COSMIC MAGNIFICATIONthe other weak lensing

signal

Jes Ford UBC graduate student

In collaboration with: Ludovic Van Waerbeke

COSMOS 2010Jes Ford

Jason RhodesAlexis Finoguenov

Alexie LeauthaudHendrik Hildebrandt

Motivation

• Dark Matter Halo measurements constrain cosmological parameters and structure formation

• Weak Lensing has become an excellent cosmological probe of halos, but so far only to modest redshifts (zL < 1)

• Future surveys attempt to optimize the lensing science, requiring good photo-z’s

• Magnification can be measured too, without needing additional data

COSMOS 2010Jes Ford

Magnification Basics• Galaxies behind a foreground matter overdensity are gravitationally lensed

• 2 competing effects of magnification:– Flux Amplification: sources get brighter– Dilution: solid angle on the sky is stretched

• Who wins? Depends on slope of source number counts at that magnitude…

COSMOS 2010Jes Ford

Dilution & Amplification

• Point: text• Point: text • Point: text

COSMOS 2010Jes Ford

Lensing conserves surface brightness

Shear & Magnification

• Why Shear?– Signal-to-Noise: factor ~ 5 higher for shear (same sources)

• Why Magnification?– Don’t need shapes, just magnitudes & photo-z’s– Can probe much higher redshifts where source galaxies are unresolved (S/N factor ~ 2-3 higher for shear)

• Why not both? – Completely different systematics – Can break degeneracies in M & 8 – Magnification comes along basically for free

COSMOS 2010Jes Ford

The vast majority of weak lensing studies measure shear (shapes)

Theory

• Dilution vs Amplification:• WL limit: magnification ≈ 1, and to first order depends only on convergence

• Slope of number counts:– (-1) > 0 : amplification wins - we see more sources

– (-1) < 0 : dilution wins - we see less sources– (-1) = 0 : effects cancel out - no change in source density

COSMOS 2010Jes Ford

€

N(> f ) = μ−1No(> μ−1 f )

€

N(m,θ)dm = μα −1No(m)dm

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=2.5d logn(m)

dm

Dilution Amplification

Steps to measuring • Lenses:

– Xray selected groups1 in the COSMOS field, chosen with z < 1, M > 3.98 1013 M

– Additional Xray groups2 in CFHTLS D1, D4

• Sources: – LBG galaxies3 at z ≈ 3, from CFHTLS D1, D2, D4– COSMOS30 galaxies, 1.2 < z < 6

• Redshift separation & masking crucial• Cross-correlate: stacked lenses and sources in different magnitude bins… expect positive, negative, or no correlation depending on (-1)

• Combine magnitude bins: weighting by (-1)

COSMOS 2010Jes Ford

1. A. Leauthaud2. A. Finoguenov 3. H. Hildebrandt

Results: CFHTLS LBGsCOSMOS 2010

Jes Ford

Number counts of LBGs used

(-1) vs magHildebrandt et al. 2009

Results: CFHTLS LBGsCOSMOS 2010

Jes Ford

Number counts of LBGs used

(-1) vs magHildebrandt et al. 2009

Results: CFHTLS LBGsCOSMOS 2010

Jes Ford

Number counts of LBGs used

(-1) vs magHildebrandt et al. 2009

Bright LBGsare correlated

Faint LBGs are anti-correlated

Results: CFHTLS LBGsCOSMOS 2010

Jes Ford

Separate Mag BinsSignal from combined magnitude bins

Results: COSMOS30(preliminary)

COSMOS 2010Jes Ford

Correlation strength nicely decreases with increasing magnitude selection ie, with decreasing slope (-1)

Brightest SourceSelection

Faintest Selection

Future Work

• Optimal weighting: of (-1) on individual galaxies, not by average of magnitude bin

• Ideal source redshift selection: chosen for each foreground lens separately

• More sky coverage: will decrease uncertainties

• Dust absorption: by lenses is only ~ few % effect, but can be probed simultaneously

COSMOS 2010Jes Ford

Prospects for DM Halos • Prediction for 200 deg2 survey:

– Lenses: 135 stacked halos at z = 1, V200= 950 km/s, c200= 4.5– Sources: realistic number of LBGs, all at z = 3

• Promising method for weighing high-z dark matter halos

COSMOS 2010Jes Ford

Van Waerbeke et al. 2009

Conclusions• Magnification:

– will provide independent cosmological constraints– different systematics useful cross-check– is complementary to shear, does not replace it

• Future Wide & Deep Surveys: – will require accurate photo-z’s for shear– magnification measurements possible without additional data – If we ONLY do shear analysis, we IGNORE many unresolved galaxies whose shapes can’t be measured

Lets make full use of our shear catalogs and exploit the magnification signal as well!

Cosmos 2010Jes Ford

Thanks for Listening!