Supernova images and luminous arcs.

Supernova images and luminous arcs 

Giant luminous arcs, now envisioned as the highly magnified images of distant, extremely faint galaxies, constitute a kind of cosmic mirage. Clusters of galaxies in the foreground act as gravitational lenses, distorting the curvature of space to produce multiple images of any background feature. Such a lens provides a view of galaxies that would otherwise be too distant and faint to observe in detail directly from Earth. A recent theoretical analysis suggests that it may prove worthwhile to look for images of supernova explosions within luminous arcs. Moreover, a single supernova going off within a distant galaxy is likely to show up at three different positions in the corresponding luminous arc.

"We wanted to address the question: What would be a typical factor by which the brightness of a supernova would be increased [by a gravitational lens] to see whether this is something that would be reasonably easy to observe?" says astrophysicist Bohdan Paczynski of Princeton (N.J.) University.

"Our analysis shows that such a supernova would be quite easy to see." Paczynski and Israel Kovner of the Weizmann Institute in Rehovot, Israel, report their results in the Dec. 1 ASTROPHYSICAL JOURNAL LETTERS.

Typical luminous arcs, often as much as 300,000 light-years in extent, are blue, reflecting the color of the background galaxy (SN: 1/17/87, p.36). Although researchers aren't sure why these distant galaxies appear so blue, one possibility is that the galaxies may be in a stage of active star formation. With so many young stars, supernovas would likely occur more frequently than in normal galaxies, perhaps as often as once every few years instead of every hundred years or so.

If this scenario is correct, then it would be worthwhile taking a look at all known luminous arcs, say, once a week or once a month for signs of a supernova, Paczynski says. About half a dozen luminous arcs are known, and systematic searches now in progress will probably unveil a few dozen more examples, further increasing the chances of observing a supernova.

Triple images occur because typical galactic clusters aren't quite spherically symmetric. They act as gravitational lenses with a somewhat distorted focus, creating a diamond-shaped region wherein objects produce three distinct but distorted images. The observed luminous arcs are so spectacular because they generally consist of three overlapping, extended images of the background galaxy, located near the focus of the gravitational lens.

The recent discovery of a luminous arc in which the three images are not completely merged supports this picture. "You see not a single arc but three sections of an arc, which are clearly separated," Paczynski says.

Finding multiple supernova images would provide useful information about both the geometry of the galactic clusters responsible for producing the lens effect and the geometry of the universe. "Seeing a triple image of a single event like a supernova with reasonably good positions, intensities and time delays could produce all kinds of information, which is extremely difficult to get otherwise," Paczynski says.

In particular, because light passing through a gravitational lens follows different paths to the three image locations, the flash of a supernova would arrive at different times. "Those time delays would depend on the properties of the lens and the geometry of the universe," Paczynski says. "If the lens were perfect, you would get just one image, and you would not get this time delay."

The discovery of supernova images in a luminous arc would strengthen the argument that clusters of galaxies can act as lenses and that luminous arcs truly are multiple images of distant, background galaxies. Says Paczynski, "If you found a supernova that flashes in succession in three different parts of an arc, this would be as good a proof as you could possibly imagine."