New Theory Explains Mysterious Dark Galaxies

The darkest galaxies in the universe, made nearly entirely of matter which researchers think can zip right through normal matter with virtually no effect, now might be explained by a new scientific model that sheds light on their strange existence.

The normal matter of which the stars, planets, moons and people consist only makes up roughly a sixth of all matter in the universe. The rest is dark matter, the  existence of which is only inferred by the gravitational effects it has on light and normal matter.

Scientists have proposed that some or most dark matter interacts with normal matter very weakly, meaning it can pass right through us and the planet with virtually no effect. There are believed to be as many as 10,000 dark matter particles in any given cubic meter of space in the solar system.

Ethereal galaxies composed almost completely of dark matter are known to orbit the Milky Way and the nearby Andromeda galaxy. These ghostly galaxies, known as dwarf spheroidals, have been stripped of most luminous matter to become dark matter shadows that are almost devoid of gas and possess very few stars.

For instance, while a typical dwarf galaxy might contain several billion stars, and the Milky Way 200 billion to 400 billion stars, "a typical dwarf spheroidal comprises only a million stars," cosmologist Stelios Kazantzidis at Stanford University told SPACE.com. "Recently a number of similar systems with even fewer stars have been discovered in the vicinity of the Milky Way."

Dwarf spheroidals are thought to be ubiquitous throughout the universe. "However, they are so faint that only those in our galactic neighborhood, known as the Local Group of galaxies, have ever been observed," Kazantzidis added.

Dark matter's nature

The origins of galaxies dominated by dark matter have been a mystery to scientists. Now Kazantzidis and his colleagues have developed a scientific model that might explain their creation.

"The most important implication of these findings is the fact that the new understanding of the origin of dwarf spheroidals may soon lead to fundamental insights into the nature of dark matter," Kazantzidis said. "Elucidating the nature of dark matter is one of the grandest challenges of modern-day science."

The new model was tested via months of simulations of galaxy formation on a number of supercomputers around the world. It entails a combination of cosmic ultraviolet rays, an intergalactic version of wind resistance and gravitational tides.

Ram pressure

The origin of dwarf spheroidals began roughly 10 billion years ago when the universe was still hot with ultraviolet radiation from primordial galaxies and massive stars, the model suggests.

Dwarf spheroidal progenitors began life as normal galaxies, Kazantzidis said. Cosmic ultraviolet radiation heated their gases, making it easier to get stripped off. As the progenitor galaxies the researchers studied orbited the more massive Milky Way galaxy, they experienced ram pressure, or a sort of "wind resistance," from gas inside the Milky Way, he said. At the same time, the progenitor galaxies encountered the overwhelming gravitational forces from the Milky Way, which wrenched luminous stars away.

Over billions of years of orbits, nearly all the normal matter got stripped away from the progenitor galaxies, leaving behind dark matter that was not affected by either the cosmic ultraviolet radiation or the ram pressure, Kazantzidis said. What tidal shocking the dark matter did experience was not strong enough to pull away a substantial amount of it.

Missing satellites problem

The research suggests many more small dark galaxies may surround massive galaxies like the Milky Way than are currently observed, potentially solving what is called "the missing satellites problem."

"These galaxies could just be too dark to detect," Kazantzidis said.

There are remaining mysteries to solve regarding dark matter galaxies, Kazantzidis added. "The dwarf spheroidal Tucana represents the biggest challenge to my model because it lies far from any massive galaxy," he said. "Proposing a model for illuminating the origin of isolated dwarf spheroidals requires improving both observations and theoretical predictions."

Kazantzidis and his colleagues report their findings in the Feb. 15 issue of the journal Nature.

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