Medium-Band Photometric Redshifts
description
Transcript of Medium-Band Photometric Redshifts
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