Galaxy-Galaxy Gravitational Lensing

1. Galaxy-Galaxy Gravitational Lensing Myungshin Im Astronomy Program School of Earth and Environmental Sciences Seoul National University

2. Outline of the Talk • Introduction • Strong Lensing and Cosmology • Weak Lensing and Mass Profile of Galaxies. • Newly Discovered Strong Lens • Conclusion

3. Gravitational Lensing 1 • General Relativity – Light ray traveling through inhomogeneous gravitational field gets deflected. • Tidal gravitational field leads to the different amount of deflection on different parts of an extended source. • This results in the change in the size and shape of the extended source. • Also change in the flux (magnification).

4. Gravitational Lensing 2 • When the lens is a galaxy, and the distorted background images are also distant extragalactic objects, we call it “galaxy-galaxy” lensing. • Lens can be a cluster of galaxies, etc. • Strong lensing, weak lensing, and microlensing.

5. Strong Lensing • Lenses with easily visible distortions such as the formation of Einstein rings, arcs, and multiple images.

6. Shear due to weak lensing Weak Lensing • Lenses with much smaller distortion on the background objects which can be found by analyzing many background objects, and the distortion is of order of a few percent.

7. Strong lensing and Cosmology • Various lens related observables are sensitive to cosmological parameters.

8. Lens Separation(Im, Griffiths, and Ratnatunga 1997, ApJ 475, 457) dLS : distance btwn lens and source. dS : distance to the source. dL : distance to the lens. s*: characteristic vel. Dispersion. m: apparent magnitude of the lens galaxy. Critical radius (radius of the Einstein Ring): Sensitive to the cosmological constant (Gott, Park, & Lee 1989; M. Park 2003). ~ dLS dL / dS

9. Dependence on Cosmology DLS DL / DS (h-1 Mpc) zs=2.5, assumed. (Lens Redshift)

10. Method • The method was applied to 7 strong gravitational lenses at zL = 0.25 – 1.0 . • Critical radius was calculated assuming the singular isothermal sphere model. • The probability of a lens having the observed parameters • was computed. • Cosmological parameters that gives the maximum likelihood were found.

11. Result • = 0.64 (+0.15,-0.26), if Wm + L = 1 Reported before the Type Ia Supernova results.

12. Other Methods • Lens statistics (Fukugita et al. 1992, Kochanek 1996; Chae et al. 2002) The number of strong lens systems is senstive to cosmological parameters: 1. The critical radius is cosmology sensitive (cross section of the lensing). 2. The number of lensing galaxies is sensitive to the cosmological parameters, because of the volume effect (comoving density is assumed). 2. The number of sources behind the lens is sensitive to the cosmological parameters, because of the volume effect. Early studies (Kochanek 1996) favored zero L, but more recent studies find non-zero L (Chae et al. 2002, 2004), consistent with the WMAP-SNe result.

13. Weak galaxy-galaxy lensing Effective tool to study the mass profile of galaxies out to large radii: Rotation curve studies have limitations at the large radius because of the low surface brightness. Other applications include studying mass distribution of the Universe.

14. Weak Lensing Shear and Galaxy • Tidal gravitational field causes the differential stretching of galaxy light. • Shape changes • Orientation angle of the background galaxy relative to the lens galaxy changes  tangential alignment (<f> > 45도). f

15. Galaxy-galaxy weak lens study(Griffiths, Casertano, Im, Ratnatunga et al. 1996, MNRAS 232, 1159) • 400 ellipticals and 1200 late-type galaxies at I < 22. • Tangential shear is measured around these galaxies using 22 < I < 26 background galaxies. • Mean tangential position angle was measured as a way to detect the shear. • Several halo mass distribution models were tested against the observation • Light distirbution = mass distribution? • Truncation of mass profile – where?

16. Detection of Shear around Elliptical Galaxies Half light radius

17. No Such Signal around Stars (Control Sample)

18. Weak Lensing Study • The mass distribution that follows the light distribution cannot explain the observed amount of shear. • Truncation radius (r_t) is greater than 20-30 r_hl (half light radius). • Mass to light ratio > ~ 100 out to large radii. • Recent works confirm the earlier findings (e.g., Wilson et al. 2000; Hoeskstra et al. 2004; Y. Park et al. 2005).

19. Newly Discovered Strong Lens(Im et al. 2005, in preparation) Arc! 평범한 타원은하? Discovered from the Hubble Space Telescope Image of the Spitzer First Look Survey Field (RA=17:18:17.6, DEC=59:31:46).

20. New G-G Strong Lens z=0.08 2.2” z=0.245 Subtraction of galaxy light Strong G-G lens system with the lowest lens redshift?

21. CFHT Image of the Strong Lens s ~ 300 km/sec from lens analysis. First g-g strong lens discovered by an astronomer in a Korean institution? This kind of lenses would be useful for studying galaxy mass profile too. Strong lens searches are ongoing.

22. Summary • Strong lenses = useful for studying the cosmological parameters (Im et al. 1997; Chae et al. 2002). • Weak galaxy-galaxy lensing = good tool to study mass distribution (Griffiths et al. 1996). • More applications are out there. • A relatively large number of Koreans working on gravitational lensing.