Posted on 12/11/2002 10:04:12 PM PST by Ernest_at_the_Beach
Some very distant galaxies called quasars are lit up dramatically by powerful black holes. Oddly, their radio emissions shimmer and twinkle when detected from Earth, even though radio waves are barely scrambled by Earth's atmosphere the way visible light is.
One quasar, called PKS 0405-385, brightens and fades by 50 percent in less than an hour.
Astronomers have concocted all manner of exotic theories to explain the behavior of the faraway, compact galaxies. The real answer, it seems, is unexpectedly closer at hand but still as interesting as anything imagined.
New observations of PKS 0405-385 reveal that a layer of space stuff, perhaps marking the edge of a giant space bubble in which our Sun and some other stars reside, generates the scintillations of quasars.
Space bubble
After travelling about 10 billion light-years, the radio waves are disrupted when they interact with a denser-than-normal layer of the interstellar medium, composed of ionized gas particles in our own Milky Way Galaxy, says Barney Rickett at the University of California, San Diego.
The waves encounter tiny deviations in what's called a refractive index, Rickett explained, due to turbulent fluctuations in the density of the medium.
"This is like the perturbations in starlight as it enters [Earth's] atmosphere and travels through the turbulent eddies of air," causing stars to twinkle, he said.
The disruptive layer is about 70 light-years from Earth -- surprisingly close, Rickett said -- and is perhaps a remnant an exploded star called a supernova. A paper describing the findings will be published Dec. 10 in the Astrophysics Journal.
The discovery could provide evidence to support a theory that our Sun resides with other stars in a sort of cavity in space carved out by some supernova that would have occurred long ago. If that theory is right, then the Rickett and his colleagues figure the layer they detected marks the edge of the expanding bubble.
Rickett, who led the research, has been looking into this for a long time. He discovered the phenomenon of interstellar scintillation in 1969.
Interestingly, the phenomenon only works with compact sources of emissions. Just as a planet tends not to twinkle because it presents a relatively large disk in the sky compared to a star's point of light, only the smallest quasars will generate the effect in radio waves. One result is that Rickett was able to resolve the radio waves from the quasar with a resolution at least 50 times better than what's achievable with the best ground-based radio telescopes and 20 times better than space-based radio telescopes.
Neat fit
The new work matches conclusions reached by other astronomers examining another quasar last summer. Both studies square with suspicions that have grown in recent years regarding the cause of the scintillations.
The combined results allow finer observations of quasars and have led to a re-evaluation of quasar temperatures and sizes, all of which is now consistent with the prevailing theory that energetic jets shot out from a quasar are powered by matter falling into a supermassive massive black hole at the core a galaxy, Rickett and his colleagues say.
Here's how this is thought to work: As the matter spirals inward toward a supermassive black hole, which can weigh as much as billions of stars, it is superheated. While some of the matter continues into the black hole, some of it is converted to energy and spat out in various wavelengths. That's what makes quasars bright, in everything from radio waves to visible light and even X-rays.
Our own Milky Wayharbors a supermassive black hole, but it is not as active and so does not qualify the galaxy to be termed a quasar.
Quasars, short for quasi-stellar radio sources, were first noticed in the 1950s and '60s and were thought to be nearby stars that behaved strangely. But by measuring their redshift (the change in the wavelength of light emanating from the object based on whether it moves toward or away from us) astronomers figured out that quasars were billions of light-years away.
They are, then, relics of the early universe, since the emissions of a quasar 10 billion light-years away left the quasar 10 billion years ago, taking that long to reach terrestrial telescopes.
More Deep Space News |Astronotes
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