Dark Halo

 A dark halo is the inferred halo of invisible material (dark matter) that permeates and surrounds individual galaxies, as well as groups and cluster of galaxies.

Evidence for the existence of dark matter first came from studies of the motions of stars and gas in galaxies. For example, the quantity and distribution of luminous matter within disk galaxies cannot account for the rotation curves observed, implying a significant invisible component. In a similar manner, velocity dispersions measured in the outer regions of elliptical  galaxies are higher than expected given the luminous matter within the galaxy. Estimates based on these considerations suggest that perhaps as much as 90% of the matter present in galaxies is in the form of dark matter.

Evidence for dark matter is also found through observations of the motions of galaxies in groups and clusters. Using a similar argument to that applied to stellar motions within galaxies, the velocities of galaxies in groups and clusters are so high that the group or cluster would fly apart if only the luminous matter were present.

In addition, the X-ray emitting gas often found throughout galaxy clusters indicates that these clusters must contain large amounts of dark matter. With a temperature in excess of a million degrees, this gas would evaporate from the cluster if the visible elements were the only components.
In both galaxies and groups and clusters of galaxies, the dark matter is found to be distributed in a roughly spherical halo around the visible component – the dark halo. In the Milky Ways the dark halo appears to extend out to at least 300,000 light years and possibly even further, reaching far beyond the extent of the visible matter in the disk.

According to the standard model of cosmology, the vast majority of galaxies are surrounded by a halo of dark matter particles. This halo is invisible, but its mass exerts a strong gravitational pull on galaxies in the vicinity. A new study led by the University of Bonn (Germany) and the University of Saint Andrews (Scotland) challenges this view of the Universe. The results suggest that the dwarf galaxies of Earth's second closest galaxy cluster—known as the Fornax Cluster—are free of such dark matter halos. The study appeared in the journal Monthly Notices of the Royal Astronomical Society.

introduce an innovative way of testing the standard model based on how much dwarf galaxies are disturbed by gravitational ,tides' from nearby larger galaxies," said Elena Asencio, a Ph.D. student at the University of Bonn and the lead author of the story. Tides arise when gravity from one body pulls differently on different parts of another body. These are similar to tides on Earth, which arise because the moon pulls more strongly on the side of Earth which faces the moon.

The Fornax Cluster has a rich population of dwarf galaxies. Recent observations show that some of these dwarfs appear distorted, as if they have been perturbed by the cluster  environment. "Such perturbations in the Fornax dwarfs are not expected according to the Standard Model," said Pavel Kroupa, professor at the University of Bonn and Charles University in Prague. "This is because, according to the standard model, the dark matter halo of these dwarfs should partly shield them from tides raised by the cluster."

The authors analyzed the expected level of disturbance of the dwarfs, which depends on their internal properties and their distance to the gravitationally powerful cluster center. Galaxies with large sizes but low stellar masses and galaxies close to the cluster center are more easily disturbed or destroyed. They compared the results with their observed level of disturbance evident from photographs taken by the VLT Survey Telescope of the European Southern Observatory.

Elena Asencio says that "the comparison showed that, if one wants to explain the observations in the standard model. The Fornax dwarfs should already be destroyed by gravity from the cluster center even when the tides it raises on a dwarf are sixty-four times weaker than the dwarf's own self-gravity." Not only is this counter-intuitive, she said, it also contradicts previous studies, which found that the external force needed to disturb a dwarf galaxy is about the same as the dwarf's selfgravity.