They've named it after Eris, the Greek goddess of strife and discord, because of the decades of debate in the field about the formation of spiral galaxies. Simulating spiral galaxies such as the Milky Way is important in confirming that researchers understand the underlying physical laws of the universe.
However, all previous attempts to recreate the formation of spiral galaxies like the Milky Way faltered on one of two points: either the simulated spiral galaxies displayed too many stars at the center or the overall stellar mass was several times too big. This simulation, shown here in a video, is the most accurate thus far. From it, researchers have predicted that the ratio of cold dark matter to matter must be , not as previously assumed, and that there must be stars and gases in the outer halo of the Milky Way six hundred thousand light years away, which have not yet been observed.
Credit : A. The stars in the disks of spiral galaxies are generally younger than the majority of stars found in the bulge and halo. For this reason, disks are thought to form after the primordial collapse event responsible for the formation of the spheroidal bulge and halo, possibly through the cooling of the hot gas contained within the halo of the newly formed galaxy.
However, this cannot be the whole picture, as many spiral galaxies possess two distinct disk structures a thick disk and a thin disk which vary in content thick disks are composed entirely of stars while thin disks also contain cold gas as well as thickness. The gas and dust contained within the spiral arms of the thin disk are continually creating new stars through secular evolution processes.
Each red knot is a newly formed massive star cluster with the reddish colour the result of extinction. In the Milky Way, on average, the thick disk is older than the thin disk but younger than the bulge. Differential rotation causes any disturbance in the disk to wind up into a spiral form. The trouble with this simple explanation is that the differential rotation would cause spiral features to wind up too quickly, so galaxies would not look like spirals for any appreciable length of time.
That force helps maintain the spiral form against the tendency to wind up. Almost everyone agrees on this basic physics. There is observational evidence that nearby companion galaxies or an asymmetric, bar-shaped concentration of mass can drive a spiral wave in the disk of the galaxy.
Disks that lack such forcing features are the tricky ones to explain. One explanation centers on the fact that gravitational systems act to increase their central binding energy. Spiral arms remove angular momentum from the center of the galaxy, allowing it to achieve a state of higher binding energy. There are two main versions of the theory of spiraling: one in which the waves are steady and long-lived, the other in which spirals are transient features that come and go.
The natural, but not very easy, test is to observe spiral galaxies for a few hundred million years and see what happens. Elmegreen, staff scientist at the IBM T. Watson Research Center, have extensively studied this question. Here is their response: "Most spiral arms in galaxies are density waves, which are compression waves like sound that travel through the disk and cause a piling-up of stars and gas at the crest.
The wave is temporarily sustained by the force of its own gravity, but it eventually wraps up or gets absorbed at orbital resonances, places where random stellar oscillations have the same period as the local wave. In all cases, the stars and gas rotate around the galaxy's center faster than the wave in the inner parts of the disk, and slower than the wave in the outer parts.
Most of the galaxies that scientists have discovered so far are spiral galaxies, as opposed to the other two main categories of galaxy shapes — elliptical and irregular. The Milky Way — the galaxy that includes Earth and our solar system — is an example of a spiral galaxy. Spiral galaxies make up roughly 72 percent of the galaxies that scientists have observed, according to a Hubble Space Telescope survey.
Most spiral galaxies contain a central bulge surrounded by a flat, rotating disk of stars. The bulge in the center is made up of older, dimmer stars, and is thought to contain a supermassive black hole. Approximately two-thirds of spiral galaxies also contain a bar structure through their center, as does the Milky Way.
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