Not just another survey

The age of mega-surveys is upon us. One of the first new “big” surveys is from ESOs 4.1m VISTA telescope at Paranal, Chile. A recent announcement from a large team of astronomers describes the VISTA Variables in the Via Lactea (VVV) survey which is scanning the southern plane and bulge of the Milky Way. Via Lactea is the Latin name for the Milky Way.  The survey started in 2010 and was granted a total of 1929 hours of observing time over a five-year period.

Here are some of the amazing stats from the survey:

  • The survey image covers about 315 square degrees of the sky (a bit less than 1% of the entire sky).
  • Observations were carried out using three different infrared filters.
  • The resultant catalogue contains about 173 million objects, of which about 84 million have been confirmed as stars.

Infrared view of the central Milky Way from the VVV VISTA survey. (Credit: ESO/VVV Consortium, Ignacio Toledo, Martin Kornmesser)

What is so interesting about the Galactic bulge? The bulge is dominated by old, 10-12 Gyr stars, although some younger stellar populations have been observed close to the bulge center near the disk plane. Mergers can build the bulge population. The bulge of the Galaxy may have an elongated or boxy shape which may suggest partial creation by instabilities in the disk, and hence being built by disk stars. The dominant old population of the bulge suggests that the dominant epoch of construction was a narrow,  approximately 1 Gyr age range at z > 2.

One of the great diagnostics of stellar populations is the colour-magnitude diagram (CMD). By plotting the colour of a star (x-axis) versus the brightness (y-axis) information is gathered about stellar ages, evolutionary states and chemical compositions. Usually the CMD is presented as brighter stars towards the top, fainter towards the bottom and redder stars to the right and bluer ones to the left.

Colour-magnitude diagram of the inner Milk Way, plotting the brightness of over 84 million Galactic bulge stars from the VVV survey. (Credit: ESO/VVV Consortium)

The VVV CMD plots the brightnesses of more than 84 million stars in the central part of the Milky Way against their near-infrared colours. This is the first time that such a colour–magnitude diagram has been made for the entire Galactic bulge and creates the richest colour–magnitude diagram ever assembled. Most stars lie in the yellow regions, with fewer in the bluer parts of the diagram. More evolved red giant stars appear at the upper-right and fainter dwarfs at the bottom.

This work will no doubt provide a wealth of information about the stellar populations in the central part of our Galaxy. Overall, constraining the bulge populations allows us to build a better picture of how our Galaxy (and other similar spirals) formed and evolved.

For more information, see

[Glen Mackie]

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