# Presentation: 313796

## Photometric Redshifts: 

- Some Considerations
- for the
- CTIO Dark Energy Camera Survey

**Huan Lin**
**Experimental Astrophysics Group**
**Fermilab**

## 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***

## A sample of SDSS luminous red galaxies (LRGs)

**A sample of SDSS ****luminous red galaxies**** (LRGs)**

## photo-z offset per 0.1 mag color offset

**photo-z offset p****er 0.1 mag color offset**

**redshift**

## Red Galaxy Monte Carlo Simulations

**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**
**Filter****	****	****g****	****r****	****i****	****z**
**Exposure ****	****900****	****900****	****1700****	****900  sec**
**10****σ 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%**

## color

**color**

**redshift**

## Results of template and polynomial photo-z fits

**Results of template and polynomial photo-z fits**

**photometric redshift error σ****(z) ~ 0.03**

## Photo-z bias from using the wrong template

**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

**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

**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/CNOC2 BVRI 

**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

**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

**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

**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 r****ed (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.**