Christian Wolf

Medium-Band Photometric Redshifts

UCL Meeting16 Sep 2008

Redshift Errors & Resolution

• Objects at different redshifts• Filterset 's fixed

z = 0.843

z = 1.958

z = 2.828

€

Δz =Δλ

λ rest=

Δλ

λ /(1+ z)

400 nm 1000 nm

flux /

qef

G2 star vs.QSO z=3

€

Δ

€

€

⇒Δz1+ z

=Δλ

λ

Z-Bias & Calibration Offsets

• N = number of filters, i.e. independent data points

• Calibration offsets in N=3-D:– 1-D normalisation

– 1-D z-bias

– 1-D restframe SED bias

– 1 out of N offset dimensions causes a photo-z bias Δz

• More filters smaller Δz (proj. component ~1/√N)

• Narrow filters small Δz (larger col/z on feature)

• Spectroscopy with N~102..3: z without flux calibration

SbSb

EE

z=0.52

z=0.56

colourco

lour

Few-filter photo-z’s limited by calibration accuracy

Many-filter photo-z’s limited by number and resolution of filters

Redshift Error Regimes

• Three regimes in photo-z quality

• Saturation– Model-data calibration offsets in

test causes p(z)-biases

• Transition– Locally linear colour(z) grid

• Breakdown– Globally nonlinear

colour(z) grid€

σ z ∝σ colour ∝N

S∝

1

100.4m⇒ logσ z ∝m

stars galaxies quasars

21 22 23 24 25 21 22 23 24 25 21 22 23 24 25

100%

0%

CA+B A+B

A+BCC

21 23 25 21 23 25 21 23 25 21 23 25

1

0.1

0.01

quasars @ z<2.2 quasars @ z>2.2quiescent galaxies starburst galaxies

σΔz

R-mag

mag

SbSb

EE

z=0.52z=0.56

colour

colo

ur

Galaxies: Saturation & Transition

R=20 R=22 R=23.7

Galaxies at z~0.45

QSOs: Saturation at R<24

QSOs at z~2.8

R=23.8R=22.9R=21.5

rms 0.0087%-20%

outlierCalibration

offsets

z biases

Calibrationoffsets

line

confusion

2-SED Classification, COMBO-17

Classification ~98% complete at R<23Stars (~3,000)

White Dwarfs (~30)

Ultra-cool WD (1)

Galaxies (~30,000)

QSOs (~300)

Old Red vs. Dusty Red Galaxies

New Subject:

Empirical 2 estimation?

Code 2 NN

Model

Template Empirical

2

+ PDF Ambiguity warning

• NN– No PDF, no warning

• Template model+ Can be extrapolated in z,mag– Calibration issues– Priors’ issues

• Empirical model+ Good priors+ No calibration issues– Can not be extrapolated

Galaxies: No Ambiguities

Collister & Lahav 2004

ANN 2 template 2 empirical

~4% outliers z/(1+z)>0.1rms z/(1+z) = 0.042

Bias -0.017

~0% outliers z/(1+z)>0.1rms z/(1+z) = 0.020

Bias ~0.00

~0% outliers z/(1+z)>0.1rms z/(1+z) = 0.023

Bias ~0.00

QSOs: Strong Ambiguities

Filipe A.

ANN 2 template 2 empirical

~22% outliers z/(1+z)>0.3rms z/(1+z) = 0.056

Bias +0.015

~1% outliers z/(1+z)>0.3rms z/(1+z) ~ 0.04

Bias ~0.000

~12% outliers z/(1+z)>0.3rms z/(1+z) = 0.113

Bias ~0.00

QSO Details