How Did Dark Galaxies Emerge?

A new model tries to explain the existence of the darkest galaxies on the mysterious dark matter and their composition on a matter believed to be able to zip right through normal matter with virtually no effect.

While the normal matter that makes up the space bodies (stars, planets, moons) represents about one-sixth of all space matter, the most of the universe's matter is dark matter, whose existence is betrayed by the gravitational effects it has on light and normal matter.

As many as 10,000 dark matter particles are found in any given cubic meter of space in the Sun system and ghostly galaxies made almost entirely of dark matter were discovered around Milky Way and neighboring Andromeda Galaxy.

These galaxies, called dwarf spheroidals, have almost no gas and very few stars.

A dwarf galaxy compasses several billion stars, and our galaxy (the Milky Way) 200- 400 billion stars, "a typical dwarf spheroidal comprises only a million stars," said cosmologist Stelios Kazantzidis at Stanford University.

"Recently a number of similar systems with even fewer stars have been discovered in the vicinity of the Milky Way."

Dwarf spheroidals could be everywhere 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.

"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.

The new computer model was checked on months of simulations of galaxy formation, combining the cosmic ultraviolet rays, intergalactic wind resistance and gravitational tides.

The dwarf spheroidals emerged about 10 billion years ago in the primordial universe.

"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.

The progenitor galaxies chocked with the huge gravitational forces from the Milky Way, which pushed away the luminous stars.

"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.

The force was just not strong enough to pull away a substantial amount of dark matter.

"These galaxies could just be too dark to detect. 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."

Image credit: Stanford University. Bright clumps are dark matter. The central bright region represents the Milky Way's luminous matter (gas and stars).