Tag Archives: galaxy halo

Supernovae Destroy Dwarf Galaxies: Dark Matter is Safe

The existence of dark matter has not exactly been under threat – the ratio of dark matter to ordinary matter in the universe is well established, at about 5:1 in favor of dark matter. Consistent results are found between observations of the cosmic microwave background, observations of clusters of galaxies, and observations of the rotation curves of galaxies. (The MOND theory as an alternative to dark matter does not do well at scales greater than that of individual galaxy rotation curves.)

But there has been an issue around galaxy formation. It has been expected that many more dwarf galaxies should be seen in our Local Group, which is dominated by the Andromeda Galaxy (#1) and our Milky Way Galaxy (#2, sorry folks), along with the aptly named Triangulum Galaxy (#3).

Where are the Dwarfs?

Our Milky Way has only around 30 dwarf galaxies as companions, the best known of which are the Large and Small Magellanic Clouds. While a few more have been discovered only recently, simulations of galaxy formation have previously suggested this number ought to be more than 1000! This posed a problem for both our understanding of dark matter and our understanding of galaxy formation.

Now, from CalTech comes a much more detailed simulation of how galaxies similar to the Milky Way are formed. The researchers used over 700,000 CPU hours of supercomputer time to create the most detailed simulation ever of the galaxy formation and evolution processes.

“In a galaxy, you have 100 billion stars, all pulling on each other, not to mention other components we don’t see like dark matter. To simulate this, we give a supercomputer equations describing those interactions and then let it crank through those equations repeatedly and see what comes out at the end.”  – Caltech’s Phil Hopkins, associate professor of theoretical astrophysics.

Death by Supernova

Postdoc Andrew Wetzel and Prof. Hopkins paid special attention to the effects of supernovae. When supernovae explode they release tremendous amounts of kinetic energy. They generate powerful winds that reach speeds of over a thousand kilometers per second.

In a dwarf galaxy an individual supernova can have substantial effect. The researchers’ simulations indicate that dwarf galaxies can actually be destroyed by the effect of even a single supernova during their early history. Stars and gas that would form future stars can both be blown out of the dwarf galaxies. In addition, many dwarf galaxies in the Milky Way’s neighborhood would have been destroyed by the gravitational tidal forces of the Milky Way, the simulations show.

These advanced galaxy evolution simulations appear to solve the dark matter and dwarf galaxy problem. The authors plan to refine their results and develop even greater understanding of galaxy formation with simulations of even greater power in the future.


Simulated View of Milky Way Galaxy
The formation and evolution of the galaxy were done on a supercomputer. Credit: Hopkins Research Group/Caltech




Dark Matter on Mars?

Yes, there is most likely dark matter on Mars, and on Earth as well, and throughout our Solar System. The Curiosity rover will not be searching for dark matter, it not only does not have the right instrumentation, but it also remains on the surface, which is not the way to pursue dark matter searches. On Earth, the direct detection experiments searching for dark matter are made primarily by deploying large crystalline detectors in laboratories within deep mines or inside mountains. A lot of shielding is required. There are too many other sources such as cosmic rays and solar wind particles that would interfere with the search.

Credit: NASA/JPL-Caltech/MSSS  Curiosity Rover

“The expected rate of WIMP interactions is already constrained to be very small (less than one event per kg-year) and the expected nuclear recoil energy is very low (100 keV or less) so background rejection is crucial…  neutrons produce nuclear recoils identical to those from WIMP interactions. To eliminate the fast neutron flux induced by cosmic rays, such experiments must be located deep underground.” – http://astro.fnal.gov/projects/DarkMatter/darkmatter_projects.html

And given the expected interaction rate, one needs detectors with many kilograms of detector volume; a ton or more is desirable. They also need intensive calibration, care and feeding by scientists and technicians.

The dark matter density is expected to be comparable throughout our solar system and in the neighborhood of the Sun. The canonical value that most models use comes from measures of our galaxy’s dynamics and is 0.3 GeV per cubic centimeter (cc). This density is determined by looking at the large-scale gravitational effects of dark matter spread throughout our Milky Way galaxy, including its effect on the rotation rate as a function of distance from the galaxy’s center. It’s important to determine this number to get a handle on the predicted flux of dark matter particles impinging on a detector in one of the labs on Earth.

