BOSTON — TO the surprise and delight of astronomers, the sharp-eyed Hubble Space Telescope has spotted two unexpected cosmic gems.
New photos show that the Whirlpool Galaxy may have a black hole at its core, appropriately marked by a gigantic natural "X." Other photos show that the core of a powerful infrared radiating galaxy called Arp 220 supports vigorous star birth and supernova star explosions. This strengthens astronomers' suspicions that Arp 220 probably is the product of an earlier collision of two "parent" galaxies.
Hubble astronomer Stephen P. Maran at the National Aeronautics and Space Administration Goddard Space Flight Center in Greenbelt, Md, says: "I think that what we're learning is that every time we look at the center of a galaxy with Hubble it doesn't look like what we expected."
The Whirlpool Galaxy (M51) - the first spiral galaxy found - is a case in point. We see it face on at the cosmically near distance of 20 million light-years in the constellation Canes Venatici. Astronomers have studied it for more than a century without seeing that telltale "X." Dr. Maran says that "it's almost the last place I would have looked for the kind of activity" the new Hubble photos show.
That activity evokes the concept of a black hole - an object so dense not even light escapes its gravity. But dust, gas, or even whole stars falling into it radiate strongly. Theory predicts that infalling matter will organize itself into a doughnut-shaped ring called a torus rotating around the black hole.
Astronomers expected that the ring would hide the black hole itself from direct view. It also should channel radiation from the infalling matter into two beams shooting out through the holes of the "doughnut." This is what the Hubble photos seem to show.
The nature of the thinner bar of the "X" is not yet known. But the thick bar looks like the outer rim of just such a black-hole torus. Bright areas on either side appear to indicate emerging beams of radiation.
Commenting on the photos last month in the announcement by the Space Telescope Science Institute at Johns Hopkins University in Baltimore, institute astronomer Holland C. Ford said, "If these ideas are correct, M51 provides the first direct view of a torus which both fuels a massive black hole and hides the hole from direct view...." Dr. Ford and his colleagues now will try to determine just how massive the suspected black hole may be.
Meanwhile, astronomer Edward Shaya and graduate student Dan Dowling at the University of Maryland in College Park are concerned with a black hole in a different kind of galaxy. They also are intrigued by other objects that Hubble photos have revealed there. These include giant star clusters within 2,000 light-years of the galaxy's bright nucleus. In these clusters, stars evolve rapidly and explode as supernovae. This should happen so frequently that "over time, the core of this galaxy should resemble a s tring of firecrackers popping off," according to the institute announcement.
This galaxy, Arp 220, is about 230 million light-years from Earth in the constellation Corona Borealis. It's one of about a dozen galaxies that emit 95 percent of their radiant energy as infrared "light." A dark lane of dust bisects the galaxy's image. Astronomers have suspected that this may hide a massive black hole. Matter falling into that black hole would release energy that heats the dust so that this dust radiates infrared "light."
Dr. Shaya says, "I do see one object right in the galaxy center that is very bright and very small. The best current model for such an object is [the concept of] a black hole." However, he adds, it is now clear that perhaps 60 percent of the galaxy's radiant energy comes from the newly discovered star clusters rather than from the imputed black hole at the galaxy core.
Shaya says that, thanks to Hubble's sharp view, Arp 220 is expected to provide a cosmic "laboratory" for studying star formation and supernovae. He notes that, in spite of the Hubble mirror flaw, "After computer enhancement, we can get the telescope's full expected resolution" for bright, close-by objects.