On January 22, 2009 an article was published in Nature by Hebrew University cosmologist Avishai Dekel and his collaborators, entitled "Cold streams in early massive hot haloes as the main mode of galaxy formation." In a January 25, 2009 news release from the Hebrew University in Jerusalem it was stated: "…A new theory as to how galaxies formed in the Universe billions of years ago has been formulated by Hebrew University of Jerusalem cosmologists. The theory takes issue with the prevailing view on how the galaxies came to exist.”

The new theory, motivated by advanced astronomical observations of the galaxies in the early Universe and based on state-of-the-art supercomputer simulations, maintains that the galaxies primarily formed as a result of intense cosmic streams of cold gas (mostly hydrogen) along the Cosmic Web.  The researchers show that big galaxy mergers, formerly assumed to have played a major role in galaxy formation, had only limited influence on the cosmological makeup of galaxies and the universe as we know it.

The galaxies are the building blocks of the Universe. They come in two main flavors, blue disks and red ellipticals, the formation of which is yet to be understood. Every galaxy is embedded in a spherical halo made of dark matter that cannot be seen but is detected through its massive gravitational attraction. The exact nature of this matter is still unknown.

The new theory of Dekel and his collaborates was echoed by the international mass media. The importance of this discovery, and the general excitement it arouse stems from the fact that it presents a theory for the main mode of galaxy formation, more than ten billion years ago, when most of the stars have formed and most of the mass assembly has occurred.

The date when this discovery became public coincides with the proclamation of the IYA 2009. We have thus asked Professor Dekel to address our PhysicaPlus visitors and readers on this occasion, and to answer some of our questions.

Q.    How you consider the importance of IYA 2009 in communicating to the general public recent discoveries in cosmology, astrophysics and physics in general.

R.  The International Year of Astronomy is indeed an opportunity to bring to the public the recent progress in our understanding of the physics of the Universe.  New, advanced telescopes, from the ground and from space, operating in the different wavelengths of the electromagnetic spectrum, are providing unprecedented rich data.  The great advances in cosmology during the last decade were driven by the discovery of the acceleration of the Universe via distant supernovae and the analysis of fluctuations in the Cosmic Microwave Background radiation from the WMAP space telescope. These measurements allowed us to determine the fundamental properties of the physical Universe to an accuracy of 1%, and determine in particular its fate to expand forever in an accelerated pace and the vast extent of space well beyond the observable part of 14 billion light years. The study of how structure has formed in the Universe can now proceed on a ``safe” ground.  The telescopes now allow us, on one hand, to detect small planets orbiting stars in our Milky-Way galaxy, and on the other hand to study galaxies out to distances of 12-13 billion light years, as they were only one billion years after the Big Bang. With the parallel advance in our physical understanding and in our ability to simulate the astrophysical processes using state-of-the-art computers, we can match the new observations with solid theoretical modeling. The theoretical progress involves almost every branch of modern physics, from the microscopic string theory to the macroscopic theory of gravity.  This parallel progress in observation and theory is the origin of the rapid developments in our understanding of the Universe.  
 
Q. Does your discovery challenge the standard paradigm of hierarchal galaxy formation claiming that small galaxies form first and larger galaxies are formed through mergers of smaller galaxies?
     
R. Not at all. To the contrary, our work represents a natural next step in our understanding of how galaxies formed within the standard hierarchical scenario, which is based on the standard cosmological model where the energy density is dominated by cold dark matter and dark energy. The dark-matter halos that serve as the sites of galaxies form at the intersections of dark-matter filaments, that construct a large-scale ``cosmic web”. The cold gas, made of regular matter of protons, neutrons and electrons, flows along the dark-matter filaments and penetrates into the halo centers. There, it forms a rotating disk. The continuous intense flux of gas makes the disk fragment into giant clumps in which turn into stars very efficiently. The incoming gas streams are partly smooth and partly clumpy (see picture 1). When these incoming clumps encounter the central disk, we have a merger. So the new picture is a generalization of the old picture where the mergers were assumed to play the central role in the buildup of galaxies and in generating star formation in them.

Q.    Your discovery is based on observations and computer simulations. What are the most important predictions which should verified by observations?

R.  We predict that the cold streams that feed the galaxies in the early Universe should be detectable by observations of a specific radiation from cold Hydrogen, termed Lyman-alpha radiation. We predict the properties of the emitting sources of Lyman-alpha; their total energy and their  irregular shape with finger-like features extending out to more than 100,000 light years away from the galaxy center.  Our theory also predicts the appearance of the early gaseous galactic disks, which is very irregular and showing giant clumps (see pictures 2,3).  The stellar distribution in these galaxies is predicted to show a central spheroid, or a ``bulge”, with mass comparable to the disk. This bulge was formed by migration of clumps from the outer disk into its center. It helps explaining the existence of red spheroidal galaxies already in the early Universe. 




Q.    In your theory of galaxy formation computer simulation shows matter flowing into the center of a galaxy through three cold gas streams. What was the source of this “cold” gas and what was its temperature?
                                 
R. The gas mass density is about 16% of the total mass density in the Universe. Initially, the gas was spread throughout the Universe, following the dark-matter distribution and forming together the cosmic web.  Before entering the galactic halos the gas is at a ``cold” temperature of about 10,000 degrees. When the gas enters the galactic halos, it was believed to heat up to about a million degrees, and thus not to fall very efficiently into the central galaxies. We found that the gas remains cold because it streams along the narrow, dense filaments, and that it penetrates into the halo centers very efficiently. This different way of feeding the galaxies with new material changes the whole picture of how the galaxies grew and how they formed stars in the early Universe.  

Q. What might be the implication of your theory of galaxy formation on the rapidly developing astrobiology with the discovery of numerous extrasolar planets?

R. The formation of galaxies is a key element in the chain of events that lead   to the appearance of life in the Universe. The accumulation of gas in galaxies brings it to the high densities and low temperatures that are required for star formation.  The stars act as giant nuclear reactors, where the initial primitive Hydrogen nuclei fuse into Helium and then into heavier elements such as Carbon and Oxygen which are the building blocks of life. These heavy elements are ejected from the first stars by supernova explosions into the interstellar medium, and then serve to form new stars and planets around them, where life can form. Understanding the process of galaxy formation is therefore directly related to astrobiology.

 Images: Credit  Dr. Daniel Ceverino and Dr. Tobias Goerdt, the Hebrew University

Interviewer: Dr. Alex Manes, Science Editor – PhysicaPlus Online