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

TL;DR

This work presents KiDS+VIKING-450, a unified 9-band optical+NIR photometric dataset covering ${\sim}341-458$ deg$^2$ with $r\le25$, designed to reduce photo-$z$ systematics that limit cosmological inference. By combining KiDS optical data with VIKING NIR imaging and employing GAaP photometry, BPZ photo-$z$ with an updated prior, and Le Phare SED fitting, the authors demonstrate substantial improvements in redshift accuracy, especially at $z_{\rm B}>0.9$, and robust stellar-mass estimates that align with previous spectroscopic-based studies. The dataset enables reliable GSMFs in multiple redshift slices and is poised to enhance weak-lensing analyses and galaxy evolution studies. Public availability of the KV450 products ensures broad applicability for cosmology and astrophysics research, with future data releases expected to further improve area and depth.

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/.

Figures (14)

• Figure 1: The footprint of the post-masking KiDS-450 dataset. Top: the distribution of the KiDS-450 fields on-sky, relative to the Ecliptic and Galactic planes. The Galactic plane is plotted with a width of 20 degrees, which roughly traces the observed width of the Galaxy thick disk. Lower panels: each of the individually named KiDS-450 patches (on their own scale). The points in each patch show the distribution of KiDS-450 photometric sources that remain after applying the bright-star mask. Points are coloured according to their overall observational coverage: green points have full KiDS+VIKING optical and NIR coverage, blue points have full KiDS optical coverage but only partial VIKING NIR coverage, and orange points have KiDS optical coverage only. As such the green and blue data show the footprint of the full KiDS+VIKING-450 (KV450) sample.
• Figure 2: The individual photometric filters (black) that make up the KV450 dataset. Each filter curve is shown as an overall transmission spectrum incorporating mirror, detector, and filter effects. We also show the typical transmission spectrum of the atmosphere at Paranal (blue) for modest values of precipitable water vapor (2.2mm) and zenith angle (30 degrees). In addition, we also show the median Le Phare spectrum of all KV450 galaxies with photometric redshift $Z_B=1.2$ and magnitude $r\sim24$ (red). The $68^{\rm th}$ and $99^{\rm th}$ percentiles of these models are also shown as shaded red regions. The $1\sigma$ detection limits of each band (orange chevrons and dotted line, derived from the values in Table \ref{['tab: detection thres']}) are also shown, for reference. These model spectra demonstrate the complementarity of the KiDS & VIKING surveys; a typical galaxy at the furthest and faintest end of our analysis is still detected in all bands. It also demonstrates the main benefit of having NIR imaging within this dataset, in that it allows much more accurate constraint of photometric redshifts for $(4000$Å$)$ Balmer-break galaxies at redshifts $z \gtrsim 1$.
• Figure 3: The distributions of individual flux measurements with respect to the final flux estimate and uncertainty in KV450. Here we show per-band PDFs of $\sigma_{\Delta{i}}$, which demonstrates the accuracy of the final flux uncertainties for sources in KV450 (see text for details). We overlay on each distribution a Gaussian model that describes well the core of each distribution, providing the mean ($\mu$), standard deviation ($\sigma$), and mixture fraction of the Gaussian given the total PDF ($\lambda$). We find that the final fluxes and uncertainties are generally a good description of the individual data, with typically $>70\%$ of all individual (per-detector) flux estimates being well described by the simple gaussian statistics. The wings of these distributions are caused by the existence of non-gaussian noise components not encoded by the GAaP uncertainties ( e.g. zero point uncertainties). We note that the $J$-band, however, has uncertainties that are underestimated by roughly $30\%$. Each panel is annotated with the kernel used in the PDF estimation, showing the width of the kernel and its log-bandwidth (bw).
• Figure 4: Photometric comparison of KV450 stellar photometry in each of the ${ZY\!J\!H\!K_{\rm s}}$-bands to photometry from SDSS (for our $Z$-band only, shown as a solid line), and 2MASS in the ${ZY\!J\!H\!K_{\rm s}}$-bands. Note that as 2MASS does not cover the $ZY$-bands, comparisons there are made using an extrapolation based on the 2MASS $J$-$H$ colour, as described in gonzalez-fernandez/etal:2018; these are shown here as dashed lines in the $ZY$ comparison panels. We simultaneously fit these distributions with a single component Gaussian (blue), with the optimised fit parameters annotated. With the exception of the Y-band extrapolation (which has a $0.02$ mag residual), all directly comparable fluxes are in perfect agreement.
• Figure 5: Comparison between the colours of KV450 stars and the same sources measured by 2MASS. The reduction in scatter of the distribution indicates that the KV450 NIR data have significantly reduced uncertainties.