Quasars: Beacons in the Cosmic Night

Overview

The term quasar is used to describe quasi-stellar radio sources that are the most distant, energetic objects ever observed. Quasars are enigmatic. Despite their great distance from Earth, some are actually brighter than hundreds of galaxies combined, yet are physically smaller in size than our own solar system. Astronomers calculate that the first quasar identified, 3C273 (3rd Cambridge catalog, 273rd radio source) located in the constellation Virgo, is moving at the incredible speed of one-tenth the speed of light and, although dim to optical astronomers, is actually five trillion times as bright as the Sun. Many astronomers theorize that very distant quasars represent the earliest stages of galactic evolution. The observations and interpretation of quasars remain controversial and challenge many theories regarding the origin and age of the Universe. In particular, studies of the evolution and distribution of quasars boosted acceptance of Big Bang-based models of cosmology (i.e., theories concerning the creation of the Universe) over other scientific and philosophical arguments that relied on steady-state models of the Universe.

Background

In 1931 American engineer Karl G. Jansky's (1905-1950) discovery of radio waves emanating from the central region of the Milky Way Galaxy laid the foundations for the development of modern radio astronomy. Six years later, another American engineer, Grote Reber, constructed the first radio telescope in his own backyard and over the course of the next decade, radio telescopes increasingly were used to explore the Cosmos. The information they provided astronomers served to shape one of the greatest mysteries of astronomy, the discovery of emission of strong radio waves from dim stellar sources. By the mid-1950s an increasing number of astronomers using radio telescopes sought explanations for these energetic radio emissions.

Puzzled astronomers suspected a variety of star-like objects might be the source of radio waves. In 1960 astronomers Allan Sandage (1926- ) and Thomas Matthews identified a source of strong radio and ultraviolet emissions that appeared to originate from an optically faint star. In 1962 British radio astronomer Cyril Hazard, in an attempt to fix the location of radio emissions, used the Moon as a occultive shield to measure the duration of eclipsed radio waves. With the help of other astronomers, Hazard discovered strong radio source traceable to a seemingly ordinary single star-like object. Optical telescopes looking at that same region of space pinpointed a faint star-like object (subsequently designated quasar 3C273) with the same unusual spectral emissions.

In 1963 Maarten Schmidt (1929- ), an astronomer working at the Mount Palomar Observatory, explained the abnormal spectrum from 3C273 by suggesting that its seemingly bizarre wide emission lines were really the highly redshifted spectral lines normally associated with hydrogen. Astronomers Jesse Greenstein and Thomas Matthews, conducting an independent study of what was later known to be quasar 3C48, also noted that their object's strange spectrum was explainable by an extremely large red shift.

Red shift describes the shift of spectral emission lines toward longer wavelengths. Determination of an object's red shift allows the calculation of a object's recession velocity. Because the rate of recession increases with distance, the velocity is a function (known as the Hubble relation) of the distance to the object. Schmidt's keen analysis made a profound impact because it meant that 3C273 had to be three billion light-years away from Earth. Because 3C273 was so far away, it must also be thousands of times more luminous than a normal galaxy to appear as optically bright as it did. Ascribing the 3C273 spectrum to Doppler-like red shift also implied an immense velocity of recession (a spreading of space in all directions).

Impact

In the wake of Schmidt's unraveling of the mystery of 3C273, subsequent studies of the evolution and distribution of quasars indicate that they were more abundant when the Universe was younger. Although astronomers have yet to fully unravel the mystifying radiation of such enormous energy from objects no larger than our solar system, the observable and measurable differences between the younger Universe and the Universe as it exists now heralded the decline of steady state models of the Universe.

In addition, prior to more direct observations, the discovery of quasars provided tacit proof of the existence of black holes. Russian scientist Yakov Zeldovich proposed that only the theoretical properties associated with black holes could explain how quasars outshine galaxies composed of hundreds of billions of stars. Black holes are created when giant stars collapse to tremendous density and thereby create a gravitational field so intense that not even light can reach the required escape velocity. Such a black hole, if located near the center of a galaxy, would begin to consume the galaxy's gas and stars and, just outside the black hole in an area called the accretion disk, intense radiation would be emitted as matter accelerated toward oblivion in the black hole.

