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, Ω Λ = 0.7 cosmology ● Photometric Errors – Filtergriz – Exposure sec – 10σ mag – 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: 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.