Two Israeli researchers, Ranan Barkana (Tel University) and Avi Leib (Harvard University in the USA) have for the first time found direct evidence of the formation of galaxies the size of the Milky Way when the universe was only a billion years old
With the help of huge telescopes that are aimed at the sky, astronomers study the distant past when the galaxies were still young and created the first generations of stars. One of the big questions that bothered researchers was: how long did it take for the first galaxies to form after the big bang? The answer - very quickly! Two Israeli researchers, Ranan Barkana (Tel University) and Avi Leib (Harvard University in the USA) for the first time found direct evidence of the formation of galaxies the size of the Milky Way when the universe was only a billion years old.
Spotlights at the edge of the universe
To learn about the early universe, astronomers study the most distant objects - galaxies and quasars whose light reaches us billions of years after being emitted. Quasars are the brightest objects known in the sky. Their brilliant light is thought to be produced by giant black holes in the centers of galaxies. Huge amounts of gas are drawn towards the black hole, and under the enormous force of gravity the gas is compressed and heated and the enormous radiation it emits can be seen for huge distances.
Many studies of galaxies close to us have shown that there is a connection between each galaxy and the black hole at its center. That is, large galaxies contain large black holes, and small galaxies contain small black holes. Researchers expected the same relationship to hold for the most distant black holes in the early universe, but until recently there was no evidence of this. Barkna and Leib have now provided the first evidence.
Astronomers observed different quasars and measured the spectrum in each one - the intensity of light in different wavelengths (or colors). In some quasars the spectrum showed the shape of two peaks, but this finding has not attracted much attention until now. Barkana and Leib created a computer model that interprets the shape of the spectrum as a result of absorption by hydrogen gas.
Intergalactic hydrogen drawn into the galaxy falls in its direction and absorbs some of the light emitted by the quasar. The infall is caused by the gravitational force of the galaxy, so the infall velocity measures the total weight of the galaxy. This is the first measurement of such an early galaxy.
Dark matter halos
Previous studies that have considered galaxies close to us have shown that the galaxies sit inside large halos of dark matter. It is an invisible substance, made of unknown ingredients, but it can be detected by the gravitational force it exerts on stars and gas. In fact, most of the matter in the universe is dark, and the known chemical elements make up only about a fifth of all matter. The new measurements of the ancient galaxies find an extremely heavy weight and strengthen the hypothesis that dark matter was dominant in the early universe as well.
According to the hierarchical model of galaxy formation, small galaxies were formed first and then merged and absorbed more material to form larger galaxies. This process takes time, so it is interesting to discover that galaxies as large as our Milky Way galaxy were formed in less than a billion years. These galaxies are probably only the tip of the iceberg. It is widely believed that most galaxies at that time were much smaller, but they produce fainter light and are therefore difficult to see.
Meanwhile, Barkna and Leib applied their model to two cases of distant quasars for which observations of sufficiently high quality were available. High-resolution observations of the spectra of additional quasars are needed to validate the model.
This study is published in the January 23, 2003, issue of the journal Nature.
Big galaxies, after only a billion years
By Marit Selvin
Israeli researchers have shown for the first time that galaxies the size of the "Milky Way" formed around quasars in the early universe
One of the questions that bothers astronomers studying the young universe is how much time has passed since the big bang, which according to the accepted opinion happened about 14 billion years ago, until the formation of the first galaxies. According to an article by two Israeli researchers, published in the latest issue of the journal, "Nature, the answer is - a very short time. Ranan Barkana from the School of Physics and Astronomy at Tel Aviv University and Avi Leib from Harvard University in the USA found for the first time direct evidence of the formation of galaxies the size of the Milky Way galaxy, when the universe was only a billion years old.
To learn about the early universe, astronomers study the most distant objects - galaxies and quasars whose light reaches Earth billions of years after being emitted. A quasar, the brightest celestial object, is a body located at the center of a galaxy with a supermassive black hole at its center. Huge amounts of gas are drawn towards the black hole, and under the influence of the enormous gravity the gas is compressed and heated. Before the gas falls in, it emits enormous radiation. Although due to the tremendous light emitted by the quasar, the stars around it are not visible (except in galaxies close to the Earth), the assumption is that there are also galaxies around ancient quasars. However, so far no direct evidence of their existence has been found and their size has not been measured.
Scientists believe that small galaxies formed early in the life of the universe and over time absorbed more and more material to form larger galaxies. This process requires time, and therefore the accepted opinion was that under the conditions that prevailed in the early universe, a large number of large galaxies could not have formed. "Galaxies as large as our Milky Way galaxy are very rare in the early universe," says Barkana. "The ones that exist were probably formed from a particularly dense area, which for hundreds of millions of years added a lot of material to it. In the end, a dense area was created, rare in size - like Mount Hermon in the State of Israel, where there are only small mountains. Even more distant matter, which is not part of the galaxy, felt its gravity and began to fall towards it. This fall is an important part of the accepted model of the formation of galaxies, but so far they have not been able to prove that it actually occurs," says Barkana.
The study by the Israeli researchers was based on data collected in a quasar survey conducted in recent years in which several ancient quasars were discovered. In each of the quasars, the spectrum is measured, that is, the intensity of light in different wavelengths (or colors). The intensity of the light indicates the age of the quasars and the chemical elements found around them. In some quasars the spectrum manifested itself in a certain characteristic that most researchers did not pay special attention to. But it turns out that precisely this characteristic provides an important layer for understanding the organization of galaxies in the early universe, as Barkana and Leib showed.
"Such a characteristic is obtained when the material that makes up the galaxy pulls gas towards it, and the gas absorbs part of the light emitted from the quasar located in the center of the galaxy. Based on that characteristic, we could calculate the speed at which the falling gas moves towards the center of the galaxy, and we found that the gas moves at a speed of about two million kilometers per hour. This speed indicates that the galaxy attracting the gas is about as big as the Milky Way galaxy."
Once you know the size of the galaxy around the quasar, you can study the relationship between the galaxy and the black hole at its center. Many studies of galaxies close to our own have shown that large galaxies contain large black holes, while small galaxies contain small black holes. Researchers hypothesized that the same relationship would also hold for the most distant black holes in the early universe. Barkana and Leib's research is the first evidence that such a relationship is indeed preserved.
Barkna and Leib applied their model to two distant quasars observed in sufficiently high quality. In both cases, it turned out that large galaxies formed around the ancient quasars when the universe was only a billion years old. Barkana emphasizes that observations of additional quasars are needed to verify the model and continue to develop it.
