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Seeing supernovas in galactic 'chimneys.'

Seeing supernovas in galactic 'chimneys'

Recent theories hold that clusters of hundreds to thousands of exploding stars, or supernovas, hurl fountains or cannonballs of gas and dust out of the disks of spiral galaxies. Such expulsions would affect the galaxy's shape and ratation speed, while redistributing its chemical elements (SN: 11/11/89, p.310). In the "chimney" theory, barrages of supernovas go off like dynamite explosions at a construction site, blasting steep-sided pits that pockmark the interstellar medium of gas and dust forming the galactic disk.

Now, astonomers present the first observational indication of these supernova-containing galactic chimneys. Evidence that exploding stars occupy such pits comes from records of supernova sightings dating back to 1885, say Sidney van den Bergh and Robert D. McClure of the Dominion Astrophysical Observatory in Victoria, British Columbia.

In two classes of spiral galaxies -- called Sc and SD -- the Canadian researchers found a sharp rise in supernova frequency among disks that appear nearly face-on as viewed from Earth, at a tilt no greater than 25[degree]. This, they propose in the Aug. 20 ASTROPHYSICAL JOURNAL, indicates that "a significant fraction" of the supernovas in Sc and Sd galaxies remain hidden in steep-walled pits, obscured by intervening gas and dust, because the galaxy tilts too much for a direct view. McClure and van den Bergh think such galactic cavities form when "superbubbles" created by the stellar explosions burst out of the disk, leaving behind smoking chimneys rich in supernova cinders.

Optical and radio astronomers have captured images of material erupting out of supposed chimneys, and radio astronomers have found holdes -- sans supernovas -- in spiral disks, notes astrophysicist Carl Heiles of the University of California, Berkeley. But "nobody has ever seen the holes [with the supernovas in them]," he says. "this is the first time there has been any sort of concrete optical evidence for these [supernova-occupied] holes." In 1979, Heiles pioneered the idea of superbubbles created by supernovas.

Some theorists regard the new finding as an unexpected bonus. "It's a natural and consistent result, but we didn't predict it," says astrophysicist Colin A. Norman of the Johns Hopkins University in Baltimore. Norman originated the chiney theory in 1989 with Satoru Ikeuchi of Japan's National Astronomical Observatory in Tokyo.

Others challenge the chimney interpretation of the new data analysis. David Branch of the University of Oklahoma in Norman calls the connection "very speculative" and suggests a simpler model for the tilt effect. "What comes to mind immediately is just a very clumpy interstellar medium," he says.

To van den Bergh and McClure, the sudden jump in supernova frequency for nearly face-on galaxies -- whatever its explanation -- indicates that they under-estimated supernova rates, which astronomers use to help gauge the intensity of star formation in galzies. The Canadian researchers conclude that Sc and Sd galaxies contain two to three times more supernovas than they originally tough

Indeed, the team began analyzing the observational data specifically to try to resolve the difference between their estimate of the supernova rate in Sc and Cd galaxies and a higher extiante ny Guctos A. TAmmann of the Univerisity of Basel, Switzerland. Tammanns' sampling of galaxies that were not oriented face-on. Applying thet were not oriented face-on. Applying the inclination correction to their own estimate brought it into line with Tammann's, not just for the Sc and Sd spirals but for all types of galaxies, van den Bergh says.

Branch and others argue that the newly matching estimates may still be too low, missing many faint supernovas that aren't even visible head-on.
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Author:Weiss, Peter L.
Publication:Science News
Date:Sep 1, 1990
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