Two astrophysicists believe they have found the first direct evidence for how large galaxies were able to form in the early stages of the Universe. Abraham Loeb at Harvard University and Rennan Barkana at the University of Tel Aviv believe that a signature in the emission spectrum of certain quasars may provide clues to the mechanisms that allowed these early galaxies to grow so quickly (R Barkana and A Loeb 2003 Nature 421 341)
Quasars are the oldest known astronomical objects and can thus provide important information on the state of the early Universe. They are thought to exist at the centres of giant host galaxies and may be powered by supermassive black holes, which would explain why they are the brightest objects in the sky.
There is now compelling evidence that galaxies as large as the Milky Way had already formed less than a billion years after the Big Bang. However, existing models of galaxy formation cannot explain how so much matter could have assembled at such a rapid rate.
Loeb and Barkana have now studied the absorption spectra of two very distant quasars, found at high redshifts of 4.79 and 6.28 by the Sloan Digital Sky Survey in 2001. The redshift is a measure of how fast an object is receding from the Earth due to the expansion of the Universe — a higher redshift means that the object is further away. The spectra contained emission lines resulting from excited hydrogen atoms with characteristic “double-horn” peaks that could not be explained.
Loeb and Barkana believe that these peaks are unique “signatures” which may provide direct evidence that quasars are embedded in massive host galaxies. Such a host galaxy gravitationally pulls in large quantities of gas from its surroundings, which would absorb some of the light from the quasar. The researchers say that, based on the absorption signature, they can estimate the amount of gas falling into the host galaxy. This enables them to calculate the gravitational force exerted by the host galaxy and consequently its total mass.
The researchers show that the two quasars lie in galaxies that weigh about 1012 solar masses — which is about the size of the Milky Way. They calculate the total in-fall rate of gas into these galaxies to be about 1300 solar masses per year for the z=4.79 quasar and 2900 solar masses per year for the z=6.28 quasar. From these rates they estimate that the host galaxies of these quasars could have been formed in about 300 million years for the z=4.79 quasar and 900 million years for the z=6.28 quasar. This ties in well with the age of the Universe, which is estimated to be about 14 billion years.
The team admit that more observational data are needed to test their model and now hope to look at other quasars. “We are working on similar signatures for less massive galaxies – those that may host other sources of light such as gamma-ray bursts which are visible at greater distances,” Loeb told PhysicsWeb.