Subsequent studies of Quasar 3C273 showed that it blasts jets of visible and x-ray energy tens of thousands of light-years into space, a phenomena that could only be explained by the presence of one rotating, supermassive object, with galactic matter orbiting in an accretion disk. One explanation for the emission of radio waves postulates that as electrons in the accretion disk are accelerated to speeds near the speed of light, they also move in the presence of a magnetic field along helical paths and thereby emit radio waves by a process termed synchrotron radiation. Waves similar to the waves emanating from quasars are observed on Earth when physicists shoot high energy electrons through synchrotron particle accelerators.

After decades of observation there is a growing consensus that quasars represent a class of galaxies with extremely energetic nuclei. Large radio emissions seem most likely associated with large black holes with a large amounts of matter available to enter the accretion disk. Less vigorous radio sources (e.g., Seyfert galaxies or QSOs—"quasi-stellar objects"—that offer a similar optical appearance to quasars but that are radio quiet or silent) may be accounted for by smaller black holes or by black holes in smaller galaxies with less matter available for their consumption.

The nature and location of quasars, as well as the existence of objects that might be associated with them, garnered and consumed considerable research attention. The limitations of ground-based telescopes, however, frustrated astronomers searching for clues to unravel the mysteries. In fact, the need and ability to study quasars was often cited as one of the principal reasons to build the Hubble Space Telescope launched by the United States in 1990. In particular, astronomers wished to determine whether quasars were associated with galaxies.

Photos from the Hubble Space Telescope subsequently determined that quasars did reside in galaxies. In addition, studies of brighter quasars that act as a powerful electromagnetic back lighting allow astronomers to examine intervening absorbing material. Using such quasars, astronomers are able to study the primitive gas clouds found in the early Universe.

The impact of quasars upon cosmology (the study of the nature and origin of the cosmos) cannot be understated. The discovery of quasars provided an enormous boost to cosmological models based on the Big Bang theory. Because of the finite speed of light, the discovery of quasars allowed astronomers to look back into the history of the Universe. Although challenged by American astronomer Halton Christian Alp, most astronomers now accept that the large red shifts of quasars indicate their great distance and that, correspondingly, their light emissions present us a picture of the early Universe as it existed within a few billion years after the Big Bang.

Other compatible interpretations of quasars assert that they were formed in large numbers in the young Universe and that these quasars evolved into normal galaxies. Accordingly, they are visible only in distant, ancient light.

Prior to the discovery of quasars, a rival cosmological model (i.e., a model describing the creation of the cosmos) was termed the steady state model. The steady state model relied on a Universe that was the same in all directions (when averaged over a large span of space) and at all times. To account for Hubble's discovery of an expanding Universe the steady state model postulated a continuous creation of matter in the space between the stars and galaxies so the density of the Universe was maintained in a steady state. Although many astrophysicists rejected the steady state model because it would violate the law of mass-energy conservation, the model had many eloquent and capable defenders, including British physicist Fred Hoyle. In addition, the steady model was seen to be compatible with many philosophical, social, and religious concepts centered on the concept of an unchanging Universe that has existed in much the same state as it had been created.

Problematic for skeptics of the steady state model was the fact that the proposed rate of "mass creation" required to support the steady state model was far too small to be detected by experimental observation. To maintain the Universe in a steady density state, slightly less than one hydrogen atom per cubic centimeter would have to be created every millennium. In essence, there was no way to absolutely disprove the steady state model of the Universe, and thus the steady state theory and the Big Bang theory competed with each other for scientific and philosophical favor.

As astronomers studied quasars they discovered an early Universe that contained many more quasars than exist now. This change in the Universe over time (e.g., specifically the rate of formation and existence of quasars) contradicted the steady state model. Along with the discovery of a permeating cosmic background radiation, the discovery of quasars tilted the cosmological argument in favor of Big Bang-based models.

Building on the work of Schmidt and others, recent discoveries concerning quasars aid astronomers, cosmologists, and philosophers refine models concerning the history, structure, and future of the Cosmos. As observation and detection techniques improved, so did the discovery of new quasars that stretch the frontiers of the Universe. In 1979, a x-ray quasar was found with a red shift of 3.2, meaning that the velocity of recession was 97% the speed of light. Another enigma surrounding quasars came with the apparent discovery of quasars approximately at a distance of 17 billion light years. Although seemingly fantastic, these measured distances would make these quasars older than the current upper estimates for the age of the Universe and thereby challenge existing cosmological theory.

K. LEE LERNER