Naming the Unknown #1

Naming the Unknown is a new series where I highlight interesting papers in astrophysics. Research papers which I find compelling or of general interest will be spotlighted. The title 'Naming the Unknown' comes from accusation that cosmologists have simply begun to come up with names for those things which are not understood; yet, I do not think that anyone would claim that science is at times anything other than coming up with names for the unknown. Scientists define the unknown in terms of the other unknowns and as time passes the first unknown has a context, but ultimately all we have a is a self referential group of symbols that isn't necessarily any more logically sound than where we begun. The relation of fundamental particles (μ⁺ compared to a π⁺, obvious relation no?), the definition words in a dictionary, and all information suffers from the flaw of self referential formalisms. I digress. On to the first paper.

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Fermi Bubbles: Giant, Multibillion-Year-Old Reservoirs of Galactic Center Cosmic Rays. Roland M. Crocker , and Felix Aharonian. DOI: 10.1103/PhysRevLett.106.101102

The Fermi telescope recently discovered evidence for giant gamma ray lobes associated with our Milky Way (you should see my first posted about the Fermi Bubbles here if you are not familiar with this remarkable discovery). The original paper on the Fermi bubbles was an observation description of the evidence for the bubbles; it was found that there are lobes extending above and below the plane of the Milky Way symmetrically with an extent of ~10 kpc and a unique energy spectrum. Several possible formation scenarios for bubbles were put forward, but no single theory was advanced a definitive. In a paper published this month in Physical Review Letters Roland Crocker and Felix Aharonian conclude that the bubbles are naturally explained due to a population of relic cosmic ray protons and heavy ions injected into the bubbles by high density star formation in the galactic center.

There are two general lines of thought as to the source of the bubbles. In one scenario the black hole at the center of our milky way is somehow responsible for the gamma ray lobes. The black hole paradigm can further be broken down into two sub-categories: tidal disruption of a star and active galactic nuclei.

• The tidal disruption of a star occurs when the super massive black hole at the center of our Milky Way, Sagatarius A*, disrupts, or basically eats a wandering star. The tidal disruption of stars by black holes is viable and certainly does occur (see work by Guillochon et al. 2009) with the release of energy, hot plasma, wind, and shocks which could heat up the halo and produces thermal x-rays (see ongoing work by Cheng et al. 2011, unpublished). However, this explanation for the Fermi bubbles is slightly ad hoc and it would have to occur on a periodic basis to account for the bubbles.
• Saggatarius A* is dormant now, but if a star cluster or gas cloud fell/accreted into it in the past it may have undergone an active galactic nuclei like phase which could  emit sufficient radiation and cosmic rays to explain the bubbles. This active galactic nuclei scenario would also have to occur periodically  (10 million years or so) to explain the presence of the bubble.

Lets forget super massive black holes and look at a simple alternative. Crocker and Aharonian invoke ongoing star formation in the galactic center to explain the hard-spectrum, uniform intensity, vast extension, and the energetics seen in the bubbles. Extremely long time scale star formation (on the order a billion years) will have with it an associated cosmic ray population which will be injected into the bubbles by a wind. Cosmic rays are hadrons (mainly protons and some heaver ions) accelerated by non thermal process such as supernovae shocks which move at extremely high velocities and thus carry lots of energy. The cosmic rays will lose energy primarily though collisions with other protons (pp collisions) in the low density plasma of the bubbles and subsequently produce gamma rays electrons, positrons, and neutrinos (intermediate meson particles are also created). This would explain the gamma ray emission at >100 Mev seen form the galactic plane. Under many conditions this kind of gamma ray emission would be expected to trace the underlying ambient density of matter with which the cosmic rays are colliding with, however, in the case of the Fermi bubbles the time scale for proton collisions and time scale for the particles to escape from the system are comparable. The bubbles would be a saturated system wherein the gamma ray luminosity is only proportional to the power injected independent of the gas density; this is a vital point in explaining the morphology of the lobes: they have a hard spectrum out to their edge and then end abruptly.

Based on IRAS satellite data the galactic center star formation rate is ~0.08 solar masses per year in turn implying a rate of ~0.04 supernovae per century. These supernovae inject power at a rate of 10³⁹ ergs per second into cosmic rays. These cosmic rays are removed the from the immediate vicinity of the galactic center and transported into the bubble regions by a super wind generated by the ongoing star formation and supernovae themselves. A wind such as this is observed commonly in many other star forming galaxies such as NGC 3079. The wind speed is ~1200 kilometers per second and has sufficient velocity to escape locally, but it has been shown that the wind should stall at a height less than ~15 kiloparsecs above the plane and this would explain the exact height of the bubbles.

That continuous star formation (and subsequent supernovae) could be responsible for the Fermi bubbles is an Occam's razor kind of solution. It reproduces a number of observations seen in the bubbles and predicts some further properties. For example the electrons and positrons which are created along with the gamma ray emission will synchrotron radiate because of ambient magnetic fields with a luminosity of ~10²⁶ ergs per second which is exactly what is seen in the 20-60 Ghz band by the WMAP satellite (the so called WMAP haze). This kind of after the fact observation is not so impressive, but the authors make various predictions which will be testable in future observations.

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A large aside on the mess of publishing, press releases, and open access. When NASA made the press release on the Fermi bubbles and I first blogged about them I stated that I didn't have anything to go on besides the press release because no paper was available. How wrong was I! The first paper was published in The Astrophyical Journal, titled Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic Wind?, in November of 2010 (at the same time as the press release), but it was posted on the arxiv on the 29th of may 2010. This paper I mentioned here today was published the 16th of August 2010 on the arxiv and then published just a few days ago in March in physical review letters. It is astounding how long the peer review process took for each of these papers, but it is deplorable that NASA doesn't make readily available links to the actual paper. I could have told you about the Fermi bubbles and given a natural explanation for them about a year ago if I had been on top of this.

References


Crocker, R., & Aharonian, F. (2011). Fermi Bubbles: Giant, Multibillion-Year-Old Reservoirs of Galactic Center Cosmic Rays Physical Review Letters, 106 (10) DOI: 10.1103/PhysRevLett.106.101102