Oct. 10, 2001
Paul Wiita, professor of astronomy
Stacie Sutton, University Relations/science writer
ASTRONOMERS FIND NEW CLUES ABOUT GALAXY FORMATION
ATLANTA - Astronomers at Georgia State University and the National Centre of Radio Astrophysics in India have proposed a new mechanism to explain the formation of galaxies during the history of the universe.
The theory has the potential to revise conventional ideas about star formation, said Paul Wiita, an astronomy professor at Georgia State. Traditional star formation theories rely strongly on the concept of galaxy mergers, in which smaller galaxies form and then collide to make larger galaxies.
"We're arguing that many more galaxies can be formed nearly simultaneously when radio galaxy lobes squeeze gas clouds," said Wiita. He and his colleague Gopal Krishna of India will publish their findings in the Oct. 20 issue of The Astrophysical Journal Letters.
Unlike optical galaxies, the brightest of which can be seen with the naked eye, radio galaxies give off power at much longer wavelengths and can't be seen without special radio telescopes. The lobes are the outer portions of the radio galaxies that contain magnetic fields and relativistic electrons, or electrons moving close to the speed of light.
A variety of astronomical observations performed using the most sensitive radio and optical telescopes have shown that the process of gas clouds being converted into stars and galaxies - a process very intense during the youth of the universe - sharply decreased in the past 8 billion years.
The two astronomers now suggest that much of the galaxy formation activity occurring in the universe during the quasar era, about 8 to 10 billion years ago, was triggered when lobes of radio galaxies engulfed gaseous clouds. The radio galaxies' high-pressure relativistic plasma led to a rapid collapse of the clouds into stars and galaxies.
This mechanism also provides an explanation for another phenomenon discovered recently by German and Canadian astronomers, who found that during the quasar era the space between the galaxies was filled by magnetic fields so strong that their origin presents a major theoretical mystery.
"It's been assumed for a long time that fields between galaxies are extremely weak, but recent observations suggest some magnetic fields may be much stronger," said Wiita. "Our theory of large magnetic fields between galaxies agrees with the German and Canadian astronomers' findings."
Most astronomers now believe that the central cores of massive galaxies contain gigantic black holes ranging from a million to a few billion times the size of our sun. Yet, at any given time only a small fraction of massive galaxies are in an "active state."
Active states occur when a black hole ejects two oppositely directed beams of relativistic electrons and either protons or positrons. During the active phase, the beams literally inflate a pair of high-pressure lobes filled with relativistic plasma and magnetic field. The lobes can eventually extend millions of light-years across.
Lobes can be traced because of the intense radio-frequency radiation they emit by the synchrotron process, which occurs from the spiraling motion of relativistic electrons around magnetic field lines. These radio emissions allow radio galaxy lobes to be detected even if their parent galaxy is located 10 to 12 billion light-years away.
By making observations of the radio lobes and their parent galaxies, it becomes possible to trace the evolutionary history of the universe.
Radio astronomers discovered that many years ago the number of radio galaxies was roughly a thousand times higher than the present era, even when correcting for the expansion of the universe. In spite of this, astronomers have maintained that radio lobes filled only a small fraction of the volume of the universe and therefore had a negligible direct role in the formation of galaxies in the universe.
But Gopal Krishna and Wiita now have shown that the fractional volume of the "relevant universe" - the part that contains the matter now in galaxies - filled by radio lobes during the quasar era has been underestimated by a factor of nearly 1,000.
One key reason for this discrepancy is that many large high-pressure radio lobes were present in the quasar era even after their synchotron emission faded below detectable limits.
Gopal Krishna and Wiita's arguments incorporate the most recent theoretical models of the radio galaxies and the process by which cosmic material organizes itself into filamentary structures.