"This may imply that collisions and merging are important in the formation and evolution of galaxies", said Francois Hammer of the Paris Observatory in France, one of the research leaders.

Using the new GIRAFFE instrument on the 8.2-meter Kueyen telescope, part of the European Southern Observatory's Very Large Telescope array in Paranal, Chile, the scientists studied the effects of dark matter on distant and therefore older galaxies, to see how they differed from the ones nearby.

"Dark matter, which composes about 25 percent of the universe, is a simple word to describe something we really do not understand," said Hector Flores, research co-leader. "From looking at how galaxy rotates, we know that dark matter must be present, as otherwise these gigantic structures would just dissolve."

In nearby galaxies, including the Milky Way, the astronomers found a steady ratio between the amount of dark matter and the number of ordinary stars: for every kilogram of stellar material there is roughly 30 kilograms of dark matter.

This calculation required measuring the velocity in different parts of distant galaxies, however. Up to now, most Earth-based instruments could not probe these galaxies in sufficient details, because they could only observe in a single direction across the galaxy.

GIRAFFE changed things. It can obtain high-quality spectral data from a large variety of celestial objects, from individual stars in the Milky Way and other nearby galaxies to very distant galaxies. It functions by means of multiple fiber-optic lines that guide the light from a telescope's focal plane into the entry slit of the spectrograph, where the light is dispersed into its different colors.

GIRAFFE and the fibers function as an integral part of the advanced Fiber Large Array Multi-Element Spectrograph, or FLAMES facility, which allows a field of view of about 25 arc minutes, or almost as large as the full Moon. The instrument can obtain simultaneous spectra of smaller areas of objects such as galaxies or nebulae.

"GIRAFFE on ESO's VLT is the only instrument in the world that is able to analyze simultaneously the light coming from 15 galaxies," said Mathieu Puech, lead author of one the papers presenting the results. "Every galaxy observed in this mode is split into continuous smaller areas where spectra are obtained at the same time."

When the astronomers used GIRAFFE to measure the velocity fields of several dozen distant galaxies, they found as much as 40 percent of those galaxies were "out of balance" - their internal motions were very disturbed, a possible sign they still exhibit the aftermath of collisions.

When they limited themselves only to galaxies that apparently have reached their equilibrium, the team found the relation between dark matter and stellar content does not appear to have evolved during the last 6 billion years.

Thanks to its powerful spectral resolution, GIRAFFE also allows astronomers to study for the first time the distribution of gas as a function of its density in distant galaxies. The team found possible outflows of gas and energy driven by intense star formation within galaxies, and a giant region of very hot gas in one galaxy that produces many stars.

"Such a technique can be expanded to obtain maps of many physical and chemical characteristics of distant galaxies, enabling us to study in detail how they assembled their mass during their entire life," Hammer said. "In many respects, GIRAFFE and its multi-integral field mode gives us a first flavour of what will be achieved with future extremely large telescopes."

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