Astronomers suggest that there may be a large number of dark galaxies. In fact, galaxies made almost entirely of dark matter may outnumber luminous galaxies like our Milky Way.
This conclusion is based on a study of dark matter properties in 43 galaxies ranging from the most luminous spirals to the faintest galaxies known. It shows that dark matter properties correlate with luminosity and that it is normal for the smallest galaxies, which contain just a faint scatter of stars, to have high densities of dark matter. These galaxies look gossamer, but they are like cannonballs: they contain a much higher density of dark matter than do giant galaxies. Almost-dark galaxies like these are the most common ones known. Darker galaxies - ones with too few stars to be discovered - may be more common still.
These results are being presented today at the 193rd Meeting of the American Astronomical Society in Austin, Texas, by John Kormendy of the University of Hawaii and Kenneth C. Freeman of the Australian National University and Mount Stromlo Observatory.
In particular, they help us to understand the growing evidence that the smallest known galaxies, companions of our Milky Way that have only 1/100,000 of its luminosity, have massive dark halos. It has not been clear whether these are normal or a fluke or even a mistake in how we interpret the observations. The dark matter correlations show that these dwarfs are entirely normal for their low luminosities.
Kormendy emphasizes the contrast between what we see and what is there: "We barely see the faintest dwarfs - they contain hardly any stars. But the central dark matter density is about 1 solar mass per 30 cubic light years. This is about 100 times larger than the dark matter density in a giant galaxy and several times larger than the density of stars and gas in the disk of our Milky Way. These dwarfs may look insubstantial, but they are like cannonballs to a giant galaxy."
Their dark matter densities are closely connected with the average density of the Universe when they formed. Freeman concludes that "The high densities of these dwarfs suggest that they formed very early in the history of the Universe, when it was 1/3000 or even 1/10000 as old as it is now. The faintest dwarfs are almost pristine remnants of the earliest time of galaxy formation."
Recent work also shows that smaller galaxies are much more dominated by dark matter. Our Milky Way is a large galaxy, and its main body is about 50% dark. Galaxies with 1/100 of the luminosity of the Milky Way are about 90% dark. The smallest dwarfs that we know about are almost completely dark: only 1% of their matter is in the form of stars. Astronomers believe that they understand why: less massive galaxies have a weaker gravitational hold on their contents, so the first stars that die in supernova explosions eject more of the remaining gas in smaller galaxies. These explosions have little effect on the dark matter. So small galaxies have less gas with which to make stars and therefore low stellar densities despite their high dark matter densities.
The smallest galaxies have so few stars that they are hard to see and harder to discover. Kormendy notes: "These galaxies do not know that holding onto 1% of their mass in gas is magic - that turning this 1% into stars will allow us to discover them. The first stars that form in many dwarf galaxies may eject so much gas that we cannot find the empty halos that are left."
Freeman cautions that "It is a long way from predicting dark galaxies to proving that they exist. But looking for them may motivate a lot of astronomers, including us."
Not surprisingly, such objects are exceedingly hard to find. One way may be to look for very low star densities using computerized searches.
The dark matter correlations provide a new handle on this slippery problem. For example, they disprove one theory about how most dwarfs form. Some astronomers have postulated that dark galaxies are produced when large galaxies collide. Tides pull long tails of gas and stars out of the parent galaxies. If small lumps in these tails are dense enough, gravity can hold them together. They can survive the collision and look like little galaxies afterward. The idea is almost certainly correct. And the vicinity of our Galaxy is a good place to look for tidal remnants, because we are currently interacting with two small neighbor galaxies, the Large and Small Magellanic Clouds. But Kormendy points out that "Large galaxies have low-density dark halos, and tides can only stretch what is already there. So tidally made dwarfs should have lower dark matter densities than their parent galaxies. Instead, the smallest dwarfs have higher dark matter densities than either the Milky Way or the Magellanic Clouds. Therefore they are real galaxies, not tidal fragments." This means that the number of dwarf galaxies is not tied to the limited number of galaxy collisions that have happened since big galaxies formed.
This work was supported by the National Science Foundation.