Evidence for Dark Matter What is Dark Matter

Evidence for Dark Matter

What is Dark Matter? Dark matter is matter which feels and exerts the gravitational force, but which does not feel or exert the electromagnetic force, so it does not interact with (absorb or emit) light. n Dark matter also (probably) does not interact strongly with other matter or with itself. n

How do you find it? n n n Since dark matter doesn’t interact with light, you can’t photograph it. Dark matter can be found by its gravitational effects. Dark matter was first found by looking at the way that stars orbit in spiral galaxies. The stars orbited the galaxies too fast to be explained by normal matter – dark matter was exerting a gravitational force to speed up the stars!

1 E 0657 -56 The object 1 E 0657 -56 is a pair of galaxy clusters that have “recently” collided. n When clusters collide, the galaxies themselves pass right by each other, and are not really effected. n But the hot gas in the clusters (which makes up 90% of the visible mass) does interact. n

1 E 0657 -56 in Visible Light

1 E 0657 -56 in X-Rays

Finding the Dark Matter So when the clusters of 1 E 0657 -56 collided, the hot gas slowed down because it interacted, while the galaxies kept moving, because they didn’t interact. n The dark matter should have stayed with the galaxies, since dark matter doesn’t interact with other matter very well. n

Finding the Dark Matter So the dark matter should now be offset from the visible matter – they should be separated. n If we can find where the gravity in the system is concentrated, we can find whethere’s dark matter. n

Finding the Dark Matter Gravity bends light. n So light from a galaxy that has passed a large object (like a galaxy cluster) will produce a stretched image – like the light has passed through a lens. n Both visible matter and dark matter have this effect. n

Gravitational Lensing

Mapping out the Gravity By analyzing the shapes of background galaxies (much farther away than the cluster), the shape of the gravitational lens can be found, so the location and amount of mass can be found. n The lensing mass was found to be offset from the visible mass! n

Location of the Dark Matter

Dark Matter Offset from Visible

1 E 0657 -56 Collision

Finding Dark Matter Remember, 90% of the visible matter in the cluster is in the hot x-ray gas (not in the galaxies themselves). n The mass producing the gravitational lens was located with the galaxies – it was not the visible mass (the x-ray gas). n So the most of the mass is in dark matter with the galaxies. n