When it comes to microwaves from the sky, the primordial cosmic background radiation gets most of the publicity, while everything that originates nearby is lumped into the category of “foregrounds.” But those foregrounds are interesting in their own right; they tell us about important objects in the universe, like our own galaxy. For nearly a decade, astronomers have puzzled over a mysterious hazy glow of microwaves emanating from the central region of the Milky Way. More recently, gamma-ray observations have revealed a related set of structures known as “Fermi Bubbles.” We’re very happy to host this guest post by Douglas Finkbeiner from Harvard, who has played a crucial role in unraveling the mystery.
Planck, Gamma-ray Bubbles, and the Microwave Haze
“Error often is to be preferred to indecision” — Aaron Burr, Jr.
Among the many quotes that greet a visitor to the Frist Campus Center at Princeton University, this one is perhaps the most jarring. These are bold words from the third Vice President of the United States, the man who shot Alexander Hamilton in a duel. Yet they were on my mind as a postdoc in 2003 as I considered whether to publish a controversial claim: that the microwave excess called the “haze” might originate from annihilating dark matter particles. That idea turned out to be wrong, but pursuing it was one of the best decisions of my career.
In 2002, I was studying the microwave emission from tiny, rapidly rotating grains of interstellar dust. This dust spans a range of sizes from microscopic flecks of silicate and graphite, all the way down to hydrocarbon molecules with perhaps 50 atoms. In general these objects are asymmetrical and have an electric dipole, and a rotating dipole emits radiation. Bruce Draine and Alex Lazarian worked through this problem at Princeton in the late 1990s and found that the smallest dust grains can rotate about 20 billion times a second. This means the radiation comes out at about 20 GHz, making them a potential nuisance for observations of the cosmic microwave background. However, by 2003 there was still no convincing detection of this “spinning dust” and many doubted the signal would be strong enough to be observed.
In February 2003, the Wilkinson Microwave Anisotropy Probe (WMAP) team released their first results. The COBE satellite had discovered the CMB anisotropy not 10 years before, and now WMAP had so refined the measurements that several cosmological parameters (the age of the universe, the mean density of matter, etc.) could be read off the splotchy pattern on the sky. This was the main goal of WMAP, but it also provided information on the “foregrounds” — the other sources of microwave emission from our own galaxy. One of these was the spinning dust that I was interested in, and I thought WMAP should be sensitive enough to see it.
When the WMAP data became available to the public, I dove right in. I immediately saw the signal I was looking for, though with a somewhat different spectrum than Draine and Lazarian had predicted. But there was also some extra emission in the inner galaxy, within about 30 degrees of the Galactic center. I tried everything I could think of to make the signal go away, but in the end I had to include it in my fit as a distinct emission component to make sense of the data. This put me in an awkward position: I was about to claim a detection of spinning dust, which many thought plausible but nobody yet believed, but my fit required something new and unanticipated. I called it the microwave “haze.”
Could it be dark matter?
In 2004, a few months after the haze paper was accepted, Amber Miller from Columbia called me to ask what the synchrotron signal from dark matter annihilation would look like in microwaves. In many models, weakly interacting massive particle (WIMP) dark matter can annihilate to gamma rays and other high energy particles. Because the annihilation rate depends on the WIMP density squared, any gamma-ray signal will be strongest where the dark matter density is the highest, e.g. in the center of the Milky Way, or in other galaxies and galaxy clusters. WIMPs generally also make high energy electrons (and positrons), and these particles spiral around in the Galactic magnetic field, producing synchrotron radiation at radio and microwave frequencies. Electrons also scatter low energy photons up to high (gamma-ray) energies, a process known as inverse Compton scattering. So a population of high energy electrons should appear in both microwaves and gamma rays. This is the signal Miller was asking about.
Having just stuck my neck out with the spinning dust and haze, I had little appetite for more controversy. I tried to talk her out of this idea for a while, but she persisted. Finally I agreed it would be a good idea to check if the haze signal in WMAP could originate from dark matter. I sketched out the calculation on the back on an envelope (a large one), and found that for a perfectly plausible spatial distribution of dark matter with plain vanilla properties, a signal like the haze could be produced. I was astonished, because this seemed too good to be true. I triple checked my results, solved the equations in multiple ways, discussed it with colleagues. I thought about that old quote from Burr. Maybe the best way forward was to put this hypothesis out there, and see what it would take to prove or disprove it. I posted my paper on the arXiv, and waited for the response.
