The Z Machine Makes Stars and Art

I met Don Winget years ago on a cloudy night in the control room of the Otto von Struve Telescope. His enthusiasm and excitement was overflowing. I could hardly see his face, lit eerily by red lights, but his words painted a picture of far away white dwarf stars. These stars are pulsating, cooling, and perhaps intertwined with mysterious undiscovered axion particles. He continues extraordinary pursuits. He is looking for white dwarfs on earth with the Z Machine. The Z Machine releases a powerful electrical discharge over a brief amount of time to create plasma, X-rays, shock waves, and an electromagnetic pulse. The Z Machine releases several times the combined energy output of all power plants on earth for a few brief nanoseconds with each shot. Usually it does nuclear weapons research, but this wonderful research aims to simulate aspects of white dwarf stars on earth and it is inspiring art.

Discovering the Higgs Boson

LINDAU, Germany — Tommrrow CERN will make an announcement, likely about the Higgs boson. The Higgs boson is a key part of the standard model of physics and this is a rather exciting discovery. You can read my article about the prospects for the discovery of the Higgs boson over at the Nature Lindau blog where I am writing.

Disassociate Galaxy Clusters

A dissociative galaxy cluster is a cluster of galaxies that just can't keep it together any longer. This may sound like an unnecessary anthropomorphication of galaxies, but it is actually a description of galaxy clusters which have collided and experienced stratification of their constituent parts. In the standard and successful model of cosmology the largest scale structures in the universe, like super clusters of thousands of galaxies, form via the merger of filamentary structures composed of smaller clusters of galaxies. Gravity keeps pulling clusters together along highways of galaxy clusters. Occasionally it is expected and observed that galaxy clusters meet each other head on in cosmic train wrecks moving at thousands of kilometers per second. These traumatic merging events scar the galaxy clusters for life. Their post traumatic stress afflictions include hot shocked X-ray gas and galaxies displaced from their gas halos. Lets consider the three main constituents of a galaxy cluster: stars, gas, and dark matter.

• Clusters are made of aggregates of hundreds or thousands of galaxies and each galaxy is made of hundreds of billions of stars. The stars of the galaxy cluster are conspicuous in that they shine and are observable in pictures, but they account for only about 5% or less of the cluster's mass. The luminous stars of galaxies don't interact much during a collision with another cluster of galaxies and so they act like people in two crowds which are moving in opposite directions. Stars are part of the cosmic ghost train.
• The gas in galaxy clusters accounts for about 10% of the regular (or baryonic) mass in clusters. Gas does interact during a collision. The gas clouds in colliding galaxy clusters slams together like two waves of water meeting and stalls out, but not without undergoing a process known as shock heating first which raises the gas temperature to millions of degrees.Gas is part of the cosmic train wreck.
• The dark matter in galaxy clusters is the most dominant part of the cluster by mass making up about 90% the mass. Dark matter does not interact much. The dark matter halos travel right through each other like ghosts when two clusters collide. However, it is possible that the dark matter does interact slightly and dissociative collisions are a powerful tool in constraining this dark matter interaction. The dark matter halos of the colliding clusters should sail right past each other like two ghost trains, but if the trains slow down even in the slightest it may indicate something strange.

These so called dissociation mergers are difficult to observe and analyze. They require telescopes in space, follow up observations on the ground, observations in multiple wavelength regimes, and algorithms to predict the distribution of dark matter. So far there are six such dissociation mergers systems detected. You would think it would be obvious to spot some of the most massive structures in the universe smashing into each other, but spotting galaxy clusters is actually very difficult because of their great distance. Perhaps in an optical survey, like that in the image below taken by the Hubble Space telescope, over densities of galaxies are detected.

In practice many times it is easier to first identify galaxy clusters through their gas content because the gas content is more massive than the stellar component. Many new clusters are identified by observing the cluster gas's effect in the microwave regime or in the X-ray regime. In the image below taken by the the NASA Chandra X-ray observatory the hot intracluster gas is seen in pink. This image corresponds to exactly the same field of view on the sky as the optical image above.

