Professor Avishai Dekel (Credit: HUJI)

Galaxies, the building blocks of our universe, are each comprised of billions of radiant stars, which spread out across over 50,000 light years. Each galaxy has an unseen “spherical halo” of dark matter, which can only be detected through its massive gravitational attraction. Cosmologists usually divide the galaxies into two types – spiral and elliptical (although other types exist as well). Spiral galaxies, such as our Milky Way, rotate like a disk, constantly producing newborn stars, which give these galaxies a blue tint. Elliptical galaxies, on the other hand, primarily inhabit old, red stars that are devoid of gas and are often referred to as “dead stars.”

 

Astrophysicists have long been trying to understand the way in which these two types of galaxies are formed. Some experts say this question is the primary challenge facing modern cosmological researchers today, as galaxies formation is “an essential stage in the cosmological process that leads to the formation of life.” Until now, the standard model explained galaxy formations by spherical gas infall into a central disk, followed by mergers between disks. Stars were assumed to form slowly within the gaseous disks, which converted into globes as 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 per year.

Spiral galaxy (Credit: NASA)

Recent astronomical observations, however, put the accepted theory to question as new data was collected using advanced telescopes, which allowed scientists to examine the galaxies as they were 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 at all a result of galactic mergers” - said Avishai Dekel, professor of theoretical physics at the Racah Institute of Physics in the Hebrew University. The observations led researchers to ask the natural question – “How is it that these galaxies were able to form stars so quickly and in large quantities at such an early stage without massive galactic mergers?”

The scientists turned to computer simulations, which were carried out using one of the most powerful supercomputers in Europe, in hopes to accurately simulate how galaxies formed in the early universe. The resulting model suggests that galaxy formation is a direct result of a continuous flow of cold gas along a few narrow streams, not a by-product of 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 through the dark-matter halo of each galaxy and the hot gas that fills it and reach the center, where they become a rotating disk. These disks, each subject to its own, local, gravitational forces, break into a few giant clumps in which the gas converts into stars very efficiently” – explained the researchers. 

The giant elliptical galaxy ESO 325-G00 (Credit: NASA / J. Blakeslee, Washington State University)

One of the interesting results presented in the Nature article is a match between the rate of star formation as derived from the proposed theory and that which is actually observed. The researchers, who now refer to the massive star formers as “Stream-Fed Galaxies,” have laid out a physical theory that explains the formation of giant clumps in the early massive disks, and how they are driven by the cosmic streams.

 

The scientists predict that the migration of these clumps to the disk centers led to the formation of elliptical galaxies already in the early universe, independent of galaxy mergers, concluding that “the role of cosmic gas streams is not limited only to the formation of star-forming disks, but that these streams are also responsible for the subsequent formation of the red-and-dead elliptical galaxies.”

 

TFOT has previously covered another related research by Professor Avishai Dekel regarding a dark matter filament detected near the Milky Way. A different research study covered by TFOT was performed by researchers at the University of Arizona and looked at dark matter density in our solar system. The results of this research make it evident that the dark matter distribution in the solar system does not conform to the galactic halo distribution.

 

More information on Avishai Dekel’s research can be found here.