Gravitational Lensing - astro.sunysb.edu · Gravitational Lensing -Microlensing Microlensing •lenses too small for image distortions may still show up by magnification effect •source

Stony Brook University

Graduate Seminar Fall 2007

David Keitel

Gravitational Lensing

p. 2/24


What is it?

• deflection of light by massive objects...

• ... or large scale mass distributions

• an important test of General Relativity

• a powerful tool for all fields of astronomy

p. 3/24

Overview

• Basic mechanism

• Lensing by the sun

• Strong Lensing

• Weak Lensing

• Cosmic Shear

• Microlensing

• Results, current and future projects


[SHA]

p. 4/24

Gravitational Lensing – Basic mechanism

How does lensing work?

• light travels along geodesics of space-time

• large masses distort space-time

• multiple images with different positions on the sky

• distortions and magnifications may occur

p. 5/24


[SCH]

p. 6/24


• multiple solutions � multiple images

• beams of a light bundle will be deflected

differentially

• this gives distortion and, via Jacobian,

magnification:

p. 7/24


• lines in lens plane with detA=0: critical curves

• mappings of these in source plane: caustics

• highest magnifications occur near caustics

• image number changes by 2 on caustics

p. 8/24


Caustics Critical curves

[AHK]

p. 9/24


Einstein rings

[NBL]

• sources on axis are imaged into a ring

• Einstein angle:

• star-star lensing:

~ 1 mas

• galaxy-galaxy lensing:

~ 1 as[BOL]

p. 10/24


Mass models

• deflection angle depends on surface mass density

• a priori unknown

• models solved analytically or numerically

• results compared to observations, parameters tuned

p. 11/24

Gravitational Lensing – Lensing by the sun

First observation

• 1919: solar eclipse

• deflection of stars near

solar rim confirmed Einstein

[WC1]

p. 12/24

Gravitational Lensing – Lensing by the sun

Albert

EinsteinArthur Stanley

Eddington

A fun anecdote:

Einstein 1911:

Einstein 1915:

[NPO] [WC2]

p. 13/24

Gravitational Lensing – Strong Lensing

Strong Lensing

• High mass needed: big galaxies, galaxy clusters

• multiple images common, Einstein rings possible

• sources on cluster caustics are imaged into

giant luminous arcs

• arc radius yields Einstein angle and thus,

with model, cluster mass

p. 14/24


QSO 0957+561,

„twin quasar“

[YOU] [HAR]

Multiple quasar

images and

host galaxy

Einstein ring

[IMP]

p. 15/24


Abell 2218

[FRU]

p. 16/24

Gravitational Lensing – Weak Lensing

Weak Lensing

• Clusters also lens background sources far from caustics

• no strong effects, but shape distortions

• arclets or at least increased ellipticity

• statistical analysis can yield cluster mass

Gravitational Lensing – Weak Lensing

Abell 1689

[BEN]

p. 17/24

p. 18/24

Gravitational Lensing – Cosmic Shear

Cosmic Shear

• deflection not only by concentrated masses

• large scale structure has statistical influence

• measured by ellipticity distribution of large fields

• structure formation, cosmological parameters

p. 19/24

Gravitational Lensing - Microlensing

Microlensing

• lenses too small for image distortions may still

show up by magnification effect

• source moving behind lens gives characteristic light curve

• low probability, monitoring of large field required

• statistics give information about galactic structure

• surveys towards LMC estimate number of MACHOs

Gravitational Lensing - Microlensing

first microlensing

event towards LMC

[ALC]p. 20/24

p. 21/24

Gravitational Lensing – Results and projects

• predictions of General Relativity proven

• cluster and galaxy masses reconstructed

• tons of indications for dark matter

• major MACHO contribution to dark matter ruled out

• extrasolar planets found

• models of structure formation checked

• independent values of Hubble constant

and cosmological parameters

Results so far

p. 22/24

Gravitational Lensing – Results and projects

• extrasolar planet searches

• cosmological parameters from cluster lensing

• weak lensing search for filaments between clusters

• wide field surveys

• James Webb Space Telescope will enhance weak lensing

+ cosmic shear � dark energy, compare to Planck CMB

Current and future projects

p. 23/24


References

• [SHA]: K. Sharon, E. Ofek, ACS, HST, NASA / ESA (2006)

• [SCH]: P. Schneider - Gravitational Lensing, Lecture Notes (2007)

• [AHK]: H. Asada, T. Hamana, M. Kasai - A&A 397, 825-829 (2003)

• [NBL]: R. Narayan, M. Bartelmann - Lectures on Gravitational Lensing (2007)

• [BOL]: A. Bolton, ACS, HAST, NASA / ESA (2005)

• [WC1]: commons.wikimedia.org/wiki/Image:1919_eclipse_positive.jpg

• [NPO]: nobelprize.org/nobel_prizes/physics/laureates/1921/index.html

• [WC2]: commons.wikimedia.org/wiki/Image:Eddington_2.jpeg

• [YOU]: P. Young et al. - ApJ 244, 736 (1981)

• [HAR]: M. Harvanek et al. - AJ 114, 2240 (1997)

• [FRU]: A. Fruchter (STScI) et al., WFPC2, HST, NASA (2000)

• [BEN]: N. Benitez (JHU) et al., ACS, HST, NASA (2002)

• [ALC]: C. Alcock et al. - Nature 365 621 (1993)

• [MCN]: M. Markevitch et al., D. Clowe et al., NASA/STScI (2004)

• [JFN]: M.J. Jee, H. Ford, ACS, HST, NASA / ESA (2004)


Any Questions?

„bullet cluster“

„dark matter ring“

[JFN]

[MCN]

Gravitational Lensing - Appendix


• PLANET collaboration since 1995

• About 500 events a year

• 170 planets identified so far

Search for exoplanets

References:

• planet.iap.fr

• Dominik et al., proceedings of the XIXth IAP colloquium, Aps. Conf. Ser. (2004)

• Beaulieu et al., Nature 439, 437 (2006) - arXiv:astro-ph/0601563v1


Cosmological parameters


Reference: Spergel et al., ApJ (2007) - arXiv:astro-ph/0603449v2


CLASS, Chae et. al 2002:

Soucail et al. 2004:

Allen et al. 2004:

Voevodkin & Vikhlinin 2004:

Some values from Lensing:

Hubble Key project:

Hubble constant:

From strong

lensing on

clusters

(Freedman et al. 2001)

for different galaxy lens models (Schneider 2007)

Lensing:

Gravitational Lensing - astro.sunysb.edu · Gravitational Lensing -Microlensing Microlensing •lenses too small for image distortions may still show up by magnification effect •source

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