Ringing in an estimate of a galaxy's mass.

Ringing in an estimate of a galaxy's mass

One of the striking manifestations of the ability of massive astronomical bodies, such as galaxies, to act as gravitational lenses and bend the path of light is known as an Einstein ring. This type of circular image results when a massive object with a simple geometry stands perfectly aligned between an observer and a distant light source. Astronomers have now used the characteristics of one such system to estimate the mass of the intervening galaxy, which acts as a gravitational lens.

"This is the most distant galaxy whose mass has been measured," says Glen I. Langston of the National Radio Astronomy Observatory in Charlottesville, Va. "It's also the first time that we've been able to reliably measure the mass of a [gravitational] lens." Langston and his collaborators report their results in the March 1 NATURE.

This particular Einstein ring, designated MG1654+1346, is ideally suited for such a measurement because the lens is simply a single, relatively bright, elliptical galaxy. To produce the Einstein ring, the galaxy magnifies and distorts radio waves travelling from one of two regions of radio-wave emissions that extend out on either side of a more distant quasar.

Because the galaxy itself is invisible at radio wavelengths, astronomers can observe it and the Einstein ring independently. The radio ring consists of a narrow, circular band with two bright, semicircular components. The quasar's second radio-emission region, which is too far from the line of sight to be affected, appears normal.

Knowing the ring's apparent size and the distances to both the background quasar and the intervening galaxy, astronomers can calculate the galaxy's mass. Langston and his colleagues obtain a total galactic mass about 300 billion times the sun's mass. "It's a reasonable number," Langston says. "Our results say that at this distance, which is a reasonable fraction of the observable universe away, galaxies still seem similar to what we see nearby." The results also suggest this galaxy contains a substantial amount of nonluminous matter.

Gravitational lenses can create not only radio rings but also enormous luminous arcs and multiple images (SN: 12/3/88, p.357; 12/9/89, p.375). The study of such images provides a useful probe of the makeup of a variety of celestial bodies.

Gravitational lensing may explain, for example, the puzzling, quasar-like properties of so-called BL Lacertae objects sometimes found embedded in nearby galaxies. In the March 1 NATURE, Jeremiah P. Ostriker of Princeton (N.J.) University and Mario Vietri of the Osservatorio Astrofisico Arcetri in Florence, Italy, cite new astronomical and statistical evidence suggeting that a significant fraction of these bright, compact objects may be the images of more distant quasars substantially amplified by the gravitational effect of individual stars in intervening galaxies that happen to be near Earth.

The result is an optical illusion that puts a bright, compact source of light in the midst of a nearby galaxy. The passage of stars across the line of sight to the quasar could account for the rapid brightness changes typically associated with BL Lacertae objects.

The notion that not only luminous galaxies but also invisible matter could act as gravitational lenses has prompted a number of searches for multiple images and for unusually bright quasars. Such observations could eventually help settle the question of how dark matter is distributed within a galaxy and across the universe (SN: 1/27/90, p.52).

"Rarely in astronomy can such a simple theory as gravitational lensing be applied to such a wide range of observations," writes Daniel W. Weedman of Pennsylvania State University in University Park in a commentary accompanying the research reports. "The bizarre manifestations of these lenses should continue to provide other wise unobtainable information about dark matter in the Universe."