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Searching for Dark Matter in Dwarf Galaxies

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Searching for Dark Matter in Dwarf Galaxies

Image of Regulus (bright blue on top) with the dwarf galaxy Leo I below. These objects appear close, but Leo I is about 11,000 times farther away from earth.

Image Credit: NASA

The image included in this article is certainly stunning. However, I must apologize is is a bit misleading given the title of this missive. The beautiful ball that dominates this image is actually Regulus, which is a main sequence star and one of the brightest stars in the night sky.

The dwarf galaxy is actually that cloudy blob right below it in the image. That is the dwarf elliptical galaxy Leo I. It is part of our local group of galaxies (54 in all), and is perhaps the most distant of the Milky Way satellite galaxies.

It is quite deceiving as both of these objects appear in the same field of view, but where as Regulus is merely 75 light years from Earth, Leo I is roughly 820,000 light-years; about 11,000 times farther away.

Using Dwarf Galaxies to Search for Dark Matter

While all this is fine and dandy, what does this have to do with dark matter? Well, I've fielded a few questions lately related to this and found a lot of misinformation around the net pertaining to it, so I thought I would try and clarify how this is done.

The search for dark matter is one of the most popular and fascinating areas of current research in cosmology and astrophysics. So far it has proven difficult, but this wasn’t really unexpected.

By definition, dark matter does not interact electromagnetically, so our usual tools for doing astronomy are out, at least in terms of doing direct observation.

And when we think that we’ve gotten some good data sets that show the distribution of dark matter, another result seems to cloud the issue.

So currently we are trying to measure dark matter in several different ways. One way, for example, is doing direct searches wherein we build complicated underground detects to measure interactions as dark matter bumps into atoms in massive chambers. The problem here is that these interactions are not very common and even when they do occur the signal is difficult to distinguish from noise.

As yet, we have not been able to detect dark matter passing through Earth with any confidence. The other methods of dark matter searches study distant objects, looking at either how the dark matter effects the environment around it, or by looking for actual dark matter interactions in these distant objects.

This is where dwarf galaxies come into play. Since dwarf galaxies are small the objects within them are easy to identify and catalog, especially at certain wavelengths. Take the gamma-ray band for instance, there isn’t much activity in dwarf spheroidal galaxies that produces gamma-rays so we can easily identify those.

So the trick would be to isolate a dark matter interaction that produces gamma-rays and search for that signal. Luckily, some dark matter theories, with WIMPs being the currently favored flavor, predict that dark matter is really a class of Majorana particles.

Such particles are their own anti-particles, so when they come in contact with a like particle they annihilate and produce pure radiation. In the case of dark energy they would produce gamma-rays at specific energies.

These signals are expected to be weak, so the closer the galaxy is to our own, the better. This is why we survey the galaxies in our local group, using gamma-ray observatories like VERITAS and Fermi, looking for these gamma-ray signals.

Currently we do not see any gamma-ray signals that can be traced, definitively, to dark matter interactions. But even though these galaxies are close, deep exposures are needed to have a hope of detecting them.

But even if we never find a signal from these galaxies, which should be full of dark matter, that wouldn’t be a terrible result. It would still give us information about the nature of dark matter and perhaps lead us to conclude that WIMPs are not Majorana particles after all.

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