photometric redshifts: some considerations for the ctio dark energy camera survey
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Photometric Redshifts: Some Considerations
for the CTIO Dark Energy Camera Survey
Huan Lin
Experimental Astrophysics Group
Fermilab
● 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
Outline
My focus is on effects of photometric errors on photo-z measurement
A sample of SDSS luminous red galaxies (LRGs)
photo-z offset per 0.1 mag color offset
redshift
● Model SEDs– Non-evolving CWW elliptical; normalized to apparent
magnitude z=23 at redshift=1– Evolving Pegase-2 model from Jim Annis; a 0.5 L* galaxy for
redshifts 0-1– Flat, ΩΛ= 0.7 cosmology
● Photometric Errors– Filter g r i z– Exposure 900 900 1700 900 sec– 10σ 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%
Red Galaxy Monte Carlo Simulations
color
redshift
Results of template and polynomial photo-z fits
photometric redshift error σ(z) ~ 0.03
Photo-z bias from using the wrong template
bias is about the same as the statistical uncertainty σ(z) ~ 0.03
Photo-z bias from 0.03 mag offset in given filter
bias looks negligible
Photo-z bias from 0.1 mag offset in given filter
bias is about the same as the statistical uncertainty σ(z) ~ 0.03
● 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 < 0.7 and R < 22
CFH12K/CNOC2 BVRI Photometric Redshifts
BVRI photo-z results: CNOC2 0223B4 field from CFH12K
photometric redshift error σ(z) ~ 0.05
Photo-z bias from 0.03 mag offset in given filter
bias relative to original solution looks negligible
Photo-z bias from 0.1 mag offset in given filter
bias relative to original solution is about the same as the statistical uncertainty σ(z) ~ 0.05
Conclusions and Outlook
● Simple Monte Carlo simulations indicate photo-z error σ(z) ~ 0.03 for red (cluster) galaxies, but with some redshift dependence and some degeneracy at z > 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 σ(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
Conclusions and Outlook (cont’d)
● Photometric calibration errors of 0.03 mag introduce negligible bias, while errors of 0.1 mag introduce biases comparable to photo-z statistical error σ(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.
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