Mapping the Milky Way

Australian and German scientists have used the world’s largest steerable radio telescopes — the 100 m Max Planck telescope at Effelsberg and the 64 m telescope at Parkes — to create the most detailed map yet of the Milky Way.

Collaborators on the HI4PI project used the radio telescopes to map the detailed structure of neutral hydrogen across the Northern and Southern Hemispheres — a task that required more than one million observations. Dozens of terabytes of raw data were recorded and processed by astronomers at Germany’s University of Bonn, with the final results published in the journal Astronomy and Astrophysics.

The project has provided the most sensitive and detailed view of all of the hydrogen gas in and around the Milky Way, improving the previous neutral hydrogen study, the Leiden-Argentine-Bonn (LAB) survey, by a factor of two in sensitivity and a factor of four in angular resolution. In particular, the survey has revealed for the first time a wealth of fine details of the large-scale structure of the Milky Way’s gas distribution.

“Very small gas clouds appear to have helped form stars in the Milky Way over billions of years,” said Professor Naomi McClure-Griffiths, team leader for the Australian survey from The Australian National University (ANU).

HI4PI data will be freely accessible on request from interested persons all over the world via CDS, the Strasbourg data centre. It will thus serve as a major resource for researchers working with observational data at all wavelengths, as well as for astrophysicists studying the Milky Way gas distribution itself.

Professor McClure-Griffiths said her research group will use the data map to hone their work with the Square Kilometre Array (SKA) and the Australian Square Kilometre Array Pathfinder (ASKAP), which will provide even more detailed maps of the Milky Way. This will enable them to answer the big questions about the Milky Way and neighbouring galaxies.

“How does the Milky Way get the new gas it requires to continue forming stars? And where are all of the small dwarf galaxies that must surround our Milky Way?” Professor McClure-Griffiths asked. “The next steps will be exciting.”

Image caption: The entire sky in the light of neutral atomic hydrogen (HI) as seen by the Parkes and Effelsberg radio telescopes. Our host galaxy, the Milky Way, appears as a luminous band across the sky with the galactic centre in the middle. The Magellanic Clouds (Large and Small Magellanic Cloud) are prominently visible in orange colours below the Galactic plane. They are surrounded by huge amounts of gas, which was forcefully disrupted from their hosts by gravitational interaction. The HI emission of the Andromeda galaxy (M31) and its neighbour, Triangulum (M33), is also easy to spot as bright purple ellipses in the south-western part of the map. The gas motion is colour-coded with different hue values, and the visual brightness in the image denotes the intensity of the received HI radiation. ©HI4PI Collaboration