Dark Galaxies

Ghostly galaxies speckle the universe. Unlike normal galaxies, these extreme systems contain very few stars and are almost devoid of gas. Most of the luminous matter, so common in most galaxies, has been stripped away, leaving behind a "spectral" shadow.
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These intriguing galaxies-known as dwarf spheroidals are so faint that, although researchers believe they exist throughout the universe, only those relatively close to Earth have ever been observed. And until recently, no scientific model proposed to unravel their origin could simultaneously explain their exceptional content and their penchant for existing only in close proximity to much larger galaxies.

Using supercomputers to create novel simulations of galaxy formation, Kazantzidis and his collaborators found that a "dark matter" dominated galaxy begins life as a normal system. But when it approaches a much more massive galaxy, it simultaneously encounters three environmental effects - "ram pressure," "tidal shocking" and the cosmic ultraviolet background-that transform it into a mere shadow of its former self.

About 10 billion years ago, when the gas-rich progenitors of "dark matter" dominated galaxies originally fell into the Milky Way, the universe was hot with a radiation called the cosmic ultraviolet background. As a small satellite galaxy traveled along its elliptical path around a more massive galaxy, called the host, this radiation made the gas within the smaller galaxy hotter. This state allowed ram pressure - a sort of "wind resistance" a galaxy feels as it speeds along its path - to strip away the gas within the satellite galaxy.

Simultaneously, as the satellite galaxy moved closer to the massive system, it encountered the overwhelming gravitational force of the much larger mass. This force wrenched luminous stars from the small galaxy. Over billions of years of evolution, the satellite passed by the massive galaxy several times as it traversed its orbital path. Each time its stars shook and the satellite lost some of them as a result of a mechanism called "tidal shocking". These effects conspired to eventually strip away nearly all the luminous matter gas and stars, and left behind only a shadow of the original galaxy.

The remaining matter, on the other hand, was nongaseous and therefore unaffected by the ram pressure force or the cosmic ultraviolet background, the scientists posit. It did experience tidal shocking, but this force alone was not strong enough to pull away a substantial amount of the remaining matter or "dark matter".

Scientists elucidate the origin of the darkest galaxies in the universe
from Stanford University news (Image courtesy of Stanford University)

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Stem cells determine the daughter cells' fate
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Intestinal stem cells (ISCs) in the gut of the fruit fly, Drosophila melanogaster, directly determine the fate of their daughter cells. The signaling protein called Delta, seen here in red, determines what type of cell the ISCs will produce.

Large amounts of Delta signal the daughter cells to become gut-lining enterocytes (left panel), while small amounts of Delta signal them to become hormone-generating enteroendocrine cells (see image). (Credit: Images used with permission of the American Association for the Advancement of Science, Science, February 16, 2007)
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From roundworm to human, most cells in an animal’s body ultimately come from stem cells. When one of these versatile, unspecialized cells divides, the resulting “daughter” cell receives instructions to differentiate into a specific cell type. In some cases this signal comes from other cells. But now, for the first time, researchers at the Carnegie Institution’s Department of Embryology have found a type of stem cell that directly determines the fate of its daughters.

Stem cells can participate actively in determining what type of cell their daughters will become right at the moment of stem cell division, suggesting that tissue stem cells might not just be a source of new cells, but could actually be the ‘brains’ of the tissue - the cells that figure out what type of new cell is needed at any given moment.

Because they truly can become any cell in the body, “embryonic” stem cells tend to receive a lot of attention. Yet “adult” stem cells remain in fully-developed organisms, where they replace specific cell types lost to age or disease.

Read more Stem Cells Determine Their Daughters' Fate

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arxiv find dark matter from Sean @ Cosmic Variance
In Search of Dark Matter Galaxies from Centauri Dreams
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