I'm forever blowing bubbles

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.

A multiwavelength view of Centaurus A. The bright colors show the varying intensity of radio emission from the powerful jet launched by the galaxy's active SMBH. From extragalactic.info

A multiwavelength view of Centaurus A. The bright colors show the varying intensity of radio emission from the powerful jet launched by the galaxy's active SMBH. From extragalactic.info

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.

An artist's depiction of an active galactic nucleus. The orange accretion torus swirls around the unresolved SMBH at the center. Plasma jets (white) are launched into the intergalactic medium.

An artist's depiction of an active galactic nucleus. The orange accretion torus swirls around the unresolved SMBH at the center. Plasma jets (white) are launched into the intergalactic medium.


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).

M82 is a prototypical starburst galaxy. Dust and gas is launched out of the central 500 parsecs (1500 light years) of the galaxy by the explosions of numerous core-collapse supernovae. From Wikipedia. M82 was also the host of the Type Ia supernova SN2014J, one of my favorite supernovae to argue over (see my previous blog post, a retrospective of IAUS322)

M82 is a prototypical starburst galaxy. Dust and gas is launched out of the central 500 parsecs (1500 light years) of the galaxy by the explosions of numerous core-collapse supernovae. From Wikipedia. M82 was also the host of the Type Ia supernova SN2014J, one of my favorite supernovae to argue over (see my previous blog post, a retrospective of IAUS322)


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 Fermi Bubbles revealed by the FERMI gamma ray satellite in 2010. The Bubbles stretch above and below the plane of the galaxy, with a total extent of 16 kiloparsecs (~40,000 ly) from North to South. From kavlifoundation.org

The Fermi Bubbles revealed by the FERMI gamma ray satellite in 2010. The Bubbles stretch above and below the plane of the galaxy, with a total extent of 16 kiloparsecs (~40,000 ly) from North to South. From kavlifoundation.org


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)

Retrospective: IAU Symposium 322 - The Multi-messenger Astrophysics of the Galactic Centre

Sun, sand and serious science: the prescription for a week in the tropical paradise of Far North Queensland as, for the first time ever, the IAU symposium on the Galactic Center visits Australian shores.

The weather even cooperated with the organisers, with an entire week of almost wall-to-wall rain keeping the 150 or so astronomers from all over the world focussed on a wide range of interesting talks as opposed to long walks on the beach.  

To pick some of my favorite talks to highlight is somewhat self-indulgent and selfish, given my relatively narrow field of interest. I’ll try to be as unbiased as possible (so, still quite biased!).

To kick off, Wednesday’s second session was home of an outstandingly good natured set of talks on positron astrophysics. Thomas Siegert, who has been responsible together with the SPI/INTEGRAL team at MPA Garching for most of the recent analysis of the 11 years of data on positron annihilation in the Galaxy, gave an excellent overview of the topic. The new analysis of the SPI data has revealed some surprising truths about the location of most annihilating antimatter in the Galaxy, in particular the fact that as much antimatter is being annihilated in the Galactic disk as the Galactic bulge. This is contrary to the results from the first 5-8 years of data, which showed a very faint disk component to the emission, half of which could be explained by the annihilation of positrons being produced from the long-lived radioisotope Aluminum-26. Now as it stands, barely 10% of Galactic positrons can originate from this isotope, synthesised by massive stars, and the origin of most bulge positrons remains unexplained. Siegert gives a convincing argument for multiple origins for Galactic positrons, an argument I’ve certainly disagreed with in the past. Also just in are his excellent results on detecting 511keV emission in dwarf satellite galaxies of the Milky Way, which almost certainly deserves a blog post of it’s own. Read the just-published A&A paper here.

I was perhaps a little star-struck by meeting Nidhal Guessoum, whose work on positron microphysics and transport I’ve been using to inform my own work on simulating positron transport, energy losses and annihilation. Guessoum’s talk focussed on the plausibility of a source close to Sgr A* being the source of Galactic bulge positrons: positrons produced by Sgr A* itself, or a source nearby, can be diffusively transported out of the Galactic nucleus by MHD turbulence. The preliminary results presented look exciting, and I’m looking forward to seeing the final results.

Moving away from positrons and back to my old stomping ground of the Nuclear Star Cluster, I feel the need to highlight two excellent talks. First by Marion Grould, who discussed how observations using the GRAVITY instrument on the ESO’s VLT of the star S2, which occupies an elliptical orbit with a period of ~15 years about the central supermassive black hole in our Galaxy, can be used as a test of General Relativity. The simulations seem to say yes, so it will be exciting if the precession of the star due to the warped spacetime around Sgr A* can indeed be detected with this instrument despite the Newtonian perturbations on the star due to unresolved members of the NSC.

The origin of the NSC was also up for debate in two talks that occupied the Friday morning session. Oleg Gnedin’s talk on the formation of the NSC had me almost wishing I had taken the offer of a PhD place at University of Queensland investigating just that. The theory talk by Gnedin was beautifully backed up by a talk by Tuan Do on the observational constraints on the NSC’s formation. Do described the incredibly detailed data now available on the massive stars that orbit the monster black hole in the Galactic Center, and the take home message seems to be “watch this space” as these data are added to and utilized to investigate whether the NSC formed in-situ, or from the disruption of a Galactic bulge globular cluster.

David Nataf took an informative step back from 5 days of focus on, largely, the central 200pc of the Galaxy with his comparative review of the Galactic Bulge stellar population. Highlighted was this lovely work by Melissa Ness and Dustin Lang on the X-shaped feature in the Galactic bulge revealed in beautiful detail in WISE data.

Another particular highlight was the afternoon dedicated to a detailed discussion of the Galactic GeV excess. An observed excess of gamma rays at ~2GeV, a signal observed by the Fermi telescope that is sometimes referred to as the “Hooper Bump” has had two posited explanations: annihilation of a ~10GeV dark matter particle, or an unresolved population of millisecond pulsars. Convincing arguments were presented from both sides, from Richard Bartels, Dan Hooper and Chris Gordon. Francesca Calore presented an updated analysis of the gamma ray data while Doug Finkbeiner presented a thorough Bayesian look at the data.

I’ve somewhat skipped over some of the other topics covered. Remarkable things are being done in radio observations, probing accretion onto Sgr A*, the structure of the SMBH itself and the radio-emitting filaments of gas in the central regions of the Galaxy using incredibly high resolution studies. X-ray data from the likes of NuSTAR is also shedding new light on the Galactic Center region, although the untimely demise of ASTRO-H has disappointed many who were hoping that the new instrument would obtain more detailed observations of the 3.5keV line near the Galactic Center. The Event Horizon Telescope, which sees many of the world’s radio telescopes joined together as a colossal interferometer to probe in fine detail the structure of Sgr A*, was also discussed, along with how the new Cherenkov Telescope Array will provide an even sharper look at VHE cosmic rays being emitted from the Galactic Center region and perhaps shed light on the “Pevatron”.

The great strength of the conference, which brings together scientists with an interest in anything related to the central regions of our Galaxy, is that it opens up the opportunity for interdisciplinary collaboration. Theorists present their work alongside observers, and as the name of the conference suggests, every observation technique from radio through to gamma rays is well represented. The outcome of most of the well-attended panel discussions clear: We don’t really understand the central regions of our Galaxy (or most of our Galaxy, really) and in order to do so, we need to build a unified, self-consistent picture of how it works.