SLOAN DIGITAL SKY SURVEY

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Photometric Redshifts Some Considerations for
the CTIO Dark Energy Camera Survey
Huan Lin Experimental Astrophysics Group Fermilab
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Outline

• Photometric Redshifts for Red (Cluster) Galaxies
• Illustration using the SDSS luminous red galaxy
  (LRG) sample
• Monte Carlo simulations based on model spectral
  energy distributions (SEDs)
• Photometric Redshifts for the General Galaxy
  Population
• Illustrative example from CFH12K imaging of a
  CNOC2 redshift survey field

My focus is on effects of photometric errors on
photo-z measurement
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A sample of SDSS luminous red galaxies (LRGs)
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photo-z offset per 0.1 mag color offset
redshift
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Red Galaxy Monte Carlo Simulations

• Model SEDs
• Non-evolving CWW elliptical normalized to
  apparent magnitude z23 at redshift1
• Evolving Pegase-2 model from Jim Annis a 0.5 L
  galaxy for redshifts 0-1
• Flat, O? 0.7 cosmology
• Photometric Errors
• Filter g r i z
• Exposure 900 900 1700 900 sec
• 10s mag 25.0 24.5 24.3 23.3
• Simply scale S/N assuming constant noise from sky
  background, plus a calibration error floor of 3

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color
redshift
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Results of template and polynomial photo-z fits
photometric redshift error s(z) 0.03
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Photo-z bias from using the wrong template
bias is about the same as the statistical
uncertainty s(z) 0.03
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Photo-z bias from 0.03 mag offset in given filter
bias looks negligible
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Photo-z bias from 0.1 mag offset in given filter
bias is about the same as the statistical
uncertainty s(z) 0.03
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CFH12K/CNOC2 BVRI Photometric Redshifts

• CFH12K Images (CNOC2 0223B4 field)
• 3.6m CFHT
• Exposure times comparable B 1320s, V 900s, R
  900s, I 900s
• But seeing is better than typical CTIO 0.5-0.9
  arcsec
• Not shown today, but z-band data, plus other
  fields, are also available for photo-z tests
• CNOC2 Redshift Survey
• General field galaxy population
• Caveat calibrators mainly limited to z lt 0.7 and
  R lt 22

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BVRI photo-z results CNOC2 0223B4 field from
CFH12K
photometric redshift error s(z) 0.05
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Photo-z bias from 0.03 mag offset in given filter
bias relative to original solution looks
negligible
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Photo-z bias from 0.1 mag offset in given filter
bias relative to original solution is about the
same as the statistical uncertainty s(z) 0.05
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Conclusions and Outlook

• Simple Monte Carlo simulations indicate photo-z
  error s(z) 0.03 for red (cluster) galaxies, but
  with some redshift dependence and some degeneracy
  at z gt 0.8
• Should acquire spectroscopic cluster calibration
  sample to accurately determine cluster red galaxy
  SED
• General field galaxy sample from example
  CFH12K/CNOC2 field indicates s(z) 0.05, but
  with color/magnitude dependence, and
  redshift-dependent systematics
• Will also need to test using real/simulated
  galaxies at both fainter magnitudes and higher
  redshifts

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Conclusions and Outlook (contd)

• Photometric calibration errors of 0.03 mag
  introduce negligible bias, while errors of 0.1
  mag introduce biases comparable to photo-z
  statistical error s(z)
• Work is in progress on improving photo-z fits and
  reducing systematics, using CFH12K/CNOC2 and SDSS
  samples
• Not yet addressed question of how many
  calibrators are needed to sufficiently
  characterize photo-z error distributions for
  cluster counts, weak lensing, etc.