![what is dark matter made of?](../../images/GrabItarrowbD.gif)
How do we know it exists?
(continued)
![](../../darkmatter/images/lente1a.jpg) |
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![](../../darkmatter/images/archi_2218.jpg) |
Fig.
3: Double image to show effect of gravitational lens. The photo, taken by the
Hubble Space Telescope
(HST 14164+5215)shows in the centre the "lens"galaxy placed between earth and the source.
(Credits:
Kavan Ratnatunga
(Carnegie Mellon Univ.) and NASA)
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Fig.
4: The Abell 2218 cluster of galaxies. This cluster is so dense and compact that light coming from very far away,
situated beyond the cluster in the direction of observation, is amplified and distorted in the form of luminous arches.
(Credits:
NASA,
Andrew Fruchter and the ERO Team [Sylvia Baggett (STScI), Richard Hook
(ST-ECF), Zoltan Levay (STScI)] (STScI)) |
- Gravitational lenses. MACHO.
When the lens is made up of an astronomical body with a mass less than that of the sun (we talk
in this case of a microlens, and these celestial bodies are called
MACHO
),
as in the case of an isolated planet, for example, the angular separation of the images is too small to be able to see
the double image. However, if the source is fixed and the lens is moving in relation to the star, we can observe apparent
variations of luminosity in the light emitted by the source star: luminosity is best when the source, lens and observer are
all in line; luminosity is reduced (and this is the true luminosity) when the lense is moved out of line. By measuring
how much the light of the source has been amplified, we can "weigh" the non-visible object that generates the gravitational lens.
![](../../darkmatter/images/MACHO.gif) |
|
Fig.
5: Artist's impression of a gravitational lens produced by a MACHO. Click on figure to see animation.
(If you can't see it, download Flash Player [qui]).
(Credit:
Davide Centomo, Dip. di Fisica Universitą di
Bologna) |
There has been a systematic analysis
of the light variations of the stars in the Grande Nube di Magellano, a small satellite galaxy in our
Milky Way
. In some cases,
light variations in the stars have been observed and interpreted as being due to lenses made up of invisible
objects with a mass similar to that of large planets which, by moving on the edge of our galaxy, intercept
the light of certain stars. A certain number of these bodies has been identified, but their mass and number
contribute only in a small way to dark matter .
Gravitational lenses help us to obtain information about the presence of non-visible matter located in specific
places, such as large planets,
black holes ,
and galaxies. This dark matter should be of the same type as visible matter (ordinary matter,
baryonic matter ).
![](../../darkmatter/images/globe-small.gif) |
(How do we know… - page 2 of 2)
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