One of the true wonders of the Space Age is that astronomers can now see the universe through all the wavelengths of the electromagnetic spectrum, from radio waves at the low-energy end to the powerful, don't-mess-with-us gamma rays at the high end.
Launched in June 2008, NASA's Fermi spacecraft isn't the first to view the gamma-ray cosmos, but its Large Area Telescope has been finding sources with unprecedented sensitivity and pinpointing their locations to within 1 arcminute (1⁄30 the diameter of a full Moon). Fermi's LAT creates an all-sky map every three hours, and over time its cumulative exposures are revealing never-before-seen details.
One such detail, it seems, is that the Milky Way has been blowing bubbles recently. Meng Su, Tracy Slatyer, and Douglas Finkbeiner (Harvard-Smithsonian Center for Astrophysics) found a matched pair of gigantic, 25,000-light-year-long lobes after sifting through Fermi's data. The bubbles weren't recognized earlier because they aren't very obvious — the team coaxed them into view only after masking bright sources and subtracting a "fog" of gamma-ray emission that pervades the entire sky.
Interestingly, clues to the existence of these bubbles have been lying around for decades. During the 1990s the German-built X-ray observatory Rosat traced out two sets of cone-shaped arcs that coincide with the bubbles' edges. Later, NASA's WMAP satellite detected a "microwave haze" that corresponds to the bubbles' interiors. Other space observatories have spotted telltale evidence too. "There were prior hints," Finkbeiner explains, "but they were only hints."
The real breakthrough came last year when Gregory Dobler (University of California, Santa Barbara), Finkbeiner, and others spotted hazy pools of gamma-ray emission above and below the galactic center that mapped onto those seen by WMAP. "We had barely submitted that one," Finkbeiner recalls, "when I suggested we dig into the data further and see if the edges we could barely see were actually real. By about December 2009 I thought there was probably something there."
Lots of galaxies exhibit two-lobed jets of matter and energy erupting from their cores, but the "Fermi bubbles" defy easy explanation. One possible cause is a titanic burp or energy from the supermassive black hole lurking at the Milky Way's center, perhaps coming after gobbling up something massive in its vicinity. As the blast wave expands into the galactic polar regions, it energizes electrons that then give off gamma rays. However, as Meng and his colleagues point out in their just-accepted Astrophysical Journal article, such bipolar jets rarely display such perfect symmetry or the wide, rounded shapes seen by Fermi's LAT.
Another possibility is that there's been a recent burst of star formation in our galaxy's crowded center. That's apparently what happened in NGC 3079, a galaxy with tall pillars of gas extending from its core.
Fermi's bubbles do have sharp edges, perhaps marking expanding shock fronts, but the gamma-ray spectrum recorded by Fermi is very "hard" (dominated by especially high-energy emissions) — not what theorists would expect from starburst-triggered outflows.
One head-scratcher is that the twin bubbles are remarkably uniform — they're not especially bright along their edges (which would favor the starburst hypothesis) or in their centers (better fit to a black-hole outburst). "There is likely more than one thing going on here," admits Finkbeiner. Either way, the bubbles must have formed only within the last 10 million years and perhaps much more recently.
"These hypotheses require us to live at a special time," Finkbeiner comments. "We always like to start by assuming everything is in steady state and there is nothing special about being alive right now. Given the appearance of these structures, we have to reconsider that."
More observations from Fermi, while useful in clarifying the bubbles' exact shape, probably won't resolve how they formed. Help might come from microwave observations now under way by the Planck space observatory and by a yet-to-be-launched German experiment called EROSITA.