Bright galaxy sample in the Kilo-Degree Survey Data Release 4 Selection, photometric redshifts, and physical properties

We present a bright galaxy sample with accurate and precise photometric redshifts (photo-zs), selected using ugriZYJHK(s) photometry from the Kilo-Degree Survey (KiDS) Data Release 4. The highly pure and complete dataset is flux-limited at r < 20 mag, covers similar to 1000 deg(2), and contains about 1 million galaxies after artifact masking. We exploit the overlap with Galaxy And Mass Assembly spectroscopy as calibration to determine photo-zs with the supervised machine learning neural network algorithm implemented in the ANNz2 software. The photo-zs have a mean error of vertical bar delta z vertical bar similar to 5 x 10(-4) and low scatter (scaled mean absolute deviation of similar to 0:018(1 + z)); they are both practically independent of the r-band magnitude and photo-z at 0:05 < z(phot) < 0:5. Combined with the 9-band photometry, these allow us to estimate robust absolute magnitudes and stellar masses for the full sample. As a demonstration of the usefulness of these data, we split the dataset into red and blue galaxies, used them as lenses, and measured the weak gravitational lensing signal around them for five stellar mass bins. We fit a halo model to these high-precision measurements to constrain the stellar-mass-halo-mass relations for blue and red galaxies. We find that for high stellar mass (M-* > 5 x 10(11) M-circle dot), the red galaxies occupy dark matter halos that are much more massive than those occupied by blue galaxies with the same stellar mass.

Automated summary

The study presents a bright galaxy sample selected using accurate photometric redshifts from the Kilo-Degree Survey Data Release 4, with precise photo-zs determined through a supervised machine learning neural network algorithm. The data shows low mean errors and scatter, allowing for estimation of robust absolute magnitudes and stellar masses. Analysis of the data reveals that red galaxies at high stellar mass occupy dark matter halos that are significantly more massive than those occupied by blue galaxies at the same stellar mass.