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Gal-G11: Composite Cluster Red Sequence Tests

Composite red sequences of galaxies in clusters of known redshift have a measurable intrinsic scatter of <0.05 mag RMS in g-r and <0.03 mag RMS in r-i and i-z.

This test makes use of the existing composite red sequence calibration code for the redMaPPer cluster finder. Given a set of clusters with spectroscopic redshifts, the calibration measures the color, slope, and intrinsic scatter of the red sequence over a wide area. These tests were used to show that the DC6B color measurements were introducing an additional color scatter of ~6% (g-r) which would make it not possible to detect clusters with any reasonable purity or completeness.

Due to the availability of pre-survey data, this test will be the most interesting at 0.2<z<0.7 in the Stripe 82 overlap region. The number of cluster spectra (for clusters with >~10 red galaxies) at higher redshifts is very limited in the SV region.

Prerequisites

  • Full-depth coadded images over >20 deg^2 of SDSS Stripe 82 with DETMODEL colors AND/OR
  • Individual images over >20 deg^2 of SDSS Stripe 82 with DETMODEL colors.
  • Mangle masks useful but not required.
  • Set of cluster spectra, including XMM-LSS clusters in overlap region and Stripe 82 redMaPPer clusters (0.2<z<0.7).
  • To be useful, we require >~ 5 clusters per 0.05 redshift bin (and more is better). This requirement is met with >20 deg^2 of overlap with Stripe 82.

Procedure

  • Run redMaPPer red sequence calibration to measure the color, slope, and scatter of the composite red sequence for the clusters with spectra.
  • The intrinsic scatter of the red sequence in g-r should be ~0.05 at z<0.4, and the scatter in r-i and i-z should be 0.02-0.03 at z<0.7.

Verdict

If the red sequence scatter is larger than cluster finding is going to be impacted, perhaps significantly. The source of the problem may be the color measurement (DETMODEL magnitudes) or field-to-field variations, as the composite red sequence is implicitly a wide-area test of the photometry.