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Phys 43
Spring 2011
May 9, 2011
Roman Savinov
Karan Kanagasabapathy
Carlos Plascencia
Attention! must be seen in the full screen mode. Hit
• What is Gravitational Lensing
• Types of Lensing
• Why do we care
• How these things are found
What is Gravitational Lensing
Light does not always travel in straight lines…
… massive objects deform the
fabric of space itself...
… when light passes one of
these objets, such as a cluster
of galaxies, its path is changed
What is Gravitational Lensing
This effect is called gravitational lensing.
It is only visible in rare cases
and only the best telescopes
can observe the related
What is Gravitational Lensing
What is Gravitational Lensing
simulation of gravitational lensing
caused by a Schwarzschild black hole
passing in front of a background
Unlike an optical lens,
maximum 'bending'
occurs closest to, and
minimum 'bending'
furthest from, the center
of a gravitational lens.
So lensing effect can magnify and distort the image of the
background source.
The surface brightness of the source image remains constant, but
its angular size vary, hence producing an amplification of the
galaxy luminosity as seen by a distant observer. Maximum
amplification occurs when the galaxy (or in this case a bright part
of it) is exactly behind the black hole.
What is Gravitational Lensing
Einstein Ring Gravitational Lens:
SDSS J162746.44-005357.5
If the distant galaxy, the lensing mass, and the observer lie in a
straight line, the original galaxy will appear as a ring around the
massive lensing object, called Einstein ring.
If there is any misalignment the observer will see an arc
segment instead.
Einstein Ring Gravitational Lens:
SDSS J120540.43+491029.3
What is Gravitational Lensing
More commonly, when the lensing mass
is complex (such as galaxy groups and
clusters), the observer may then see
multiple distorted images of the same
Got it?
…. the number and shape of
these images depend upon
the relative positions of the
source, lens, and observer,
and the shape of the
gravitational well of the
lensing object.
Types of Lensing
There are three classes of gravitational lensing
•Strong lensing:
lensing effect that is strong enough to produce
multiple images, arcs, or Einstein rings
•Weak lensing:
Einstein Cross. Four images of the
same distant quasar appear around a
foreground galaxy due to strong
gravitational lensing.
In this type of lensing the distortions of background
sources are much smaller and can only be detected by
analyzing large numbers of sources to find coherent
distortions of only a few percent. Thus it is an
intrinsically statistical measurement.
no distortion
in shape
be seen
the amount
of light
Weak gravitational
in the path
of areceived
light ray
a background
in time.
as it crosses
universe. This
the path
light isfrom
only a few
galaxies (blue
at the back
of the
box) as by
it journeys
Why do we care
Abell 2218
Gravitational lenses can be used as gravitational telescopes.
The concentrated light from objects seen behind gravitational lenses
makes very faint objects appear brighter, larger and therefore more
easily studied.
One of the most distant object in the universe was discovered by the gravitational lensing
Abell 2218, a cluster of galaxies about 2 billion
light-years away, was used as lens to see a galaxy
some 13 billion years old
This galaxy at the background of the Abell 2218
A faint galaxy is outlined by a white ring in the shape of a
Abell 2218
is seen from Earth as it would have been just 750
sausage. The image is elongated due to the gravitational-lensing
million years after the Big Bang.
effect of a closer galaxy cluster.
Why do we care
Observations of gravitational lensing can also be inverted to examine the lens itself.
Gravitational lensing is particularly useful if the lens is
for some reason difficult to see.
If the foreground lensing star has a planet, then that
planet's own gravitational field can make a detectable
contribution to the lensing effect.
Gravitational lensing can probe the amount and distribution of mass.
Mass of the universe is dominated by invisible dark
The mass of lens’s constituent particles and their crosssection affect the distribution of dark matter in lens
systems. Thus it is possible to determining how much
dark matter they contain
If the foreground lensing star has a planet, then that
planet's own gravitational field can make a detectable
contribution to the lensing effect.
3D map of the large-scale distribution of dark matter,
reconstructed from measurements of weak gravitational
lensing with the Hubble Space Telescope.
How these things are found
In the past most of the gravitational lenses have been discovered accidentally
Today the search is done at 20 GHz frequency in the radio domain of the
electromagnetic spectrum
A big chunk of known lenses today were
discovered by VLA -radio astronomy observatory
in New Mexico
The lensing is easier to detect and identify in
simple objects compared to objects with
complexity in them
Very Large Array (VLA) - a radio astronomy
observatory in New Mexico. Was used to discover
most of the gravitational lenses known today.