# KiDS+VIKING-450: A new combined optical & near-IR dataset for cosmology and astrophysics

## TL;DR

This paper introduces a comprehensive combined optical and near-infrared dataset from KiDS and VIKING surveys, enabling improved cosmological measurements with reduced systematic uncertainties and publicly available data products.

## Contribution

The paper presents the creation and validation of a new, large, multi-wavelength dataset that enhances photometric redshift accuracy and stellar mass estimates for cosmology and astrophysics.

## Key findings

- Reduced systematic uncertainties in photo-z compared to optical-only data.
- Over 80% of sources have significant detections in all 8 bands.
- Photometric stellar mass functions agree with spectroscopic studies.

## Abstract

We present the curation and verification of a new combined optical and near infrared dataset for cosmology and astrophysics, derived from the combination of $ugri$-band imaging from the Kilo Degree Survey (KiDS) and $ZY\!J\!H\!K_{\rm s}$-band imaging from the VISTA Kilo degree Infrared Galaxy (VIKING) survey. This dataset is unrivaled in cosmological imaging surveys due to its combination of area ($458$ deg$^2$ before masking), depth ($r\le25$), and wavelength coverage ($ugriZY\!J\!H\!K_{\rm s}$). The combination of survey depth, area, and (most importantly) wavelength coverage allows significant reductions in systematic uncertainties (i.e. reductions of between 10 and 60\% in bias, outlier rate, and scatter) in photometric-to-spectroscopic redshift comparisons, compared to the optical-only case at photo-$z$ above $0.7$. The complementarity between our optical and NIR surveys means that over $80\%$ of our sources, across all photo-$z$, have significant detections (i.e. not upper limits) in our $8$ reddest bands. We derive photometry, photo-$z$, and stellar masses for all sources in the survey, and verify these data products against existing spectroscopic galaxy samples. We demonstrate the fidelity of our higher-level data products by constructing the survey stellar mass functions in 8 volume-complete redshift bins. We find that these photometrically derived mass functions provide excellent agreement with previous mass evolution studies derived using spectroscopic surveys. The primary data products presented in this paper are publicly available at http://kids.strw.leidenuniv.nl/.