Quasars

In the 1950s astronomers began to appreciate the potential for radio astronomy and a whole series of objects in the sky that emitted radio waves were catalogued systematically. One catalog of such objects was called the Third Cambridge Catalog, and objects in it were designated by "3C", followed by a number. As these radio sources were catalogued, a concerted effort was made to correlate the objects emitting radio waves with sources visible in optical telescopes. These associations were sometimes difficult because the resolution of the single-dish radio telescopes in use at the time was much poorer than that of large optical telescopes. Therefore, there were often many possible optical sources that might potentially be correlated with the fuzzily known position of a radio source. Nevertheless, rapid progress was made in identifying newly discovered radio sources with visible objects.

"Radio Stars"

Most radio sources were found to be correlated with certain galaxies or some with nebulae, but a few of the optical sources identified with the radio sources appeared to be stars (point sources with no obvious extension). These were often called radio stars, but under closer investigation they proved to have very strange characteristics for stars. In 1963, the first two of these "radio stars" were associated with the radio sources 3C48 and 3C273, respectively. Although they appeared to be stars in optical telescopes, they had spectra unlike any stars that had ever been observed. First, there was a very strong continuous distribution across many wavelengths. This distribution appeared to be non-thermal in character (recall our discussion of synchrotron radiation and nonthermal spectra in Chapter 5). Sitting on top of this continuum were emission lines, but they were very broad. Furthermore, their wavelengths did not correspond to the lines for any known atom, molecule, or ion.

Rediscovering Hydrogen

Later in 1963, Dutch astronomer Maarten Schmidt came to an important discovery. While studying the spectrum of 3C273, he realized that the strange emission lines were really very familiar. They were known lines of the Balmer series of hydrogen (and a line in ionized magnesium). However, they were redshifted by a very large amount, corresponding by the Doppler formula to velocities away from us that was about 15 percent of the speed of light. Once this was realized for 3C273, it quickly became apparent that the spectrum of 3C48 could be interpreted in the same way, but with a redshift that was even larger. The reason that it took some time to come to this conclusion about these objects is that they were thought initially to be relatively nearby stars, and no one had any reason to believe they should be receding from us at such velocities.

Star-Like Radio Sources

These objects were named Quasistellar Radio Sources (meaning "star-like radio sources"), which was soon contracted to quasars. Soon other quasars were discovered and it became apparent that they constituted a class with the following characteristics.

Quasars appeared to be star-like in the initial images. Most of the first discovered were also radio sources, but we know now that most are not (see the right panel).

They exhibit a non-thermal continuum spectrum that is stronger than most other sources at all wavelengths, varies substantially in time, and exhibits the basic characteristics of synchrotron radiation (nonthermal and polarized).

Because quasars emit strongly in the ultraviolet, they are distinctly blue at optical wavelengths.

They usually exhibit emission lines that are very broad. If the broadening of the lines is attributed to random motion of the emitting regions, velocities in the range of 10,000 km/s are indicated by the widths of the emission lines.

Quasars exhibit large redshifts, implying by the Hubble law that they are at great distances and that the light that we see from them was emitted when the Universe was much younger than it is now.

The following figure shows the quasar 3C 273, which was not only the first quasar identified but also is the quasar with the greatest apparent brightness. It has a redshift of 0.158, corresponding to a recessional velocity of almost 15 percent of the speed of light (nearly 44,000 km/s, meaning that its separation from us increases by about the distance from the Earth to the Sun every hour). This places it at a distance of nearly 700 Mpc by the Hubble law, with a look-back time of about 2 billion years. Thus, when the light that we now see from 3C 273 left the quasar, the Earth was only about 2.5 billion years old.

Despite its enormous distance, 3C 273 has an apparent visual magnitude of +12.9, which implies that it must be incredibly luminous. At visual wavelengths it is about 3 trillion times more luminous than the Sun (its absolute visual magnitude is -26). But, in fact, 3C 273 emits most of its light at nonvisual wavelengths (we shall see its spectrum in the next module on active galaxies). When summed over all wavelengths, the luminosity of 3C 273 is about 20 trillion times that of the Sun. This is typical and implies that the average quasar is some 1000 times more luminous than bright normal galaxies.

Jets and Fuzz

Although initial observations indicated that quasars were point-like (like stars), more careful study shows fuzziness and faint jets associated with some quasars. The left image of 3C 273 above shows the quasar and a jet. The right image, which has been rotated and enlarged relative to the left image, superposes on this optical image contours of radio frequency intensity. The optical jet clearly coincides with a jet structure in the RF map. We will find a general correlation in quasars between jets and radio emission. (The sharp radial lines from the quasar are optical spike artifacts common in quasar images because of their star-like brightness.) The adjacent right figure shows an X-ray view of the quasar PKS 0637-752 and an associated jet, as imaged from the Chandra X-ray Observatory. This quasar is almost 2000 Mpc (more than 6 billion light years) distant.