With their giant telescopes pointed toward the heavens, astronomers can look back to a time when young galaxies were just beginning to coalesce and when the first generations of stars were forming. Since the speed of light (300,000 kilometers per second) is finite, light from the most distant objects - galaxies and quasars - has traveled for billions of years before reaching Earth. Quasars, the brightest known astronomical objects, are believed to be powered by huge amounts of gas falling in towards super-massive black holes. Enormous gravitational forces compress and heat the gas, producing blasts of vast quantities of radiation and creating a beacon visible over great distances.
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| A nearby quasar surronded by its spiral host galaxy, seen head-on. (Credit: Hubble Space Telescope, NASA). | Artist's illistration of a quasar, showing a broad disk and a narrow jet of hot gas around a central black hole. (Credit: NASA). |
In the nearby universe, every quasar lies at the center of a much larger galaxy. Furthermore, big galaxies contain relatively big black holes, while smaller ones contain smaller black holes. Astronomers, however, have been unable to measure sizes of distant (early) host galaxies or even provide direct evidence for their existence. The enormous brightness of the central quasar drowns out the light of the host galaxy, except in a few nearby cases (see photograph).
Dr. Rennan Barkana, of the TAU Faculty of Exact Sciences, in collaboration with Prof. Avi Loeb of Harvard University, has recently reported the first measurement of the mass of distant galaxies, in fact, the host galaxies of two of the most distant quasars known. Their work, published in Nature (2003), attracted widespread scientific attention and media coverage. As black-hole researcher Laura Ferrarese of Rutgers University noted, "This is something that hasn't been done before. This really ties it all together."
The researchers accomplished their feat by examining the spectra of quasars, a record of the intensity of their light of different colors (wavelengths). These spectra can indicate when the quasars formed (by an optical equivalent of the Doppler effect) and what chemical elements are found in the surrounding gas (by absorption patterns). Some early quasars, however, have a curious spectral feature that most astronomers failed to notice. Barkana and Loeb created a computer model which found that this spectral feature is, in fact, crucial. It results, in their model, from intergalactic hydrogen gas falling toward the quasar's host galaxy. Since this infall is caused by the gravitational pull of the galaxy, it can be used to measure the host galaxy's mass. The researchers found that the two quasars they examined lie in galaxies about as massive as the Milky Way, implying that large galaxies had already formed before the universe was a billion years old. Observations of additional quasars are required both to confirm the model and to develop it further.
