Ghost Galaxies dominate the Universe

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Smaller galaxies are observed to be more dominated by dark matter, with the smallest galaxies known are at least 99 % dark with an incredibly gossamer appearance as shown above. In reality, they are like cannonballs, with a much higher density of dark matter than do giant galaxies. When their first stars died in supernova explosions, they may in many cases have blown away so much of the remaining gas that too few stars were ever formed for us to find the empty halos that are left.

Tiny dSph galaxies outnumber large galaxies like our Milky Way. Since almost-dark galaxies are the most common ones known, darker galaxies may be even more common.

The Fornax dwarf spheroidal (image top of page) is about 1/1400 as bright as the Milky Way and still very easy to see. Meanwhile, the Sculptor is about 1/11000 as bright as our Galaxy; it is noticeably fainter than Fornax, but still easy to discover. The Carina is only 1/56000 as luminous as the Milky Way; it is hard to see but still reconizable at a glance as a cluster of stars.

In contrast, the Sextans dSph, although about the same luminosity as Carina (1/47000 of the Milky Way), is lower in surface brightness and so it is virtually invisible. Sextans was discovered only recently 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, which 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. 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.

Astronomers have long known that small galaxies are much more numerous than large ones. The faintest are believed to be the most numerous galaxies in the Universe.

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, with only 1% of their matter is in the form of stars. 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.

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The smallest galaxies have so few stars that they are hard to see and harder to discover. But 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.

Our Milky Way, a giant barred spiral galaxy, creates stars both large and small, and astronomers had assumed other galaxies did the same. But a new study of Fornax, a dwarf galaxy orbiting our own, suggests that might not be true for for small "ghost galaxies."

Stars of different masses create different chemical elements. Barium tends to form in stars too puny to explode, while very massive stars forge a lot of iron when they detonate.

Fornax has high levels of barium relative to iron, and Takuji Tsujimoto of the National Astronomical Observatory of Japan in Tokyo suggests this is because the galaxy never formed many stars weighing more than 25 suns.

"It's an intriguing suggestion," says Rosemary Wyse of Johns Hopkins University in Baltimore, Maryland. "I don't think it's definitive. But it points to how useful dwarf galaxies really are in terms of trying to understand how stars form."

So how could the size of a galaxy determine the size of the stars it gives birth to? Mark Krumholz of the University of California, Santa Cruz, and Christopher McKee of the University of California, Berkeley, have suggested that the density of a gas cloud might affect the size of stars it can create.
A Chandra x-ray image of the Fornax galaxy cluster below reveals that the vast cloud of ten-million-degree Celsius gas surrounding the cluster core has a swept-back cometary shape that extends for more than half a million light years. This geometry suggests that the hot gas cloud is moving through a larger, but less dense cloud of gas, creating a ram pressure, or intergalactic headwind.

Optical studies of Fornax have identified a large group of galaxies on the outskirts of the cluster that appear to be on a collision course with the cluster core. The motions of this group and the cluster core indicate that they lie along a large, unseen, filamentary structure composed mostly of dark matter that is collapsing and flowing toward a common center of gravity (see the accompanying illustration).

Most galaxies, gas, and dark matter in the universe are thought to be concentrated in such structures, and galaxy clusters are believed to form where the structures intersect.


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