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Dark tales from the neighborhood

How close galaxies lost their luminous matter

Munich, 02/16/2007

Draco, Ursa Minor and Andromeda IX are satellites of our Milky Way and the Andromeda galaxies. They are members of a class of faint galaxies, devoid of gas, known as dwarf spheroidals – and share one characteristic: They have by far the highest ratio of dark to luminous matter, making them the darkest galaxies in the universe. None of the models proposed to unravel their origin can simultaneously explain their exceptional dark matter content and their proximity to a much larger galaxy. As reported in “Nature”, an international team of researchers, among them Dr Chiara Mastropietro at the Observatory of Ludwig-Maximilians-Universität (LMU) Munich, developed simulations on supercomputers that show which factors allowed the origin of these extreme galaxies. “We find that a combination of tidal shocks and so-called ram pressure swept away the entire gas content of these satellites’ gas-dominated progenitors,” says Mastropietro. “This was only possible because heating by the cosmic ultraviolet background radiation kept the gas loosely bound which prevented star formation.”

About ten billion years ago the gas-rich progenitors of the satellite galaxies entered into the orbit of a much more massive galaxy. At that time the universe was filled with hot radiation which made the gas within the satellite hotter and prevented it from forming many stars. “As the satellite approached the massive galaxy, it experienced two environmental effects, ram-pressure and tidal forces,” reports Mastropietro. “Ram-pressure is due to the presence of hot gas around the big host galaxies. Gas in the satellite systems will feel it when moving close to a bigger galaxy. In the proximity of the center of the host galaxy, ram pressure can be very strong and is able to strip away most of the gas within the satellite. Simultaneously, the satellite experienced the tidal forces induced by the proximity of a much larger mass. These effects contributed to removal of nearly all the luminous matter like gas and stars, but left behind the dark matter.”

That is because this component of the progenitor galaxies is only affected by tidal forces which by themselves are not strong enough to strip away much of this still mysterious material. Even to remove gas and stars from the progenitor dwarf galaxies ram pressure, gravitational forces and UV background radiation had to act together. “Therefore, similar dwarf systems that fell into the Milky Way halo later, when the Universe was colder, can continue forming stars, and tidal shocks will produce periodic bursts of star formation,” says Mastropietro. “These newcomers should account for those dwarf spheroidals that have fairly normal mass-to-light ratios and extended star formation histories.”

Dr. Chiara Mastropietro
Observatory at Ludwig-Maximilians-Universität (LMU) Munich
Tel.: +49-89-2180-6029
Fax: +49-89-2180-6003


L. Mayer, S. Kazantzidis, C. Mastropietro & J. Wadsley
Early gas stripping as the origin of the darkest galaxies in the universe
Nature, February 15, 2007

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