Ring around a gravitational lens.

Ring around a gravitational lens 

Early this year, two astronomers announced the discovery of what appeared to be huge arcs of luminous matter associated with two small clusters of distant galaxies (SN: 1/17/87, p.36). If real, these sausage-shaped arcs, which are hundreds of thousands of light-years long, might have been an entirely new kind of astronomical object or evidence of fantastically energetic processes going on at the cluster.

Now, however, the two astronomers announce that spectroscopic study of one of the arcs convinces them that it is a kind of optical illusion, an image formed by a gravitational lens.

A strong gravitational field, like that of this galaxy cluster called Abell 370, will bend rays of light coming from a more distant object. It can act like a lens distorting the image of the more distant object.

The spectrum of an object like the arc characteristically shows a continuous rainbow background with bright or dark lines representing resonant emissions or absorptions by different chemical substances superimposed. As the expansion of the universe is carrying distant galaxies away from us, these resonance lines will appear shifted to the red by some amount from the wavelengths at which they appear at rest in the laboratory. The trick is to identify the lines properly and determine the amount of redshift.

In the spectrum of this arc is one prominent emission line, note Vahe Petrosian of Stanford University and C. Roger Lynds of Kitt Peak National Observatory in Tucson, Ariz. If the arc is real matter and associated with the cluster, that line can be identified as an emission of ionized helium. However, as Lynds puts it, "Since September we've been processing the data with greater and greater care to try to reveal and confirm any spectroscopic features other than the one strong emission line.' This study showed that the spectrum lacks the pattern of fainter lines usually associated with the helium line.

The alternative is to identify the bright line as an emission of oxygen, for which an accompanying pattern of faint lines does appear in the arc's spectrum. That means, however, that the light in the arc is actually coming from something twice as far away as the cluster Abell 370.

The theory of gravitational lenses says that if the lensed object, the lens and the observer are perfectly in line, the lens should make an image in the form of a circular ring. This appears to be almost such a case, with part of the ring present. The rest may be suppressed by the presence of a second lens, Petrosian suggests. Abell 370 actually has two closely spaced concentrations of mass, he points out.