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Star Formation Formed the Fermi Bubbles

by Sarah Scoles


You may recall the Fermi Bubbles: Those 20,000-light-year-tall Cadbury eggs of gamma radiation blowing upUh, you've got a little something, um, on your, yeah. Right there (Credit: NASA/Fermi). from the center of our galaxy. But, like you do when gigantic, high-energy monsters appear in nightmares, you have to ask yourself, "What causes these?"

In the case of your monsters, the answer is likely the temperature of your bedroom or your consumption of Doritos before sleep. But in the case of the Fermi Bubbles, there are two possibilities:


  1. The supermassive black hole in the middle of the Milky Way (Sagittarius A*) is not dormant, and its activity could, and could have in the past, powered outflows like this.
  2. Star formation is intense in the center of the galaxy. There's a lot of stuff in a small space, and the closer together stuff is, the more often "stuff" changes into "stars." These processes could be energizing the outflows, releasing high-energy cosmic rays and causing plasma to shoot up and down from the Milky Way's disk.


Evidence from a new paper titled "Giant magnetized outflows from the center of the Milky Way" suggests that #2 is the winner. The research team, led by Ettore Carretti of Australia's CSIRO, looked at the the regions where the Fermi Bubbles are, but instead of looking for gamma rays, they looked for radio waves.

Why would radio waves help clarify?

Just try to tell me that doesn't look like fun (Credit: daviddarling.com)

All non-neutral particles will interact with magnetic fields. When the charged particles encounter a magnetic field, they spiral around the field lines. The curving of their path causes them to emit radio waves called "snychrotron radiation."

In areas where supernovae are going off and stars are forming, electrons are sometimes accelerated to very high speeds (making them "cosmic rays"), and then they make these loops and emit all those identifiable radio waves.

The Parkes Radio Telescope mapped the radio waves above and below the galactic center.

And guess what? The map lined up nearly perfectly with the gamma rays of the Fermi Bubbles, suggesting that whatever is causing one is also causing the other. And the qualities of the photons suggested that they were not just any old radio waves, but synchrotron radiation.

It's a match! You win! The gamma rays are the colors, and the dotted lines represent the extent of the radio emission (Credit: Carretti).

So the question then is: 

So why do they think it's star formation?

The Bubbles have a "narrow waist," as the Nature paper says--a waist whose diameter happens to correspond to the size of the star-forming gas ring around the center of the galaxy and also approximately a size 4 at J. Crew.

That star-forming area is also "missing" a bunch of radio and gamma-ray emission. Based on the amount of infrared light that busts out of there, scientists expected to see a lot more radio and gamma right in that region. But it's not there. They think that it might instead be jetted up and down into these Bubbles, hiding right under their noses. In fact, if they take the Bubble radiation pretend to put it back into the star-forming region, the level is almost precisely what they expected to see.

For the cosmic rays to get to out that far before losing their energy and to make the shape we see, they need to be going about 1,000-1,100 km/s. For that to happen, and for the magnetic field to be as strong as we observe it to be, 10^39 erg/s of magnetic energy needs to be "injected" into the system every few million years (what a prescription, huh?). That's about as much as 10,000,000,000,000,000,000,000,000,000,000 refrigerators. Not that that means anything to you. Anyway, that's how many refrigerators' worth of energy the star formation (simulations say) would put out.

But that's not a totally constant number, and the Fermi Bubbles, symmetric as they are, are not homogeneous. They do have structure.

On StructureThose curves are not just the outflow: They also trace out the magnetic field lines the outflow was following (Credit: Carretti, et al.).

The structures--called "ridges" in the paper--must have formed at a time that is not now, when the energy injection was different and the magnetic field was different. They show us (if we use some careful backtrack-extrapolation) what star formation was like as many as 10 million years ago. 

Fast fact

If you could see these lobes, they would span 2/3 of the sky from horizon to horizon. But you can't see them. And that's why it's important to have telescopes that see the universe differently from the way our eyes do. Our eyes are great at seeing oncoming cars and what kind of saber-toothed tiger is lurking outside the cave, but they are not great at keeping their shutters open for long periods of time and detecting the rays and radiation that would kill us if we were close-by. Happy New Year, telescopes.


ResearchBlogging.org Carretti, E., Crocker, R., Staveley-Smith, L., Haverkorn, M., Purcell, C., Gaensler, B., Bernardi, G., Kesteven, M., & Poppi, S. (2013). Giant magnetized outflows from the centre of the Milky Way Nature, 493 (7430), 66-69 DOI: 10.1038/nature11734

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Reader Comments (1)

No need for black holes, nor for dark matter to explain the milky way's haze. Star formation is proven to be by magnetic fields. The universe is ruled and shaped by fundamental electromagnetic forces.

January 23, 2013 | Unregistered CommenterHalo

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