It may dawn on you that by the very definition of dark matter there is no telescope which can observe it directly. The only in way in which dark matter interacts strongly is through gravity and thus that is how astronomers look for it. Through theoretical predictions and confirmed observations we know that gravity bends light and thus massive galaxy clusters will bend the light of even more distant galaxies. Thus through weak gravitational lensing the dark matter betrays its presence. A careful statistical analysis of galaxy shapes in the optical image above reveals that the galaxies which are confirmed not to be in the foreground cluster are slightly distorted in shape via the gravitational force of the dark matter which is in the foreground. A reconstruction of the total mass in the clusters is shown in the image below where the parts of the cluster which have the most mass are shown in blue. This image corresponds to exactly the same field of view on the sky as optical and X-ray images above.

Finally, a superposition of all the data allows us to glimpse at what a crisis this merging cluster is in. Note that the optical image remains in its original color, the gas is in pink, and the mass is in blue. The image below is known as the Musket Ball Cluster. The actual collision of galaxies occurred about 700 million years ago. We can rewind the collisions in our heads and envision that blue/optical cluster on the right of the image was once on the left and so the blue/optical cluster on the left of the image was once on the right; the clusters collided head on and the gas stopped dead at the center, but the galaxies and dark matter hardly stopped. There are several other images below of other dissociative cluster mergers with the same color scheme. Notice the different morphologies and distributions of mass, stars, and gas. The collisions are not always so straight forward.

Musket Ball Cluster. X-ray: NASA/CXC/UCDavis/W.Dawson et al; Optical: NASA/STScI/UCDavis/W.Dawson et al.

Train Wreck Cluster. X-ray: NASA/CXC/UVic./A.Mahdavi et al. Optical/Lensing: CFHT/UVic./A.Mahdavi et al.

Bullet Cluster. Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.

The awesome thing about these cosmic mergers is how they can constrain the dark matter self-interaction cross-section. That is, exactly who much does dark matter interact with itself? The interpretation of these collisions is not always simple such as in the Train Wreck Cluster (seen above) where there seems to be an extra dark matter core not associated with any bright galaxy at the center of the image, but nonetheless these mergers can be thought of as astrophysical laboratories of dark matter. It would be very interesting to discover that dark matter self-interacts at all, however dissociate clusters will only be one piece of the extraordinary evidence necessary to make that claim.



Dawson, W., Wittman, D., Jee, M., Gee, P., Hughes, J., Tyson, J., Schmidt, S., Thorman, P., Bradač, M., Miyazaki, S., Lemaux, B., Utsumi, Y., & Margoniner, V. (2012). DISCOVERY OF A DISSOCIATIVE GALAXY CLUSTER MERGER WITH LARGE PHYSICAL SEPARATION The Astrophysical Journal, 747 (2) DOI: 10.1088/2041-8205/747/2/L42

Jee, M., Mahdavi, A., Hoekstra, H., Babul, A., Dalcanton, J., Carroll, P., & Capak, P. (2012). A STUDY OF THE DARK CORE IN A520: THE MYSTERY DEEPENS The Astrophysical Journal, 747 (2) DOI: 10.1088/0004-637X/747/2/96

Markevitch, M., Gonzalez, A., Clowe, D., Vikhlinin, A., Forman, W., Jones, C., Murray, S., & Tucker, W. (2004). Direct Constraints on the Dark Matter Self‐Interaction Cross Section from the Merging Galaxy Cluster 1E 0657−56 The Astrophysical Journal, 606 (2), 819-824 DOI: 10.1086/383178

The Boundary Between Knowledge and Belief

The director of CERN, Rolf-Dieter Heuer, talks to European Magazine.

It’s a quest for knowledge. The questions we are examining have been asked since the beginning of mankind. We are humans, we want to understand the world around us. How did things begin? How did the universe develop? That distinguishes us from other creatures. If you go outside at night and look up into the sky, you cannot help but dream. Your fantasy develops, you are naturally drawn to these questions about being and existence. And at the same time, our work has very practical consequences. When antimatter was introduced into the theoretical framework 83 years ago, nobody thought that this had any practical relevance. Yet today, the concept is used in hospitals around the world on a daily basis. Positron Emission Tomography (PET) is based on the positron, which is the anti-particle to the electron. Or take the internet. The idea of a worldwide network started in 1989 here at CERN, because we needed that kind of digital network for our scientific work. That’s the beauty of our research: We gain knowledge but we also gain the potential for technological innovation.

More here.

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