What is the first word that pops into your head when I mention "space"? Stars? Planets? Galaxies? What's the first thing I think of when someone mentions "space"?
Bubbles are possibly the most ubiquitous structures in the universe. We see them universally across all size scales - from the imperceptible "space-time foam" of quantum theory right to the massive supervoids - the unfathomably large caverns of nothingness that are surrounded by filaments in which galaxies live. At every stage of the universe's evolution, we can find bubbles. From the time of the earliest stars and galaxies, which produced strong ultra-violet radiation fields that would ionize the neutral hydrogen gas into great bubbles surrounding them, right up to the present day, where exploding stars carve out cavities into the interstellar medium. In fact, if time is "wibbly-wobbly" as the 10th Doctor says, space is almost certainly "bubbly-wubbly".
The nearby galaxy Centarus A (which also goes by the slightly less memorable moniker NGC5128) looks relatively innocuous in visible light, and the galaxy was first discovered by James Dunlop of Parramatta, Sydney, NSW, in 1847. For most of human history, we’ve only ever been capable of seeing our universe in optical light. However, the development of radar during the Second World War had some surprisingly dramatic implications for not only astronomy, but Australian astronomy in particular. Following the end of WWII, many of the devices that had previously been used to detect enemy ships in the Pacific were repurposed, and it was from Australia that many of the first astrophysical radio sources, then “radio stars”, were detected. One such object was Centarus A, one of the first localized extragalactic radio sources identified by the pioneering Australian radio astronomers John Bolton, Bruce Slee and Gordon Stanley in 1949.
Anyone with a fine pair of binoculars or a small telescope in the Southern Hemisphere can observe Centaurus A - it’s around 8 full moon diameters north of the globular cluster Omega Centauri (one of my favourite objects to target while I’m running a focussing sequence on the ANU 2.3m telescope). It has a bright central bulge, partially obscured by a prominent dark dust lane. What isn’t visible to the human eye, however, are the enormous radio lobes, or “bubbles” stretching away from the galaxy.
The radio bubbles stretch a remarkable 1.8 million light years from end to end. In comparison, it’s a mere 27,000 light years to the center of our galaxy, and four light years to the nearest star (Proxima Centauri - I promise that not all the interesting stuff in the universe happens in this one particular constellation). From Earth, the radio emission covers an angular area over 200 times greater than that of the full moon. But what is driving the radio emission so far from the galaxy itself?
The answer lurks at the very heart of Centaurus A. Astronomers understand that all galaxies contain supermassive black holes - SMBHs - in their very enters. Millions to billions of times more massive than the Sun, SMBHs have many ways of making their presence known. Like all black holes, which derive their name from the fact that nothing, not even light, can escape their gravitational pull after crossing the event horizon, the SMBH at the center of Centaurus A cannot be seen directly. However, dust and gas close to the event horizon of the SMBH creates an accretion disk - a donut of material that is accelerated in the gravitational potential of the black hole, a bit like water around a bath plughole.
With a large amount of material being fed into the accretion disk, the SMBH can’t help but be a messy eater. Thanks to the angular momentum of the swirling material, the black hole burps out massive jets of plasma - accelerated electrons and protons - into the intergalactic medium. As this jet ploughs through intergalactic space, the plasma expands outwards like a rocket exhaust, and the electrons in the jet interact with any ambient magnetic field. As electrons spiral around magnetic field lines, they emit a kind of radiation called “synchrotron” radiation. Here on Earth, we can detect this radiation as radio waves. Of course, this jet can’t keep expanding forever. Eventually, it reaches a region where it no longer has sufficient power to continue it’s collimated tunneling through the intergalactic medium, and gradually, over millions of years, a bubble forms, visible in not only radio waves, but also in gamma rays produced by electrons interacting with the radiation field that pervades the entire universe - the afterglow of the big bang.
Not all galaxies have their bubbles blown by active SMBHs. Many galaxies are actively forming new stars. For every hundred or so solar type stars that form (the most common kind in the universe), one star with a mass of greater than eight times that of the sun is formed. Star formation is often concentrated into pockets where molecular gas has accumulated, and such regions can have incredibly high specific star formation rates, resulting in the formation of clusters of stars classified as "high mass" by astronomers. While a star like our sun will live for around 10 billion years, these massive stars may only last a few million years before they exhaust their nuclear fuel, ending their lives as a spectacular core-collapse supernova. Because many of these stars will have formed around the same time, there will be a brief period where several stars will explode in quick succession (quick to astronomers being around a million years).
This "starburst" is capable of driving powerful winds out of galaxies. Because molecular material is frequently concentrated in galactic spiral arms, or galactic centers, all of the starburst activity in a galaxy can drive a bi-conical outflow from either the galactic disk or center. The result is not unlike that of the case of the galaxy with the active supermassive black hole - gigantic radio, gamma ray and X-ray emitting bubbles are spawned above or below the plane of the galaxy. The main difference being that it can take billions of years to blow up a bubble using the combined power of core collapse supernovae, compared to only a few million years to form a jet-powered bubble.
