NASA's Hubble Space Telescope has discovered a new distant class of quadruple, or cross-shaped, gravitational lenses which might eventually provide astronomers with a powerful new "magnifying glass" for probing a variety of characteristics of the universe: the distribution of dark matter, abundance of supermassive black holes, and even determining if the universe will expand forever or eventually collapse.
The two gravitational lenses were discovered in about 100 fields of sky imaged by Hubble's Wide Field Planetary Camera 2 (WFPC 2). Because the combined area surveyed is about that of a full Moon, astronomers expect that there may be as many as half a million similar lenses scattered across the heavens though Hubble is expected to only detect about three per year through snapshots of the sky.
Hubble's high resolution allows astronomers to extend the search to much fainter, and hence much farther lenses, than those few examples ground-based telescopes have uncovered relatively nearby. Hubble can explore a larger volume of space which could provide enough examples of this rare cross type of lensing to allow astronomers to address a variety of fundamental cosmological questions.
"This is a big jump for the young field of gravitational lenses which was theory until less than two decades ago. This opens up a new class of lens, which is a galaxy with well understood properties," said Kavan Ratnatunga of Johns Hopkins University, Baltimore, MD. "The distinctive cross-like pattern around an elliptical galaxy makes them unambiguous quadruple lens candidates, even before spectroscopic observations, which are typically used to confirm lenses."
The discovery is reported by Ratnatunga and other astronomers of Johns Hopkins University in the November 1 issue of the Astrophysical Journal Letters. The first cross-shaped lens was discovered serendipitously by Eric Ostrander while processing HST images for the Medium-Deep Survey, a Hubble key project led by Richard Griffiths. A second fainter and smaller lens was identified a few weeks later by Myungshin Im. Each configuration is in the form of four faint blue images situated symmetrically around a much brighter red elliptical galaxy.
A gravitational lens is produced by the enormous gravitational field of a massive object which bends light to magnify, brighten and distort the image of a more distant object. Depending on the alignment between the objects and the mass distribution of the foreground lens, the more distant object can be smeared into arcs or split into pairs, triples, or even quadruple images.
Gravitational lenses were predicted by Albert Einstein, though the resolution of ground-based optical telescopes at the time made him remark: "there is no great chance of observing this phenomenon".
Since 1979, few examples of lensing have been observed. Arc-shaped, objects are the most common, followed by pairs of lensed objects. However, it is impossible to identify the true gravitational lenses without observations which show the two objects have exactly the same spectral fingerprint and so are "multiple" images of a single object.
Bright quadruple lenses, which have a distinctive cross pattern, are extremely rare. Only two examples are known by ground-based surveys of the whole sky. The lensing objects for these cases are also unusual - the first lens, discovered in 1985 and dubbed the "Einstein cross", is a quasar lensed by a supermassive black hole at the center of a nearby bright galaxy; the other, discovered in 1988 and dubbed the "clover leaf", is a quasar lensed by an unseen mass.
More than simply showing lenses as astronomical curiosities, the new Hubble observation offers a new tool for probing the cosmos. The researchers say this may be as significant as the discovery of Cepheid variable stars earlier this century, that allow astronomers to measure cosmic distances to neighboring galaxies.
Astronomers can use quadruple lenses to estimate the density of matter in space by comparing the redshift of the lensing galaxy and lensed object with the geometric distance (as measured by the angular deflection by the lens). If the density of matter is high, then space is positively curved and the universe will eventually collapse and come to an end. A low mass-density means the universe will expand forever.
"Preliminary analysis of the two new HST lenses seems to suggest that the density of matter in space is probably low enough to allow the universe to expand forever," said Myungshin Im. A sample of at least a dozen lenses of this type is needed before a more definitive estimate can be made.
Detailed image analysis of the lens has also been used to probe the distribution of dark matter in the foreground galaxy. "The model for the observed lens configuration clearly shows that the mass of these galaxies consists of predominantly dark matter in a very elliptical distribution", said Ratnatunga. This shows that the mysterious dark matter - invisible matter of an unknown type is more than 90 percent of the mass of a typical elliptical galaxy and provides an upper limit to the masses of black holes that may dwell at the centers of these galaxies.
Hubble's Medium-Deep Survey is conducted in "parallel" mode using Hubble's WFPC 2. These pictures survey previously unobserved random regions of sky while a predetermined celestial target is being observed with another HST instrument. The survey is being used to continue an ongoing systematic search to find more gravitational lens candidates.
Space Telescope Science Institute, Baltimore, MD
Johns Hopkins University, Baltimore, MD