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Hebrew University cosmologists have formulated a new theory on how galaxies form. The new theory, published in a recent issue of Nature, arose from advanced astronomical observations and state-of-the-art computer simulations.
The research group led by Prof. Avishai Dekel of HU's Racah Institute of Physics claims that galaxies form primarily as a result of intensive cosmic streams of cold gas (mostly hydrogen) and not - as current theory holds - due to galactic mergers. The researchers show that these mergers had only limited influence on the makeup of the universe. The observations were led by researchers in Garching, Germany, headed by Prof. Reinhard Genzel of the Max Planck Institute, whose group is collaborating with the HU researchers.
Each galaxy contains about 100 billion radiant stars such as our Sun. An average spiral galaxy extends across about 50,000 light years and is embedded in a spherical halo made of mysterious dark matter whose existence is deduced because of it apparent gravitational effect. Spiral galaxies such as our Milky Way are rotating disks rich in hydrogen and constantly forming new stars, which give them a blue tint. In contrast, elliptical galaxies have a larger, more rounded shape and are made of old, red stars; they are therefore referred to as red and dead.
The attempt to understand the way in which these two types of galaxies form is the primary challenge facing modern cosmologists, and the formation of galaxies is an essential stage in the process that leads to life. The accepted model until now was based on the idea of spherical gas infall into a central disk, followed by mergers between disks. The assumption is that the stars form slowly within the gaseous disks, and that the disks became globes when they merged. In such a merger, the colliding gas clouds produce a big burst of new stars at a rate of hundreds of solar masses a year.
This model has lately been questioned as a result of observations using more powerful telescopes that enable observations of what happened some 10 billion years ago (about three billion years after the Big Bang). "The large galaxies, as they appear in this early stage, indeed created stars at a very rapid rate, but this does not appear to be a result of galactic mergers," says Dekel.
So how were these galaxies able to form stars so quickly and at such an early stage without massive galactic mergers?
In the Nature article, Dekel and his Israeli and French associates pose their new theory, which explains these observed phenomena. Based on simulations using one of the most powerful supercomputers in Europe, they made a detailed investigation of how galaxies formed in the early universe.
The picture that emerges is of galaxy-building that results from a continuous flow of cold gas along a few narrow streams rather than by mergers. These gas streams follow the filaments of the "cosmic web" that defines the large-scale structure of matter in the universe - filaments that feed the dark-matter halos in the first place. These cold gas streams penetrate the dark-matter halo of each galaxy and reach the center, where they become a rotating disk. These disks, each subject to its own, local gravitational forces, break into giant clumps in which the gas becomes stars very efficiently.
Dekel and colleagues worked out a simple physical theory that explains the formation of giant clumps in the early massive disks, and how they are driven by the cosmic streams. They predict that the migration of these clumps to the disk centers led to the formation of elliptical galaxies, independent of galaxy mergers.
They are thus making the revolutionary proposal that the role of cosmic gas streams is not limited to the formation of star-forming disks, but that these streams are also responsible for the formation of red-and-dead elliptical galaxies. New state-of-the-art simulations seem to confirm this theory.
NUTS IN CARS
A research team at Baylor University in Texas has developed a way to use coconut husks in automotive interiors, potentially providing work for poor coastal regions where the nuts grow. Coconut fiber, they say, can serve as a replacement for synthetic polyester fibers in compression-molded composites for trunk liners, floorboards and interior door covers on cars. With an estimated 11 million coconut farmers in the world making an average of $500 a year, the Baylor researchers hope to triple the farmers' income by increasing the market price for each coconut to 30 cents.
The team is working closely with a Texas-based fiber processing company that is a supplier of unwoven fiber mats to four major automotive companies. Engineering Prof. Walter Bradley said the mechanical properties of coconut fibers are just as good for automotive parts as synthetic and polyester fibers. Coconut fibers are less expensive and better for the environment because the husks would have otherwise been thrown away. The nuts also don't burn well or give off toxic fumes - crucial qualities in commercial automotive parts.
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