1. COSMIC MAGNIFICATION the other weak lensing signal Jes Ford UBC graduate student In collaboration with: Ludovic Van Waerbeke COSMOS 2010 Jes Ford Jason Rhodes Alexis Finoguenov Alexie Leauthaud Hendrik Hildebrandt

2. 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 (z L < 1) Future surveys attempt to optimize the lensing science, requiring good photo-z’s Magnification can be measured too, without needing additional data COSMOS 2010 Jes Ford

3. 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 2010 Jes Ford

4. Dilution & Amplification Point: text COSMOS 2010 Jes Ford Lensing conserves surface brightness

5. 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 2010 Jes Ford The vast majority of weak lensing studies measure shear (shapes)

6. 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 2010 Jes Ford Dilution Amplification

7. Steps to measuring  Lenses: –Xray selected groups 1 in the COSMOS field, chosen with z 3.98  10 13 M  –Additional Xray groups 2 in CFHTLS D1, D4 Sources: –LBG galaxies 3 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 2010 Jes Ford 1. A. Leauthaud 2. A. Finoguenov 3. H. Hildebrandt

8. Results: CFHTLS LBGs COSMOS 2010 Jes Ford Number counts of LBGs used (  -1) vs mag Hildebrandt et al. 2009

9. Results: CFHTLS LBGs COSMOS 2010 Jes Ford Number counts of LBGs used (  -1) vs mag Hildebrandt et al. 2009

10. Results: CFHTLS LBGs COSMOS 2010 Jes Ford Number counts of LBGs used (  -1) vs mag Hildebrandt et al. 2009 Bright LBGs are correlated Faint LBGs are anti-correlated

11. Results: CFHTLS LBGs COSMOS 2010 Jes Ford Separate Mag Bins Signal from combined magnitude bins

12. Results: COSMOS30 (preliminary) COSMOS 2010 Jes Ford Correlation strength nicely decreases with increasing magnitude selection ie, with decreasing slope (  -1) Brightest Source Selection Faintest Selection

13. 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 2010 Jes Ford

14. Prospects for DM Halos Prediction for 200 deg 2 survey: –Lenses: 135 stacked halos at z = 1, V 200 = 950 km/s, c 200 = 4.5 –Sources: realistic number of LBGs, all at z = 3 Promising method for weighing high-z dark matter halos COSMOS 2010 Jes Ford Van Waerbeke et al. 2009

15. 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 2010 Jes Ford Thanks for Listening!