Just like the dark matter that they are made of, dark halos have not been observed directly. But with the help of gravitational lensing, the detailed distribution of dark matter can be probed.
A dark matter halo is a region that has decoupled from cosmic expansion and contains gravitationally bound matter. A single dark matter halo may contain multiple subhalos.
Several cosmological models propose that dark matter halos and subhalos may contain galaxies and that they play a crucial role in their formation and evolution.
Dark matter halos surround the cores of the galaxies
The dark matter halo within a galaxy englobes the galactic disc and goes farther, passing beyond the boundaries of the galaxy. These halos’ presence can be observed through gravitational lensing.
“A gravitational lens is a distribution of matter between a distant light source and an observer, that is capable of bending the light from the source as the light travels towards the observer,” says in an article on Wikipedia.
The amount of bending is one of the predictions of Albert Einstein’s general theory of relativity. That effect could allow galaxy clusters to act as gravitational lenses.
“Unlike an optical lens, a gravitational lens produces a maximum deflection of light that passes closest to its center and a minimum deflection of light that travels furthest from its center,” according to Wikipedia.
Consequently, a gravitational lens has no single focal point, but a focal line. There are three classes of gravitational lensing:
- Strong lensing: where there are straightforwardly visible distortions.
- Weak lensing: where the distortions of background sources are shallower and can be identified by statistically surveying vast numbers of sources to find the appropriate distortions of only a few percent.
- Microlensing: where no warp in shape can be noticed, but the volume of light emitted by a background object differs in time.
The weak lensing and the distribution of dark matter within the clusters
Weak lensing can be used to reconstruct the mass distribution in the background distribution of dark matter.
Since galaxies are mostly elliptical and the weak gravitational lensing is shallow, a vast number of galaxies have to be observed during such studies.
In a new study, the astronomers surveyed approximately 3,200 galaxies with masses more significant than the Milky Way’s mass. They discovered that, even though invisible, massive dark matter halos are essential to the mass distributions of their central galaxies.
This verifies the hypothesis that massive galaxies’ cores formed along with the dark matter halos that surround them.