BY DR EMILY BALDWIN
Posted: 08 January, 2009
New research may have solved the cosmic chicken-and-egg problem of which formed first in the early Universe: galaxies, or the supermassive black holes seen at their cores.
"The big question has been whether one grows before the other or if they grow together," says Dominik Riechers of Caltech.
After studying conditions in the first billion years of the Universe's history using the National Science Foundation's Very Large Array radio telescope and the Plateau de Bure Interferometer in France, an international team of astronomers arrived at the conclusion that black holes came first. "The evidence is piling up," says Chris Carilli of the National Radio Astronomy Observatory (NRAO).
Sloan discovery image (left) and VLA image (right) showing gas in a young galaxy seen as it was when the Universe was only 870 million years old. Image: NRAO/AUI/NSF, SDSS.
Previous studies of galaxies like our own spiral Milky Way Galaxy, which have central bulges containing gas and dust surrounding a supermassive black hole, showed that the ratio of the mass of the black hole to the mass of the central bulge is nearly the same for a wide range of galactic sizes and ages. That is, for central black holes from a few million to many billions of times the mass of our Sun, the black hole's mass is about one-thousandth of the mass of the surrounding galactic bulge.
"This constant ratio indicates that the black hole and the bulge affect each others' growth in some sort of interactive relationship," says Riechers. The question has been whether this mass ratio is maintained throughout the whole evolutionary process, and the new observations throw up some interesting results.
By measuring the properties of black holes and bulge masses in several galaxies seen as they were in the first billion years after the Big Bang, it turns out that the constant ratio seen nearby may not hold true in the early Universe. "The black holes in these young galaxies are much more massive compared to the bulges than those seen in the nearby Universe," says Fabian Walter of the Max-Planck Institute for Radioastronomy in Germany. "The implication is that the black holes started growing first."
ALMA is currently under construction at 5,000 metres altitude in the Chilean Andes. Image: ESO.
The next challenge is to figure out exactly how the black hole and the bulge affect each others' growth. "We don't know what mechanism is at work here, and why, at some point in the process, the 'standard' ratio between the masses is established," says Riechers.
New telescopes such as the Expanded Very Large Array (EVLA) and the Atacama Large Millimeter/submillimeter Array (ALMA), which are now under construction, will be key tools for unraveling this mystery. "[They] will give us dramatic improvements in sensitivity and the resolving power to image the gas in these galaxies on the small scales required to make detailed studies of their dynamics," says Carilli. "To understand how the Universe got to be the way it is today, we must understand how the first stars and galaxies were formed when the Universe was young. With the new observatories we'll have in the next few years, we'll have the opportunity to learn important details from the era when the Universe was only a toddler compared to today's adult."