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The scientific results of the so-called "first light" observations of the Visible Integral-field Replicable Unit Spectrograph-W, have just been made public.
VIRUS-W is a high resolution spectrograph capable of producing 267 simultaneous spectra, one for each of its optic fibres, covering a large 1x2 arcminute field with a single pointing. The instrument is particularly suited to studying the dynamics of nearby galaxies.
The Spectrograph is the product of a collaboration between the Max Planck Institute for Extraterrestrial Physics and the University Observatory of Munich, and was first used at the focus of the 2.7 m Harlan J. Smith Telescope of McDonald Observatory. Its first target was the centre of the spiral galaxy NGC 2903, about 30 million light years away (the rectangular region in the above image).
At this distance it isn't possible to measure the velocity of individual stars, but rather the mean velocity along a given line of sight. This allows two important parameters of the velocity distribution to be measured: the mean velocity gives the large scale bulk motion, and the velocity dispersion (the width of spectral lines) gives the amount of variation of stellar velocities.
For spiral galaxies, the motion of stars around the centre tends to be rather ordered, so that the velocity dispersion at any given point is low; but for elliptical galaxies, stellar orbits
are more disordered and the dispersion is much higher.
Thanks to VIRUS-W it is possible to measure velocity differences of only 20 km/s in objects several millions of light years away. In the case of NGC 2903, within the instrument's field of view, the velocity dispersion was seen to increase from 80 km/s to 120 km/s towards the centre of the galaxy.
With these kind of data, the large scale dynamical structure of galaxies can be determined, providing a detailed picture of their history and mass distribution; something which is quite different from their more easily observed light distribution. This is especially true near the nuclei, where most galaxies harbour super-massive black holes.
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