Photometric Redshifts: Some Considerations for the CTIO Dark Energy Camera Survey

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

2. 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

3. A sample of SDSS luminous red galaxies (LRGs)

4. photo-z offset per 0.1 mag color offset redshift

5. Red Galaxy Monte Carlo Simulations • 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%

6. color redshift

7. Results of template and polynomial photo-z fits photometric redshift error σ(z) ~ 0.03

8. Photo-z bias from using the wrong template bias is about the same as the statistical uncertainty σ(z) ~ 0.03

9. Photo-z bias from 0.03 mag offset in given filter bias looks negligible

10. Photo-z bias from 0.1 mag offset in given filter bias is about the same as the statistical uncertainty σ(z) ~ 0.03

11. 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 < 0.7 and R < 22

12. BVRI photo-z results: CNOC2 0223B4 field from CFH12K photometric redshift error σ(z) ~ 0.05

13. Photo-z bias from 0.03 mag offset in given filter bias relative to original solution looks negligible

14. 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

15. 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

16. 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.