Debris Trails in Galaxies

Astronomers thank that many galaxies (including our Milky Way) grew to their present size by capturing small, nearby galaxies. Such captures may occur when the gravity of a larger galaxy pulls on a passing smaller galaxy. The smaller galaxy falls into the larger one and is gradually pulled apart by the larger one. Evidence supporting this hypothesis comes from the detection of long, thin streams of stars (called "debris trails") found in the halo of the Milky Way and other large galaxies. The picture below shows such a debris trail

Star Streams in NGC5907, a disk galaxy seen nearly edge-on


Science at Work -- Dark Matter or Maybe Not
Although the majority of astronomers think that the existence of dark matter is the best way to explain the observed high rotational speed of disk galaxies and the high speed of galaxies within clusters of galaxies, not all astronomers agree. A few have proposed that Newton?s laws of motion need a correction if the acceleration is very small, such as occurs with motions on the immense scale of galaxies. One such proposal is referred to as the MOND hypothesis, for Modified Newtonian Dynamics. Proponents of the MOND hypothesis have had good success in using the theory to explain motion within galaxies (including the Milky Way) and within galaxy clusters. Moreover, they claim that MOND models fit the observed motions better than standard models without MOND. On the other hand, observations using the theory of gravitational lenses to map the distribution of mass in galaxy clusters strongly favor the dark matter hypothesis. How can these competing ideas be resolved? One way would be the convincing detection of dark matter particles, and recently some claims to this have been hinted it. The issue is not yet settled