Ancient hole, black hole
How did galaxies form? It’s a cardinal mystery of the early universe. Microwave radiation created 380,000 years after the Big Bang shows a smooth array of molecules, spread out like a fog. The contrast to the situation one billion years later is complete: by then, matter was concentrated in stars and galaxies, separated by empty space.
Nowadays, most galaxies hide at least one super-dense black hole, whose gravitation prevents even light from escaping. Until now, nobody knew about black holes in the earliest galaxies.
Yesterday, Ezequiel Treister of the University of Hawaii and colleagues reported that most or all of the earliest galaxies also had black holes.
A problem of roots
The data illuminates the ultimate roots question – how our universe formed its present structure, and in particular, what happened during the billion years after the Big Bang banged about 13.7 billion years ago.
For 380,000 years, “During the embryonic universe, the fluctuations in density were about one-one thousandths of a percent, but over a billion years, structures developed,” Alexey Vikhlinin, author of a commentary in Nature, told The Why Files. “These galaxies are essentially the same type of objects in the present universe,” says Vikhlinin, an expert in X-ray astronomy at the Harvard-Smithsonian Center for Astrophysics.
How did we go from the primordial fog to a universe with ultra-dense galaxies, neutron stars and black holes separated by a vast nothingness where each cubic centimeter has about one lonely atom?
The vast epoch of ignorance, Vikhlinin says, “is called the dark age because little has been observed, and one of the major questions in astrophysics is how this transformation took place.” The new observations show that roughly the same proportion of matter (excluding the enigmatic dark matter and dark energy) was concentrated in galaxies and black holes then as now.
“These results show that pretty much every galaxy must have contained a substantial black hole, similar to today,” says Vikhlinin, “but this is the first observation that the relationship between galaxies and black holes that exists today, existed 1 billion years after the Big Bang.”
An extraordinary step
In the study, Treister and colleagues correlated long exposures from
Hubble Space Telescope, which can see extraordinarily distant (and ancient) galaxies, and
Chandra X-ray observatory, which picked up X-rays from distant, unidentifiable sources.
By pinpointing the source of Chandra’s X-rays on Hubble’s galactic snapshots, the scientists located ancient black holes inside some of the first galaxies.
The study benefited from three features, says Vikhlinin. “The necessary Chandra and Hubble data were taken only recently, and the observations were immediately made available to every interested scientist,” along with some money for their interpretation.
Treister also looked at the highest energy range that Chandra can detect, Vikhlinin adds. Because Chandra’s mirrors are more sensitive to lower-energy X-rays, “most people work in this region.”
The newly detected black holes produced a surprising result – that the basic structure of the universe has not changed terribly much in the 12.7 billion years since that ancient light embarked toward a planet that did not yet exist.
The “so-what?” part
Although the study shines some light on the presence of black holes and galaxies during the dark age, it does not provide a complete answer, says Vikhlinin. “It definitely seems as if galaxies and black holes have evolved in parallel. The growth of one controls the growth of the other, and vice versa, but the nature of the process and why they evolve in parallel is not entirely clear.”
Logically, a black hole would require a galaxy to provide the cold gas that it inhales. (This gas heats up as it enters the hole, creating the black hole’s X-ray signature; it also supplies material for the stars in the galaxy.)
But why a galaxy would need a black hole is less clear, Vikhlinin says. “We don’t know if galaxies can form in regions that initially don’t have the right conditions for the growth of a black hole. Maybe whenever a galaxy starts to grow actively, it makes a black hole in the center.”
Although the new evidence for an unchanging relationship between galaxies and black holes narrows the possible explanations, the formation of the first galaxies and black holes “remains one of the biggest unsolved problems in astrophysics,” Vikhlinin says.
– David Tenenbaum
Terry Devitt, editor; S.V. Medaris, designer/illustrator; Jenny Seifert, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive
- WMAP homepage. ↩
- WMAP data. ↩
- Chandra homepage. ↩
- More onChandra mission. ↩
- NASA Astrophysics. ↩
- The Big Bang. ↩
- NASA’s outreach and education site. ↩
- Compilation of Universe history papers. ↩
- Cosmic microwave background. ↩
- History of the universe. ↩
- Black holes and galaxy growth. ↩
- Science video: the birth of black holes. ↩