Quasar Studies Provide A Model

Radio surveys of objects (presumed to be galaxies) with intense radio-wavelength emissions (hypothesized as the radiation emitted by high-energy particles moving about in intense magnetic fields) were well established by the mid-1950s, and astronomers were beginning to use them to direct their optical telescopes, in hopes of further exploring their apparent sources. In 1963, while studying the spectrum of a radio source known as 3C 273, Maarten Schmidt at the California Institute of Technology was the first to identify quasars as extragalactic. Since that time, using a similar strategy, astronomers have identified thousands more. The name was coined to represent the phrase quasi-stellar radio source, indicating that, although these objects appeared starlike, they could not possibly be anything like a star. Their redshifts indicated they were far too distant to be resolvable as individual stars, nor could they be "plain vanilla" galaxies because of both the intensities (in many cases as great as that of hundreds of galaxies) and the dramatic fluctuations of their optical luminosities. Redshift measurements for 3C 273 indicate it is 1 billion to 2 billion light-years away; if so, its energy emission every second is equal to the energy our Sun generates over several hundreds of thousands of years.

Whatever they were, quasars suddenly provided a much deeper lens into the past. Explained Bechtold: "The high luminosities of quasars enable astronomers to study the universe at great distances and hence at epochs when the universe was about 20 percent of its current age. This was a period when many important events took place," not the least of which was "the rapid turn-on of quasars themselves." Quasar surveys of the sky indicate that the number of quasars per volume of space increases with the distance from the Milky Way, which suggests that whatever caused them was more common in the distant past, possibly during the era of galaxy formation. Astrophysicists have developed a consensus that quasars are the very active nuclei of galaxies, possibly the result of supermassive black holes at work. Black holes (as they are postulated) could cause the phenomenon astrophysicists detect with their spectrographs from quasars. If material were to collapse under gravity to a critical density, neither optical nor any other radiation could escape its grip. Einstein's theory of general relativity demonstrates this phenomenon with mathematical precision, and a parameter called the Schwarzschild radius provides its measure. For a mass the size of the Sun, the black hole will be compressed into a radius of 3 kilometers; black holes with larger masses increase their radii proportionally. Bechtold showed the symposium pictures of a quasar that would come from a black hole with between 107 and 109 solar masses.

"Material in the host galaxy that this black hole sits in falls into the black hole," Bechtold explained. Because it is approaching the object from its circular orbiting trajectory, "it cannot fall straight in," she added. "So, it sets up an accretion disk in the process of losing its angular momentum." The power of this reaction is incomparable, said Bechtold, and "is much more efficient in converting mass to energy than is the nuclear fusion that makes stars shine." As the material is drawn toward the center, swirling viscous forces develop and heat up the environment, "creating an incredible amount of energy that produces this continuum radiation we see" on our spectrographs. While quasars exhibit characteristic emission lines, other interesting spectral features arise in "little ensembles of clouds that live very close to the quasar," said Bechtold. ''What is really most interesting about quasars is their absorption lines.''

Because of the vast intergalactic distances traveled by quasar light, the chances of this light interacting with intervening material are great. Just as absorption lines are created as radiation from its core passes through the Sun's photosphere, so too does a quasar's emission spectrum become supplemented with absorption lines that display a smaller redshift, created when the radiation passes through "gas clouds along the line of sight between us and the quasar," explained Bechtold. "We are very fortunate that quasars have somehow been set up most of the way across the universe," she continued, "allowing us to study the properties of the interstellar medium in galaxies that are much too far away for us to be able to see the stars at all. Quasars allow us to probe galaxies at great distances and very large lookback times." As the signature quasar-emission-line radiation travels these vast distances, it almost inevitably encounters interstellar gas, as well as the gas presumed to reside in so-called haloes far beyond the optical borders of galaxies.

The most intriguing evidence from these probes, said Bechtold, is the distribution of the so-called Lyman alpha (Ly-α) absorption line, generated by hydrogen at 1215 angstroms. Analysis suggests that these data can provide vital clues, said Bechtold, to "the formation and rapid early evolution of galaxies, and the beginnings of the collapse of large structures such as clusters of galaxies and superclusters." Big Bang theory and numerous observations suggest that stars are the cauldron in which heavier elements are formed, after first hydrogen and then helium begin to aggregate through gravity. During such a stage, which may well be preceded by diffuse gas clouds of hydrogen throughout the region, no metals exist, and spectra from such objects are therefore devoid of any metal lines. Although Bechtold cautions about jumping to the conclusion that spectra without metal lines are definitive proof that no metals exist, she and other quasar specialists believe that "if there are, in fact, no metals, then these clouds probably predate the first stars. These may be the clouds

of gas in the early universe with only hydrogen and helium that later formed stars. The stars then created elements like carbon, nitrogen, and oxygen through nuclear synthesis," but these "forests" of Lyman-alpha lines may provide the portrait of a protogalaxy that Djorgovski said astronomers eagerly seek (Figure 4.1).

While definitive confirmation of the full implications of the Lyman-alpha forest is in the future, Bechtold explained that the Lyman-alpha absorption lines possess an indisputable value for cosmology today. "Because they are so numerous, they can serve as tracers of large-scale structure very well, in fact, in some ways better than galaxies." Their distribution throughout the universe is "much more uniform than galaxies in the present day," and thus they provide a possible precursor or link in the causal chain that might explain the

Figure 4.1 An optical spectrum of the quasar 0913+072 obtained with the Multiple Mirror Telescope on Mt. Hopkins, Arizona, a joint facility of the Smithsonian Institution and the University of Arizona. The dashed line indicates the quasar continuum; the deep absorption lines originate in galaxies along the line of sight, or intergalactic Lyman-alpha forest clouds. 

present structure. "They don't cluster. They don't have voids," pointed out Bechtold, which "supports the idea that they are protogalactic or pregalactic clouds that have collapsed to galaxies and later the galaxies collapsed to form clusters of galaxies." Thus quasars provide penetrating probes into the past because of their distance, which, but for their intense luminosity, would preclude optical detection. This fact, together with the phenomenon of a distant source of radiation picking up information about intermediate points along the path to Earth, provides a parallel with another recent and dramatic development in astrophysics.