Gravitational lens

In Astrophysical, a gravitational lens or preferably a gravitational mirage is a very massive object (a galaxy cluster for example) being located between an observer and a light source “” remote. The gravitational lens, printing a fort gravitational Field around it, will have as effect to make deviate the luminous rays which will pass close to it, thus deforming the images which an observer placed on the line of sight will receive.

Predicted by the General relativity of Albert Einstein, the gravitational lenses have had for summer observed of many times by, amongst other things, the Space telescope Hubble. Loins to be exotic, they are abundantly present when one makes stereotypes of fields deep of the observable Univers. They are the subject of several studies, its effects are used in particular for detection of the black Matière present in our Univers.

Origin of the deformation

A massive star, such as a star, a Black hole or a Galaxy, curve the light.

By observing some Galaxy S or some Quasar S, one attends curious optical effects sometimes: their image is duplicated, tripled or even quintupled with some seconds of arc of distance or take the shape of arcs curved around a central axis. These multiple images are in all points into perfect Corrélation S. According to the General relativity, a distribution of Masse, for example the Sun, deviates the luminous rays which pass in the vicinity. That confirms the existence of a Masse sinks deforming some share in the center of the field, between the object and the Ground.

Let us imagine that, by chance, a close galaxy and a remote quasar are aligned on the same line of sight, i.e. exactly in the same direction of the sky. The Lumière which comes to us from the quasar then is strongly deviated at the time of its passage close to the galaxy. Thus for example, the luminous rays which pass slightly above the galaxy are deviated to the bottom and give place to an image of the quasar shifted upwards. On the other hand, the luminous rays which pass under the galaxy are deviated to the top and give rise to an image of the quasar shifted downwards. In this way, the close galaxy, by disturbing the light propagation of the quasar, gives rise to several images of this one. The full number of images is determined by the shape of the galaxy and the precision of alignment. Sometimes, when alignment between the two objects is perfect, the image of the remote object can be modified at the point to take the form of a Anneau luminous surrounding the image of the close object.

In addition to multiplying the images of the quasar, the galaxy also will concentrate the light of this one and thus will produce images much more brilliant. An effect which is far from being negligible when bodies far from luminous are observed.

History

Fritz Zwicky had predicted in 1937 which the galaxies could cause of the gravitational effects on the light of the sources that they came to to occult. It applied thus the laws of the General relativity stated by Einstein.

But 1979 had to be awaited so that the first real example is observed. The British astronomer Refusals Walsh and its collaborators of the Kitt Peak cherished the obscure desire to identify optically Radiosource S. the March 29th they observed two images of a Quasar baptized Q0957+561A-B. Two separate objects of 6" were of magnitude 17,5 and rigorously presented same the spectrum, with a Décalage towards the red of 1,407. Walsh supposed that it was about the duplicated image of a single quasar. Later observations confirmed it and showed that the gravitational lens in this case was consisted a elliptic Galaxie giant four times closer to us than the quasar. For this time, tens of images multiple of quasars have been discovered, with two, three or four components.

In 1988, the radioastronomers of VLA discovered a lens gravitational in the shape of ring, MG 1131+0456, in conformity with the theory. This object was baptized “the Anneau of Einstein” in its memory. It is probably about the image of a quasar deformed by an invisible dwarf galaxy located at the foreground. In the contrary case, the physicists will have to explain the reality of its form.

In 1995, for example, the Space telescope Hubble revealed the very impressive example of the Galaxy cluster Abel 2218 which produces multiple images of a whole population of remote galaxies and gives rise to more than 120 luminous arcs.

Constant of Hubble

Ring of Einstein

In the event of perfect alignment of the source observed (such as a star), stellar object, playing the part of gravitational lens - also called deflecting - (such as a Black hole) and observer, one will not see more star like such but rather like a ring named ring of Einstein . It is noted:

\theta_E

Calculation of the ring of Einstein

The size of a ring of einstein is given by the Rayon of Einstein. Here in Radian S:

\ theta_E = \ left (\ frac {4GM} {c^2} \; \ frac {d_ {LS}} {d_L d_S} \ right) ^ {1/2}

$G$ being the Constant gravitational,

M

being the Mass of the lens,

c

being the Speed of light,

d_L

being distance from the observer to the lens,

d_S

being the distance from the observer until the source

d_ {LS}

being distance enters the lens and its

source