?
• RQ5 Can the selected red-sequence sample be reliably used in future 3×2pt cosmological analyses?

Key findings

• The angular two-point correlation function of red-sequence galaxies is successfully measured in four redshift bins using KiDS-DR4 data.
• The linear bias for both the 'dense' and 'luminous' red-sequence samples is consistent across redshift bins, supporting model robustness.
• The observed clustering is well described by the passive evolution model of red-sequence galaxies.
• The inclusion of VIKING Z- and Ks-band photometry improves red-sequence template accuracy and contamination assessment.
• The sample's purity is validated, with low contamination from stellar-like objects, especially when using Ks-band information.
• The final sample is suitable for use in future 3×2pt analyses combining galaxy clustering and cosmic shear in KiDS-DR4.