Dwarf Galaxies

There exists a sub-class of galaxies in the study of astronomy called Dwarf Galaxies. These galaxies are actually just like their larger siblings, but contain significantly fewer stars and sometimes have some different properties.

But don't mistake these smaller galaxies as something other than a galaxy - like a globular star cluster or something - because they maintain certain characteristics that mandate they be included in the ranks of galaxies.

Characteristics of Dwarf Galaxies

What defines a dwarf galaxy is somewhat nebulous, but effectively they are just smaller versions of "normal" galaxies. Generally, though, they are galaxies that may contain unto a few billion stars. This may sound like a large number, but it is quite small compared to the hundreds of billions of stars (or more) present in the regular galaxies.

There are some galaxies that appear to be in-between dwarf galaxies and typical galaxies. The Large Magellanic Cloud (an irregular galaxy), which our Milky Way is consuming, has about 30 billion stars. This is much greater than the typical dwarf galaxy, but somewhat smaller than typical irregular galaxies. Therefore its classification has been continually debated.

Because of their small size, dwarf galaxies, particularly elliptical or irregular types, can be mistaken simply for large star clusters. However, while star clusters are decidedly lacking in dark matter, dwarf galaxies are forming within dark matter fields.

(This is actually typical of all galaxies.)

Types of Dwarf Galaxies

There really is no such thing as a "typical" dwarf galaxy since they can come in a number of varieties and, as mentioned above, are sometimes even difficult to identify as dwarf galaxies because their star count doesn't fit squarely into one category or the other.

So dwarf galaxies usually fall into one of three broad categories:

  • Dwarf Spiral Galaxies: These smaller versions typical spiral galaxies contain the signature spiral arms and central bulges, but usually contain only a few billion stars, compared to the hundreds of billions that inhabit the Milky Way Galaxy, for instance. There is also a subclass of draft spiral galaxies known as Dwarf Barred Spiral Galaxies. These, like their larger siblings have much in common with spiral galaxies, except that they have a bar structure that passes through the bulge, off which the sweeping arms terminate.
  • Dwarf Elliptical Galaxies: The dividing point between a normal elliptical galaxy and the dwarf elliptical is difficult to identify. That is because these galaxies are spherical or elliptical in shape (hence the name) and are defined by a diffuse volume of stars. There is really no limits to the sizes of these galaxies so there is often debate about whether an elliptical galaxy should be considered a regular or dwarf variety. One major difference, however, might be their formation. Regular elliptical galaxies are thought to form from the merger of two spiral type galaxies. On the other end of the spectrum, the dwarf variety may have actually formed in the early Universe within distributions of dark matter, though this is not yet well understood. The alternative is that, like their larger counterparts, they formed from mergers of dwarf spiral galaxies, but they would have to have been extremely star and gas poor to make such small elliptical galaxies.
  • Dwarf Irregular Galaxies: Irregular galaxies, whether of the dwarf variety or regular, usually were at some point a spiral or elliptical galaxy and were distorted by some gravitational disturbance (like a passing galaxy or the merger of a smaller galaxy). They don't contain any identifiable form: no spiral arms, no bulge, no elliptical or spherical shape. And, like dwarf ellipticals, can sometimes be large enough to blur the line between dwarf and regular.

Dwarf Galaxies and Indirect Dark Matter Searches

At first glance dwarf galaxies would seem to be scientifically uninteresting. Because of their smaller star count and lower gas density, there isn't a whole lot going on sometimes, particularly at the high end of the energy spectrum.

However, researchers have realized that they can use this to their advantage.

Nearby dwarf galaxies, and there are a number of them in our local group (the group of galaxies closest to the Milky Way, and with which our galaxy gravitationally interacts) are pretty well understood in terms of what sources of high energy radiation are present.

This fact presents the opportunity for doing indirect dark matter searches. Some theories of dark matter - though not all - suggest that this mysterious form of matter is actually a very specific type of particle that is its own anti-particle. In that case, when dark matter interacts with other dark matter they annihilate, producing gamma-rays.

So using ground based gamma-ray observatories like VERITAS and space based gamma-ray instruments like the Fermi Space Telescope, researchers to can these dwarf galaxies for "extra" gamma-ray signatures that could be associated with dark matter collisions. This research could shed light on the nature and distribution of dark matter in galaxies.