KiDS-Legacy calibration: Unifying shear and redshift calibration with the SKiLLS multi-band image simulations

Open Access

Table 1.

Differences between the COllege (K19) and SKiLLS simulations.

		COllege (K19)	SKiLLS (this work)
Galaxies	Morphology	Sérsic models with parameters taken directly from the HST-ACS measurements (Griffith et al. 2012)	Sérsic models with parameters learned from the HST-ACS measurements (Sect. 2.1.2)
	Photometry	Single-band magnitudes from the Subaru r⁺-band observations	Nine-band synthetic magnitudes based on a semi-analytic model (Sect. 2.1.1)
	Depth	Limited by the HST-ACS measurements	Extending to 27th magnitude in the r band
	Position	Based on the observed locations in the COSMOS field	Based on the SURFS N-body simulations (Elahi et al. 2018)

Stars	Photometry	Single-band synthetic magnitudes from the Besançon model (Robin et al. 2003; Czekaj et al. 2014)	Nine-band synthetic magnitudes from the TRILEGAL model (Girardi et al. 2005)

Images	Band	the r-band images only	the full nine-band images
	Layout	32 CCDs with even gaps in between	32 CCDs with variable gaps as in the actual camera (Fig. 6)
	PSF	13 sets of spatially constant Moffat profiles, with each containing five different models corresponding to the five exposures	108 sets of spatially varying polynomial models, with each containing 5 × 32 different models
	Noise	One fixed noise level for all tiles	108 different noise levels
	Stack	Only THELI-like stacks for shape measurements	Both THELI-like and ASTRO-WISE-like stacks for shape and photometric measurements, respectively

Measurements	Shape	From the self-calibration version of lensfit with the weight bias correction of FC17	From the updated lensfit with the AlphaRecal method detailed in Sect. 4.2
	photo-z	Assigned with the KiDS observations of the COSMOS field	Measured from the simulated nine-band images following the KiDS photometric processing steps (Sect. 3.4)

Sample variance		Identical input catalogues of galaxies and stars for all the 13 realisations	Different galaxy catalogues for the 108 realisations and six stellar catalogues for the selected sky blocks (Fig. 7)
Input shears ^(a)		Eight sets of constant shears	Four sets of constant shears in the baseline simulations and a variable shear field for the blended objects (Appendix E)
Shape noise cancellation ^(b)		Each tile has three counterparts with galaxies rotated by 45, 90 and 135 degrees	Each tile has one counterpart with galaxies rotated by 90 degrees

Total simulated area		416 deg²	864 deg² in the constant shear simulations plus 7776 deg² of blending-only simulations for the correction of the ‘shear interplay’ effect (Sect. 5.2)

Notes.

^(a)

We verified that the four sets of input shears are sufficient to recover the previous results.

^(b)

Although more rotations suppress shape noise more efficiently (FC17), the selection effects diminish the actual performance of the shape noise cancellation (K19).

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