Bursting bubbles in the galactic disk appear to be source of hot gas permeating the Milky Way galaxy and its halo

By Robert Sanders, Public Affairs

BERKELEY-- Though the Milky Way galaxy is studded with dozens of superbubbles - hot, expanding spheres of gas created by exploding stars - astronomers could only speculate that these superbubbles were the source of much of the hot gas that permeates the galaxy and its halo.

Now they have found a smoking gun - a superbubble that has burst and is spewing hot gas into the galactic halo right below our feet.

The discovery is reported today (Wednesday, June 2) at the national meeting of the American Astronomical Society (AAS) in Chicago by University of California, Berkeley, astronomer Carl Heiles and a University of Wisconsin team headed by Ron Reynolds. (photo and caption)

The burst superbubble is the Orion-Eridanus superbubble centered on the well-known Orion nebula, a nursery of young blue stars sitting in the sword of the constellation Orion. The superbubble is the result of at least six or seven supernova explosions that have occurred in the past five to 10 million years, all of which have combined to blow the bubble to a diameter of about 1,000 light years, extending far beyond Orion into the neighboring constellation Eridanus. One light year is about six trillion miles.

Heiles discovered the Orion-Eridanus superbubble in the 1970s after a radio survey of the sky at a radio wavelength of 21 centimeters, where atomic hydrogen gas emits. Now he has used an updated 21-centimeter line survey done by Dutch researchers, together with an optical survey of the hydrogen line emission from hot ionized gas by Reynolds and postdoctoral researchers Steve Tufte and Matt Haffner, to look more closely at the superbubble.

By combining these observations with other observations in infrared, optical and X-ray wavelengths, he was able to put together the most complete picture to date of the superbubble.

In that picture, the near and far walls of the superbubble are readily visible because of the hot, 21-centimeter line emissions of cold atomic hydrogen and the optical line emission of hot ionized hydrogen. In one area, however, he could see no rear wall. (See area marked by big yellow X on color illustration.) Looking at the X-ray data, he clearly saw gas leaking out.

"This hole is like a champagne bottle just uncorked," said Heiles, a professor of astronomy at UC Berkeley. "The high pressure gas inside pops out the hole with explosive force."

The gas inside the bubble is very hot, Heiles says, almost 10 million Kelvin (18 million degrees Fahrenheit). Also visible, though, is a cooler component of gas at about 1 million Kelvin (1.8 million degrees Fahrenheit) that is clearly not confined to the inside of the bubble, but instead occupies a much larger area.

Heiles believes that this cooler gas is hydrogen that has popped out the hole, expanding and cooling as it escapes. The X-ray picture appears to be consistent with this idea.

"Without the combination of these datasets at different wavelengths, you cannot see the interaction between the different gas phases clearly," Heiles says. "For example, you tend to interpret the radio data on the wall gas without appreciating the close proximity and interaction with the hot interior gas, because the radio data don't see this hot gas at all."

The new evidence supports the theory that the hot gas in superbubbles is the source of the hot gas in the galactic halo - a roughly spherical ball of gas and dust centered on the disk of the galaxy but with a much larger diameter.

Another paper presented at the AAS meeting by Barry Y. Welsh and Daphne Sfeir of UC Berkeley's Space Sciences Laboratory reports a similar burst superbubble - a much older one called the Local Bubble, which surrounds our Sun.

"People have long speculated that the hot gas originates in superbubbles that grow so large that their hot interior gas escapes into the halo, but the Eridanus superbubble is not THAT large," Heiles says. "Nevertheless, gas is escaping, and it will expand into the galactic halo. It seems, then, that even modest-sized superbubbles like Eridanus help to fill the halo with hot gas."

The Orion-Eridanus superbubble is a Rosetta stone, of sorts, for superbubbles, Heiles says. It is nearby - the center is only 1,500 light years from Earth - and easy to observe, since it lies below the disk of the Milky Way with no other confusing objects along the line of sight. Thus astronomers can study it far better than any other superbubble in the galaxy.

"This work is the first detailed comparison of the different gas phases for any superbubble on a global scale, that is, a scale comparable to the size of the whole superbubble," Heiles says. "The Eridanus superbubble is perhaps the only one for which this can be done because of its proximity and its away-from-the-galactic-plane location, which make it easy to study with no ambiguity from other foreground or background objects along the line of sight."

It was formed by the explosions of at least three previous generations of stars that formed in or near the current Orion stars. The supernovae swept out a cavity containing rarefied gas heated to millions of degrees, with a cool outer wall containing gas that has been swept up from inside the cavity. It is expanding with a speed of about 20 kilometers per second (about 13 miles per second) - 100 times the speed of a jet plane and comparable to the shock speed of a powerful nuclear bomb.

The Orion-Eridanus superbubble is about one-hundredth the diameter of the Milky Way galaxy, but there are many other larger superbubbles. The biggest are some five times larger in diameter than Orion-Eridanus, produced by hundreds of supernovae.

Heiles says that gas in galactic halos should exist not only here and now in galaxies like our own, but also in very distant galaxies in the early universe. In fact, there is evidence this is true.

Spectra of high-redshift galaxies exhibit the 'Lyman alpha forest' - a series of closely-spaced absorption lines - that must arise from a large number of intervening, high-redshift galaxies.

"The only way we can possibly see so many lines is if the intervening galaxies are large, and this means that the lines must be produced by large halos of gas, since galaxies themselves are not big enough to produce so many lines unless they have huge halos," he says. "These halos probably are produced by superbubbles, and perhaps by ordinary superbubbles with holes, like Eridanus, instead of the more rare large ones that become so large that you can think of them becoming part of the halo themselves."

This work builds on previous work by many people who have studied the Orion-Eridanus superbubble in various wavelength bands and have also put different wavelength data sets together. Apart from the people involved with the various data sets, the most significant earlier work was done by the X-ray group at the University of Wisconsin, which has published several previous papers on the Eridanus superbubble that have also included multi-wavelength data sets.

This group has produced many graduate students who have now gone on to other institutions. For the Eridanus region, Steve Snowden, a former graduate student at Wisconsin now at NASA's Goddard Space Flight Center in Maryland, was the key person in publishing the Roentgen Satellite (ROSAT) data on the diffuse X-ray emission sky survey and allowing the present work to be done.

Other data came from the 25-meter diameter Dwingeloo radio telescope in Holland, which provided the 21-centimeter line observations which highlight the gas in the superbubble wall; the 100-foot diameter radio telescope of the Institute of Radio Astronomy in Argentina (IAR), which provided the southern-sky portion of the 21-centimeter line observations; a 0.6-meter automated optical telescope on Kitt Peak in Arizona, which is operated by the University of Wisconsin and funded by the National Science Foundation, and provided the H-alpha line; the Infrared Astronomical Satellite (IRAS), used for the far-infrared observations which highlight the gas in the superbubble wall; and ROSAT, used for X-ray observations that highlight the hot interior gas.

The Dutch telescope is supported by the Netherlands Organization for Scientific Research. IRAS was a cooperative effort between the United States, the Netherlands and the United Kingdom. ROSAT was a cooperative effort between Germany, the United States and the United Kingdom.

Carl Heiles and Ron Reynolds are supported by independent grants from the National Science Foundation.

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