Inception of the Virus-W spectrograph

Fig. 1: "First Light" for VIRUS-W: This image (from the Sloan Digital Sky Survey) shows the galaxy NGC2903 and the field of view of the spectrograph.
Credit: SDSS

By McDonald Observatory at University of Texas, Austin, Max Planck Institute, Garching, Germany

Published: January 28, 2011

The new observing instrument VIRUS-W, built by the Max Planck Institute for Extraterrestrial Physics and the University Observatory Munich, Germany, saw "first light" November 10, 2010, on McDonald Observatory's 2.7-meter Harlan J. Smith Telescope in West Texas. Its first images of a spiral galaxy about 30 million light-years away were an impressive confirmation of the capabilities of the instrument, which can determine the motion of stars in nearby galaxies to a precision of a few miles per second.

"When we attached VIRUS-W around midnight on November 10 to the 2.7-meter telescope, we were very happy to see that the data delivered by VIRUS-W was of science quality virtually from the first moment on," said Maximilian Fabricius from the Max Planck Institute for Extraterrestrial Physics.

"As the first galaxy to observe, we had selected the strongly barred galaxy NGC 2903 at a distance of about 30 million light-years — right in front of our doorstep. The data we collected reveal a centrally increasing velocity dispersion from about 50 miles per second (80 km/s) to 75 mps (120 km/s) within the field of view of the instrument. This was a very exciting moment and only possible because of the remarkable teamwork during the commissioning with a lot of support by the observatory staff."

As an integral field spectrograph, VIRUS-W can simultaneously produce 267 individual spectra — one for each of its glass fibers. By dispersing the light into its constituent colors, astronomers are able to study properties such as the velocity distribution of the stars in a galaxy. For this, they use the Doppler shift, which means that the light from stars moving toward or away from us is shifted to blue or red wavelengths, respectively. This effect can also be observed on Earth when a fast vehicle, such as a racing car, is driving past — the sound of the approaching car is higher, while for the departing car it is lower.

VIRUS-W´s unique feature is the combination of a large field of view (about 1 by 2 arcminutes) with a relatively high spectral resolution. With the large field of view, astronomers can study nearby galaxies in just one or a few pointings, while the high spectral resolution permits an accurate determination of the velocity dispersion in these objects. In this way, astronomers obtain the large-scale kinematic structure of nearby spiral galaxies, which will give important insight into their formation history.

Most galaxies are too distant, and the separation between their billions upon billions of stars too small, to resolve with even the best cutting-edge instruments. Astronomers, therefore, cannot study individual stars in these distant galaxies, but only the average motion along a specific line of sight.

The measured velocity distributions are characterized by two parameters: The mean velocity reveals the large-scale motion of the stars along the line of sight, and the velocity dispersion measures how much the velocities of the individual stars differ from this mean velocity. If the stars have more or less the same velocity, the dispersion is small, but if they have very different velocities, the dispersion is broad. For spiral galaxies where the stars travel in fairly regular circular orbits, the velocity dispersion is mostly small. In elliptical galaxies, however, the stars have rather disordered orbits and so the dispersion is broad.

With the high spectral resolution of VIRUS-W, astronomers can investigate relatively small velocity dispersions down to about 12 mps (20 km/s). This was confirmed by the first images taken by VIRUS-W of the nearby spiral galaxy NGC 2903.

The observing time at the telescope was made available by the VIRUS-P Exploration of Nearby Galaxies (VENGA) project, and VIRUS-W will be contributing from the beginning of 2011 onward.

The VIRUS-P instrument, on which VIRUS-W is based, has probed these 30 galaxies in a wide range of wavelengths, from ultraviolet light all the way into the red portion of the visible spectrum. This wide wavelength range allows astronomers to probe a large number of questions about these galaxies, from their star formation rate to their ages.

"VIRUS-W is an improved version of VIRUS-P," Guillermo Blanc from the University of Texas at Austin said. Astronomers will follow up the VIRUS-P studies by using VIRUS-W to look into the hearts of the brightest spiral galaxies in the sample to get what Blanc calls "exquisite measurements" of the motions of stars and gas clouds inside these galaxies. Understanding how stars and gas move will help astronomers better understand how stars form.

VIRUS-P is a prototype of the Visible Integral-field Replicable Unit Spectrograph (VIRUS) being developed for a large dark energy study called the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) led by The University of Texas at Austin. For a study of the large-scale distribution of galaxies, HETDEX will combine about 100 spectrographs at the 9.2-meter Hobby-Eberly Telescope at McDonald Observatory to form one large instrument. VIRUS-W — the W stands for a later mission at the Wendelstein telescope of the Munich Observatory — is based on the same basic VIRUS design.