Well-meaning senior colleagues reacted with concern, opening conversations with statements like, “This won’t hurt you too much if it is wrong for an interesting reason…” Even though I had a lot of confidence in the result, I had to admit: I was a young postdoc, claiming the haze might be a sign of dark matter annihilation, when many colleagues did not accept the haze was even there at all. This sounded like crackpot territory. The referee didn’t like my paper, and I got a clear message that it would be good to work on something else for a while, so I did.
Fast forward 5 years to 2009. I was a junior professor at Harvard, working with postdoc Greg Dobler on refining our understanding of the haze, and with well known particle physicists like Neal Weiner, Dan Hooper, and Nima Arkani-Hamed, filling in the details of how dark matter might produce the haze (e.g. this paper with Hooper and Dobler). Many particle theorists loved the haze, because it fit into a larger picture of astrophysical signatures of dark matter annihilation, and I drew inspiration and encouragement from them. Astronomers were still skeptical, but a new revelation was just around the corner. Excitement was in the air as the end of summer approached, and with it the long-awaited data from the Fermi Gamma-ray Space Telescope. Gamma-rays were the key, because the same high energy electrons that produce the microwave synchrotron must also scatter ambient photons up to high energies, producing gamma rays. If we could find the expected gamma-ray signal with Fermi, we would remove all doubt that the haze was synchrotron, and settle 5 years of arguments about its origin.
The moment came, and we downloaded the photon event list and worked late into the night making our first maps of the gamma-ray sky. It was clear there was an excess signal there, and over the next few weeks we (my group along with Weiner and his student, Ilias Cholis) quantified it and drafted a paper containing preliminary results on the “Fermi haze.” The synchrotron hypothesis was vindicated, and we could now see the haze was real. We raced to get the paper out.
In science, as in life, it is hard to see what you aren’t looking for. In our excitement, we failed to notice an important detail. Buried in the somewhat noisy gamma-ray images was the outline of an edge, the edge of a figure-eight, centered on the Galactic center and extending up and down 50 degrees in latitude, and up to 40 degrees wide. Our paper on the gamma-ray haze was barely out the door when we started to take this edge seriously. Over the next 6 months, my students Meng Su and Tracy Slatyer refined our analysis and measured the shape and energy spectrum of these features. Inspired by the similarity to radio or x-ray bubbles seen in other galaxies, we named this structure the “Fermi bubbles.” This raised the possibility that the Fermi bubbles were emitting most or all of the haze microwaves, but we needed better microwave data to confirm this.
WMAP took our understanding of Galactic microwave emission to a new level, and Planck is about to do it again. The Planck Surveyor satellite observes the whole sky, with better resolution and sensitivity than WMAP, and at more frequencies. This allows Planck to map the microwave haze like never before, and to tell us more about its spectrum and spatial morphology.
The public release of the Planck data will not happen until 2013, but members of the Planck team have been refining our understanding of the haze based on the latest WMAP data (see work by Dobler and Pietrobon et al.). In February, the Planck team announced (press releases here and here) a confirmation of the microwave haze, showing the cleanest map of it yet. They also juxtaposed the Fermi bubbles with the microwaves, showing convincing spatial alignment. Just recently, the Planck collaboration has posted a paper with a more thorough investigation of the haze. They provide compelling evidence that the Fermi bubbles produce the microwave haze, or at least the part of it with well defined edges. There is still room for a contribution from dark matter annihilation, but isolating such a signal in this complex part of the sky is daunting. We still don’t know exactly what the Fermi bubbles are, how they formed, or how old they are. Additional data from Planck and Fermi will help us understand these fascinating structures, and hopefully teach us something about the black hole at the heart of the Milky Way.
In the nine years since I first saw the haze, we have learned a lot about the Milky Way, and also a lot about the scientific endeavor. I tell my students to watch out for the thing that doesn’t quite fit, the detail that would be easy to ignore. It sometimes happens that the unexpected residual signal you don’t understand will lead to a discovery. More often, it is an artifact of the instrument or analysis, and even if it is real, it may not be what you first think. But the path forward is usually to choose a hypothesis, make a prediction, and test it. You may not find what you are looking for, but what you find may help you see the universe in a new way.