So what about our own Galaxy? Until very recently, it appeared that our Galaxy had never generated these bubble-like structures. However, observations made by the FERMI gamma-ray satellite revealed the so-called Fermi Bubbles in 2010, and radio structures associated with the gamma ray lobes were revealed in observations made at the Parkes radio telescope in 2012. These bubbles stretch above and below the Galactic plane with an extent of some 40,000 light years from the top of the Northern Bubble to the base of the Southern Bubble. If we could observe gamma rays with the naked eye here on Earth, they would cover almost the entire sky. But what created them?
The clues to the origin of the Fermi Bubbles lies in the gamma ray energy spectrum. Spiraling electrons produce radio waves, and the interaction of cosmic rays with the interstellar and intergalactic medium produces gamma rays. "Cosmic ray" is a collective term for the extremely high energy sub-atomic particles that bounce around in interstellar space. Cosmic rays come in two varieties: leptonic (composed of "light" particles like electrons and their antimatter counterpart, positrons) and hadronic (composed of "heavy" particles like protons, neutrons and nuclei). Leptonic cosmic rays produce gamma rays through the "Inverse Compton" process - leptons interact with the ambient radiation field photons (primarily due to cosmic microwave background radiation, the afterglow of the Big Bang, with an additional component from the starlight produced by the galaxy) and emit gamma ray photons, which can then be detected by a space based observer. On the other hand, hadrons first interact with one another in the "pp interaction", producing a type of secondary particle called a pion (a quark and anti-quark bound together). These pions then decay to produce gamma rays, which can once again be detected by space-based observers. The gamma rays from the Fermi Bubbles cannot penetrate Earth's atmosphere, so can only be detected in space.
The gamma rays produced by these two processes are not all emitted at a single energy, but rather at a range of energies. However, constraints on the gamma rays that are being emitted by the Fermi Bubbles, and the various models of the different physics behind the formation of the bubbles means that the jury is very much still deliberating on the origin of the bubbles. If the gamma ray emission is primarily due to leptonic emission, the bubbles are likely to have been formed by a jet from our galaxy's SMBH some 8-10 million years ago. However, one thing we do know is that today, our Galaxy's SMBH is mostly harmless, accreting far too little material to launch a jet (although whether our Galaxy once had an active core, or could do again, is entirely within the realm of possibility - the novel "The Inferno" by astrophysicist and anti-Big Bang campaigner Fred Hoyle deals with exactly the scenario of our Galactic Center "switching on". But don't worry, it probably won't happen within the next few million years as today we know there isn't enough material close to the Galactic Center to support a quasar any time in the near future).
Another compelling argument against the short-lived jet scenario is that the Fermi Bubbles, and their radio lobe counterparts appear to "lean over" to one side of the Galactic meridian. This is likely to be due to the Galaxy's motion through the Local Group toward the Andromeda galaxy. A jet-inflated bubble would be too young for this effect to be so apparent, while a slowly-inflated wind-blown bubble would experience this effect more markedly.
Of the models advocating the starbust-driven-wind model, proposed gamma ray emission models account primarily for the hadronic cosmic ray scenario. The center of the Milky Way galaxy is truly enigmatic: the central 600 light years are responsible for 10% of the total Galactic star formation rate, and a number of young star clusters inhabit the region. The presence of these massive stars and the high star formation rate, together with the large concetration of molecular gas in the region (the so-called Central Molecular Zone) is strong evidence that there has been continual starburst activity, capable of driving a wind with velocities in excess of 500 km/s, over the past few billion years. The shockwaves that result from core collapse supernovae in these regions would be capable of accelerating protons to incredibly high energies, and these hadronic cosmic rays would then be carried away with this wind.
The Fermi Bubbles are among the most enigmatic puzzles associated with our own Galaxy, and the work done by astronomers to better understand them is a remarkable example of cross-disciplinary research. The Bubbles have been observed at almost every wavelength, from radio waves, microwaves, optical and UV (through observation of background sources to constrain the kinematics of the outflow), x-rays and gamma rays. With better constraints on the properties of the bubbles obtained through observation, and advances in computational capabilities allowing the simulation of the Bubbles, the coming years should see the scientific community begin to close in on the origin of the bubbles: the artefact of our Galaxy's violent past, or billion-year old reservoirs cosmic rays from more benevolent starburst-driven winds.
Thanks to Prof. Ron Ekers (CSIRO) for his presentation on the life and times of John Bolton, to Adam Thomas for the delightful analogy of AGN as messy eaters, to Prof. Geraint Lewis for the book recommendation (The Inferno), to Prof. Christoph Pfrommer for the fascinating discussion of the role of cosmic rays in galactic outflows, to Dr. Roland Crocker for the phrase "billion-year old reservoirs cosmic rays" and for teaching me pretty much everything I know about the Fermi Bubbles, and to my copy editor (pictured below)