What does this 0.3 GeV per cc mean in terms of particle density? Well the mass of a proton is about 0.9 GeV where GeV is a billion (giga) electron-Volts and one electron-Volt is the energy of moving a single electron through a one Volt electron potential. This is a convenient unit of measurement for particle physicists. Since GeV is an energy, strictly speaking the mass is in units of GeV/c² (energy divided by the speed of light squared, in accordance with Einstein’s famous equation), “GeV” is used for shorthand. So if the mass of the dark matter particle were equal to the proton, that would imply about 1/3 of a particle per cc. But dark matter particles are heavier than protons according to particle physicists, significantly heavier.

On very large scales, in the early universe, slightly over-dense regions collapsed out of the general Big Bang driven-expansion due to their internal gravity (dominated by the dark matter within) and the ordinary matter in those regions formed galaxies and groups of galaxies, including clusters with up to 1000 or more galaxies. At the cluster of galaxies level the dark matter is dominant, but within an individual galaxy like our Milky Way ordinary matter can dominate due to the high degree of contraction possible with ordinary matter. Dark matter does not clump to the same degree as it can’t “cool off” via radiation.

So while dark matter dominates on the largest scales within the universe, amounting to 5 times as much matter as ordinary matter, within our galaxy the ordinary matter density is larger. Ordinary matter clumps more easily than dark matter, since it interacts with itself and light readily and undergoes cooling via radiative processes. The removal of energy via radiation allows matter to clump into molecular clouds and in turn form into stars and planets from that material.

A recent study by astronomers and astrophysicists associated with research institutes in Switzerland, Germany, the UK and China has used a new method and new data from a large sample of red dwarf stars to measure the dark matter density in the solar neighborhood. In this case what we mean by the neighborhood is up to about 3000 light-years from the Sun, and what is measured is an average number across that large region.

Their method makes fewer assumptions than other methods about the nature of the shape of our galaxy’s halo, i.e. the details of how the density of regular matter falls off as one moves away from the galaxy center. The new result is about 0.9 GeV per cc and comes with a large error bar of +/- 0.5. It does suggest the correct value may be 3 times higher than that previously assumed. Since the proton mass is 0.9 GeV, coincidentally, this would imply the equivalent of around one proton per cc in mass density. The dark matter particle is heavier, so the number of dark matter particles would be lower than one per cc.

Dark matter is thought to be due to a new particle, a WIMP (weakly-interacting massive particle) of some sort such as the lightest supersymmetric particle, which would remain stable against decay over billions of years. No such supersymmetric particle is yet detected, but the LHC (Large Hadron collider outside of Geneva) is working on the supersymmetry problem as well as the Higgs boson. The apparent discovery of the Higgs boson with mass around 125 GeV by the ATLAS and CMS experiments at LHC is consistent with supersymmetry.

While we don’t know the mass for dark matter WIMPs, the range of somewhat less than 10 GeV up to 1000 GeV is generally favored. Using the new value of 0.9 GeV per cc for dark matter density indicates that if the dark matter mass is around 10 GeV then there would be 1/10 of a dark matter particle per cc (or 100 per liter). If the mass of the particle is around 100 GeV then there would be one dark matter particle per 100 cc (which is 10 per liter).

So even with the potential  increase of a factor of 3 in actual density, this is an extremely rare particle and each one has a very low probability of actually interacting with ordinary matter and being detected. Which is why detectors are growing to 1 ton in size and multiple years of very sensitive observations are needed to place limits on the amount of dark matter or, more hopefully, obtain a statistically significant positive detection. A lot of progress is expected in the next couple of years with a new generation of larger detectors coming on line.


S. Garbari, C. Liu, J.I. Read, G. Lake 2012, Mon. Not. R. Astron. Soc., submitted; arXiv:1206.0015v2 “A new determination of the local dark matter density from the kinematics of K dwarfs”