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Up the chimney: pipes of hot gas stream from superbubbles bursting out of the disk of the Milky Way
Natural History, Nov, 2003 by Charles Liu
The Milky Way has gas--and lots of it. Throughout the flapjack-shaped spiral galaxy we live in, there's at least half a quadrillion Earth-masses' worth of free-floating gas, most of it cold, neutral hydrogen just, a few degrees above absolute zero. That's impressive, but it's still just a drop in the bucket on a galactic scale. Even excluding the ubiquitous dark matter that surrounds our galaxy [see "Dark and Darker," by Nell deGrasse Tyson, page 18], gas comprises only about 1 percent of the total mass of the Milky Way.
Still, that 1 percent packs a lot of astrophysical punch. As it flows and ebbs through the galaxy, interstellar gas serves as the raw material of creation--from the tiniest planet-bound life-forms to the grandest stars and nebulae.
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Among the most spectacular patterns of gas flow are galactic chimneys--vast rivers of hot gas thousands of light-years long that can transport matter from the galactic intergalactic space. research group McClure-Griffiths of Australia Telescope Facility in Epping, Wales, has produce detailed map ever made of a galactic chimney, and it has shed new light on the fascinating movement of life-bearing gas into, out of, and throughout the Milky Way.
Imagine the stream of smoke rising from a just-extinguished candle. At first the smoke rises straight up, but then it starts to bend, spreading outward and upward. What the plume looks like a few seconds later depends on the local atmospheric conditions around the candle. Set the candle outdoors, on a breezy day, and the smoke blows away in a formless, ashy wind. Place it indoors, in a quiet room, and the smoke becomes a cloud of wispy filaments, swirling gently until they all blend into a screen of gray.
Gas moving around in a galaxy acts like candle smoke on a cosmic scale. Nearly all the gas in a typical spiral galaxy is confined to the galactic disk. Left undisturbed, the gas moves languidly around in the disk, settling into clouds made of softly swirling wisps and loops. But if a breeze blows through it--say, a stellar wind from a giant star nearby--the gas is driven outward. Depending on the strength and persistence of the winds, the gas gets piled up into new configurations! shells, bubbles, and walls.
In extreme cases, entire clusters of hot, massive stars combine to blow superwinds outward at more than a million miles an hour. The superwinds are then further energized when the most massive stars in the cluster self-destruct in titanic supernova explosions, releasing more energy in seconds than our Sun gives off in a billion years. The result: a "superbubble" forms in the surrounding interstellar space, rapidly expanding to hundreds or even thousands of light-years across. Inside the superbubble is very sparse, hot gas; all around it is a thin, dense shell of the cooler gas that was once drifting near the central star cluster. Eventually, a weak spot on the shell may rupture and the superbubble will burst, allowing the hot gas to stream out and causing the bubble to break up.
But if a superbubble does not burst, it can grow large enough to reach an edge "above" or "below" the galaxy's disk. There, with no more cool gas to pile up against its expansion, the superbubble pops out of the disk and into the much sparser galactic halo. The hot gas pours out of this hole, spewing energy and superheated particles into the halo and sometimes beyond, into intergalactic space. A galactic chimney is born.
Well, that's one idea, anyway. But the model has its problems, and one of them comes from observations of gas in the Milky Way: there aren't enough hot, luminous stars in our own galaxy to generate the supernovae and superwinds needed to make all the shells, bubbles and galactic chimneys that have been observed. Why should other galaxies act any differently? To patch up the galactic-chimney model, energy sources other than stellar winds have been suggested over the years, but none has been altogether satisfactory. Recently supercomputer simulations have suggested that stellar winds aren't even necessary; the random swirling of the gas can give rise to galactic chimneys by chance.
One way to address the problem is to look closely at a chimney's interior walls. If they are smooth, they're more likely to have formed by gentle, fairly random processes. But if the walls have fine structures, ripples, or intrusions, they probably reflect an interaction of hot, sparse gas with dense, cold gas--what you'd expect if a superwind were at work. McClure-Griffiths and her collaborators made images of the galactic chimney designated GSH 277+00 +36; some images show structures more than 3,000 light-years long, others zoom in on details less than thirty light-years long. Studying the overviews, they noted that the chimney bifurcates, both at the top and the bottom of the superbubble, into vast "pipes" that direct the gas thousands of light-years outside the disk of the Milky Way and into the galactic halo. In the detailed images the investigators found countless loops, whorls, drips, and blips on the chimney's inside walls--like huge villi along a giant interstellar intestine. By themselves, the images don't resolve the question of how galactic chimneys form. But they do bring us one step closer to the answer--and afford us a beautiful glimpse of streaming, swirling star smoke.
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