Transcript Document

Weak lensing tomography:
the good, the bad and the ugly
Intrinsic
Photo-z
The Method
Alignements
Filipe Batoni Abdalla
Leverhulme Fellow
M. Banerji, E. Cypriano, S. Bridle,
O. Lahav (UCL), Chris Blake (Swinburne),
Rachel Mandelbaum (IAS) ,
A. Amara (Saclay), P. Capak, J. Rhodes
(Caltech/JPL), S. Rawlings (Oxford)
Cosmology: Concordance Model
Heavy elements 0.03%
Neutrinos 0.3%
Stars 0.5%
H + He gas 4%
Dark matter 20%
Dark Energy 75%
Outstanding questions:
• initial conditions (inflation?)
• nature of the dark matter
• nature of the dark energy
 Science goals for any
weak lensing project
7/17/2015
Filipe B. Abdalla (UCL)
It has been ~10 years! -> LCDM
Universe is flat(ish), dark energy exists:
Empty DA~10 kpc/arcsec
Flat DA~0.05 kpc/arcsec
Angle q = s / DA
• CMB -> Universe is flat
• High z supernovae ->
•
accelerated expansion
Other probes such confirm
this standard model:
- Integrates Sachs Wolf
Effect
- Galaxy power spectrum
- Clusters
- Weak lensing results.
Dark Energy:
Stress Energy vs. Modified Gravity
Stress-Energy:
G = 8G [T(matter) + T(new)]
Gravity: G + f(g) = 8G T(matter)
To distinguish between these choices, we must have probes of both
the geometry and the growth of large-scale structure.
Vacuum Energy: (special case, c.f. Einstein)
vac = L / 8G
vac = L/3H02
pvac = – vac w = -1
vac ~ 0.7 <--> vac ~ (0.001 eV)4
Undesirable for theoretical reasons
The Good:
The Methods statistical potential
Background sources
Dark matter halos
Observer
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Statistical measure of shear pattern, ~1% distortion
Radial distances depend on geometry of Universe
Foreground mass distribution depends on growth of structure
Background sources
Dark matter halos
Observer

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Statistical measure of shear pattern, ~1% distortion
Radial distances depend on geometry of Universe
Foreground mass distribution depends on growth of structure
Just one equation from GR
b
^
^
M
O
• ^ = 4 G M / (c2 b)
• NB. Independent of light wavelength
^ ^
Apparent deflection angle α
Cosmic shear two point tomography
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Cosmic shear two point tomography
q
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Cosmic shear two point tomography
q
Cosmic Shear & Weak Lensing
Tomography
•
Measure shapes for millions
source galaxies with z ~ 0.8
• Shear-shear & galaxy-shear
correlations probe distances
& growth rate of
perturbations
• Requirements: Sky area,
depth, photo-z’s, image
quality & stability
Huterer
Photo-z connection
The Bad:
The photo-z connection
Photometric Redshifts
• Photometric redshifts (photo-z’s) are
determined from the fluxes of
galaxies through a set of filters
• May be thought of as lowresolution spectroscopy
• Photo-z signal comes primarily from
strong galaxy spectral features, like
the 4000 Å break, as they redshift
through the filter bandpasses
• All key projects depend crucially on
photo-z’s
• Photo-z calibrations will be
• optimized using both simulated
catalogs and images.
Galaxy spectrum at 3 different redshifts,
overlaid on griz and IR bandpasses
Training Set
Methods
• Determine
functional relation
z phot = z phot ( m,c )
• Examples
Nearest Neighbors
(Csabai et al. 2003)

