Exploring What is a through Gravitational Gravitational Lensing Lens?

„ A is formed when the light from a distant, bright source is "bent" around a massive object Exploring the Dark Universe (such as a massive or cluster of ) between Indiana University the source object and the observer „ The process is known as gravitational lensing, and is 28-29 June 2007 one of the predictions of Einstein's theory of general relativity (predicted by Einstein in 1936)

General Discovering Relativity Gravitational Lenses „ The lens phenomenon exists because gravity bends the paths of light rays „ Mysterious arcs „ Bright knots on the arcs show „ In general relativity, discovered in 1986 the structure of the intrinsic gravity acts by producing distribution of brightness of the curvature in space-time (a) Cluster (left) galaxies, whose images are „ cluster z=0.37 strongly distorted „ arc redshift z=0.735 „ The influence of individual (b) Cluster C12244 (right) cluster galaxies on the detailed „ The paths of all objects, whether or not they have mass, morphology of the arc in A370, are curved if they pass near a massive body „ cluster redshift z=0.31 which coils around these „ arc redshift of 2.24 galaxies, can also be „ Prediction of bending confirmed for starlight passing near recognized. the Sun in the 1919 solar eclipse

Three Classes of Unlike optical lenses, Gravitational Lenses gravitational lenses „ Strong lensing - easily visible distortions produce multiple images • Einstein rings, arcs, and multiple images The Einstein Cross „ Weak lensing - distortions are much smaller • Detected by analyzing large numbers of objects to find „ In an optical lens, maximum bending occurs distortions of only a few percent. furthest from the central axis • The lensing shows up statistically as a preferred stretching of „ In a gravitational lens, maximum bending occurs the background objects perpendicular to the direction to the center of the lens closest to the central axis „ A gravitational lens has no single focal point „ Microlensing - no distortion in shape can be seen but the „ If the source, the lens, and the observer lie in a amount of light received from a straight line, the source will appear as a ring background object changes with around the lens time • Microlensing occurs with „ If the lens is off-center, multiple images will The Double – and extrasolar planets appear. The lensed image will always be the first gravitational lens distorted

1 Simulating Gravitational Lenses Arcs in the „ HST MDS WFPC2 HST Gravitational Lens Galaxy Simulation (mds.phys.cmu.edu/ego_cgi.html) Cluster Abell 2218 (z=0.175) cluster center

source and source and „ Several arcs surround the cluster center lens aligned lens not • Arc A0 has a redshift of 2.515; aligned • Near A2 is another image of the same galaxy „ Several arcs also surround a second concentration of mass to the „ A galaxy having a mass of over 100 billion solar masses right above the middle of the picture will produce multiple images separated by only a few „ Multiple images of the lensed galaxy can be identified by comparing arcseconds the light distribution and by the spectra of the arcs well as by the „ Galaxy clusters can produce separations of several spectral properties of their light (i.e. of the colors) arcminutes „ A detailed model of the mass of the lensing cluster can be made with so many lensed galaxies

Cluster of Galaxies Cl0024+16 Galaxy „ The reddish objects are galaxies in the Cluster lensing cluster at z=0.39 Cl1358+62 „ The bluish objects are multiple images of a distant galaxy at z=1.63 lensed by the „ The reddish arc is a lensed image of a cluster background galaxy with z=4.92 „ The distant galaxy has been reconstructed „ upper right - an enlarged version of the lensed from models of the galaxy individual pieces of the „ lower right - a reconstruction of the unlensed arc source

The Bottom Line…

cluster center „ The visible matter does not provide enough gravity to produce the gravitational lenses we see from galaxies „ A thick arc with z=0.913 and galaxy clusters „ Two more arc systems are also seen Abell „ Dark matter must be (indicated by the letters A and B) present to account 2390 • system A has redshift z=4.04 • system B has redshift z=4.05 for what we observe

2 Mapping the Mapping the Dark Universe Dark Universe

The distribution The distribution of dark matter in of dark matter in the lens can be the lens can be mapped by mapped by modeling the modeling the distortions of the distortions of the lensed galaxies lensed galaxies

1E 0657-56 – Proof that Dark Matter Exists

The Bullet Cluster „ 1E 0657-56 – A collision of galaxy clusters „ A cluster of galaxies consists of three components

1. Galaxies 2. Hot Gas 3. Dark Matter Direct Observation of Dark Matter

The gas interacts, the dark What’s going on with 1E 0657-56? matter and galaxies don’t

„ The galaxies and dark matter pass through unimpeded, but the hot gas is separated from the clusters

„ Animation of 1E: 0657-56 „ TWO clusters of galaxies collide http://chandra.harvard.edu/photo/2006/1e 0657/1e0657_bullett_anim_lg.mpg

3 A Second Cluster Cluster Collisions Collision - „ The Bullet „ ZwCl0024+1652 Cluster ZwCl0024+1652 „ Collision seen „ Collision head-on seen from „ „ Dark matter A collision between two the side massive galaxy clusters 1-2 forms a ring billion years ago and 5 billion „ Dark matter light years away formed a ring and gas are of dark matter separated „ Our view of the collision is “head-on” „ The ring measures 2.6 million light-years across

„ The distribution of the dark matter is inferred from how its gravity bends the light of more distant background galaxies „ As the two clusters collided, the dark matter fell to the center of the combined cluster and then sloshed back out. As the dark matter moved outward, it slowed down under the pull of gravity and piled up, forming a ring

In a galaxy far, far away… Lensing Review

„ The Lynx Arc „ Lensing is more evidence for dark „ Biggest, brightest, matter hottest forming region „ Lensing tells us about the ever seen distribution and properties of dark „ 12 billion light matter years away „ Stars as hot as „ Gravitational lenses act on all 140,000 K kinds of electromagnetic „ >1,000,000 hot, radiation, not just visible light massive, blue- • Strong lenses have been observed white stars (Orion in radio and x-ray images has only 4!) „ Seen through a „ Lensing provides a “telescope” to gravitational lens view the distant universe

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