Table 1.
KiDS: systematic-tracer variables for training of self-organising maps, with units and descriptions.
| Systematic-tracer variable | Unit | Description |
|---|---|---|
| MU_THRESHOLD (MU) | mag arcsec−2 | r-band detection threshold above background; the minimum surface brightness of objects detected after background-subtraction. Fainter objects will be lost from an area of observation where the detection threshold is brighter. |
| psf_fwhm | arcsec | Full-width at half-maximum of the r-band point-spread function, averaged over nside = 512 (47 arcmin2) HEALP IX(1) (Gorski et al. 2005) pixels and then interpolated to galaxy locations. The blurring of small, faint sources could cause objects to drop below detection thresholds. |
| psf_ell | dimensionless | Ellipticity (1 − q, where q is the 2D major/minor axis ratio) of the r-band point-spread function, also averaged over nside = 512 pixels and interpolated to galaxy locations. A PSF ellipticity indicates non-isotropic blurring of object isophotes, creating challenges for shape estimation and inducing a directional dependence for detections. |
| MAG_LIM_x | mag | x-band limiting magnitude (5σ above background in a 2″ aperture) at the object’s location, for each of the 9 bands observed by KiDS-VIKING. KiDS object detection is performed in the r-band. |
| EXTINCTION_r | mag | Galactic dust extinction in the r-band, derived using the Schlafly & Finkbeiner (2011) coefficients for the Schlegel et al. (1998) dust maps. Dust preferentially scatters short-wavelength light from extragalactic objects; the loss of flux could prevent detections, and the modification of galaxy spectral energy distributions (SEDs) poses other problems, e.g., for photo-z estimation. |
| gaia_nstar | count arcmin−2 | Number density of Gaia DR2 (Arenou et al. 2018) 14 < G < 17 stars in nside = 512 pixels, interpolated onto galaxy locations. High stellar densities can hamper detections as the light from stars obscures background objects, and can also result in spurious galaxy-detections through the misidentification of PSF-blurred, or blended, stars as galaxies. |
Notes.
http://healpix.sourceforge.net (Gorski et al. 2005). Systematic-tracer variables chosen from the KiDS DR4 data products, or from other public data e.g., Gaianstar. These are variables thought to trace phenomena that may impact upon the observed number density of galaxies. See Kuijken et al. (2019) for details on threshold, PSF, and magnitude-limit parameters. Maps of each of these quantities are displayed in Figs. 1 and 2. Where specified, pixelated systematic-tracer values are interpolated from pixel centres onto galaxy locations via a two-dimensional (RA/Dec) nearest-neighbour interpolation.
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