Polynomial
(Connolly et al. 1995)
Template Fitting
methods
• Use a set of standard SED’s •
•
Polynomial
Nearest
Neighbors
(Cunha et al.
in prep. 2005)
Neural Network
(Firth, Lahav & Somerville 2003;
Collister & Lahav 2004)
•
templates (CWW80, etc.)
Calculate fluxes in filters of
redshifted templates.
Match object’s fluxes (2
minimization)
Outputs type and redshift
• Bayesian Photo-z
Hyper-z (Bolzonella et al. 2000)
BPZ (Benitez 2000)
ANNz - Artificial Neural Network
z = f(m,w)
Input:
magnitudes
Collister & Lahav 2004
http://www.star.ucl.ac.uk/~lahav/annz.html
Output:
redshift
DUNE: Dark UNiverse
Explorer
Mission baseline:
• 1.2m telescope
• FOV 0.5 deg2
• PSF FWHM 0.23’’
• Pixels 0.11’’
• GEO (or HEO) orbit
Surveys (3-year initial programme):
• WL survey: 20,000 deg2 in 1 red broad band,
35 galaxies/amin2 with median z ~ 1, ground
based complement for photo-z’s
• Near-IR survey (Y?,J,H). Deeper than possible
from ground. Secures z > 1 photo-z’s
• SNe survey: 2 x 60 deg2, observed for 9
months each every 4 days in 6 bands, 10000
SNe out to z ~ 1.5, ground based spectroscopy
7/17/2015
Filipe B. Abdalla (UCL)
Surveys considered: galaxies
with
RIZ<25 considered
JPL Simulated catalogue
Av
Type
z
Know the requirements:
Catastrophic
outliers
Biases
Uninformative
region
Abdalla et al. astro-ph:0705.1437
• A case study: the DUNE satellite
• I have performed analysis within the DES framework as well: VDES
Number of
spectra needed
FOM: Results &
Number of spectra needed
• FOM prop 1/ dw x dw’
• IR improves error on DE
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parameters by a factor of
1.3-1.7 depending on
optical data available
If u band data is available
improvement is minimal
Number of spectra needed
to calibrate these photo-z
for wl is around 10^5 in
each of the 5 redshift bins
Fisher matrix analysis
marginalizing over errors
in photo-z.
Cleaned catalogues:
Method:
Motivation:
Remove systematic
effects associated
to catastrophic
outliers
Effect on the dark energy
measurements:
• Can clean a catalogue
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•
without degrading dark
energy measurements
In a cleaned catalogue
systematic effects such
as intrinsic alignments
will be smaller
An error of
dw x dw’=1/160 can
be achieved
The Ugly:
Intrinsic Alignements
Intrinsic alignements.
Additional
What we
contributions
measure Cosmic shear
What we
measure
Cosmic shear
Additional contributions
To remove these we need
good photometric redshfits
Cosmic
Shear
Intrinsic Alignments (IA)
Could bias w results by 100%
Normalised to Super-COSMOS
Heymans et al 2004
Galaxy at z1 is tidally sheared
Dark matter at z1
Hirata & Seljak
Intrinsic-shear correlation (GI)
Net anti-correlation
between galaxy
High z galaxy gravitationally ellipticities with no
sheared tangentially
prefered scale
Bridle & Abdalla
GI alignements:
Bridle & King
Different Cl contributions:
Removing intrinsic alignments:
• Finding a weighting function insensitive of
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•
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shape-shear correlations. (P. Schneider)
- Is all the information still there?
Modelling of the intrinsic effects (Bridle & King.)
- FOM definitely will decreased as need to
constrain other parameters in GI correlations.
Using galaxy-shear correlation function.
In any case there will be the need of a given
photometric redshift accuracy.
Intrinsic-shear correlation (GI)
and the galaxy-shear correlation
Galaxy at z1 is tidally sheared
Dark matter at z1
High z galaxy gravitationally
sheared tangentially
With position shear
correlation one can
know how much
alignement there is
Measurements of intrinsic
alignments using photo-z:
• Can measure intrinsic
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•
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Mandelbaum et al. 05
alignments with shearposition correlation
function.
Currently:
13000 2SLAQ gals
Proposal:
1400000 MegaZ-LRG gals
Probe z evolution
Collaborating with
S. Bridle, C. Blake and R.
Mandelbaum.
Bridle & King
High demand on photo-z
for intrinsic alignement
calibration
Abdalla, Amara, Capak
Cypriano, Lahav & Rhodes
Another way -> Modelling:
Are photo-zs good enough?
• PSF known.
• Redshifts are spectroscopic
• Given spectroscopy: Intrinsic
alignments easier to remove,
smaller systematic effect.
• But: is it feasible in practice.
Blake, Abdalla, Bridle, Rawlings 04
Explore other routes to weak
lensing:
Requires: (i) good image quality and low
systematics for measuring shear; (ii) source
density (iii) wide-field to beat down cosmic
variance (particularly away from strongly nonlinear scales); (iv) lensing tomography.
Conclusions
• Weak lensing is an important probe of cosmology.
• Today dw=1/10 prospect: dwxdw’=1/160 but there is a big
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demand on photometric redshifts, specially for future
surveys such as DUNE.
Need of around 10^5 spectra in ~5 redshift bins
Removing poor photo-z is possible, removes systematic
effects and does not hit the statistical limits of certain
surveys.
IR data can significantly improve FOM form 1.3 to 1.7
Importance of the u band filter, potentially being as
important as the IR.
It is possible to measure intrinsic alignments with
spectroscopic redshift surveys, need to assess it that is
possible with photo-z.
Future radio surveys will have much lass problems, i.e. no
photo-z issues, less GI - II issues. But is this feasible?