The best map of hydrogen distribution in the Milky Way

HI4PI survey map (Image courtesy Benjamin Winkel, Max Planck Institute, and the HI4PI collaboration.)
HI4PI survey map (Image courtesy Benjamin Winkel, Max Planck Institute, and the HI4PI collaboration.)

An article published in the journal “Astronomy & Astrophysics” describes the creation of a map of neutral atomic hydrogen in the Milky Way. An international team of scientists put together data collected by two of the largest steerable radio telescopes in the world, the 100-m Max-Planck radio telescope in Effelsberg, Germany and the 64-m CSIRO radio telescope in Parkes, Australia.

Neutral atomic hydrogen is the simplest among the atoms, consisting of only one proton and one electron. Usually it’s indicated as HI. This is the most abundant atom type in the universe and for about 13 billion years has been providing by far the largest contribution to star formation.

This new map was created through the HI4PI survey, so called because it focused on neutral atomic hydrogen (HI) in all directions of the sky (4*PI). It’s not the first map of its kind but improves in various ways the results of the best one available so far, created by the LAB (Leiden/Argentine/Bonn) survey. In 2005, the LAB survey data were presented, obtained by combining the data from the LDS (Leiden/Dwingeloo Survey) and the IAR (Instituto Argentino de Radioastronomia) survey.

It’s no coincidence that both the LAB and the new HI4PI surveys are the result of the combination of data from two different sources. The data for the northern hemisphere needs to be combined with those of the southern hemisphere. Having used different instruments, it was necessary to perform a further data processing to produce a consistent combined map.

The map obtained is the result of over a million individual observations for a total of tens of terabytes of recorded raw data. The two radio telescopes used are very sensitive but researchers must always take into account radio pollution that today is generated by various modern devices. Just consider the spread of smartphones and the amount of radio waves they generate. For this reason, the signals detected by radio telescopes must be cleaned from those from interference through sophisticated algorithms processing.

The end result of all this work is visible in the details of filamentary structures among the stars that can be seen for the first time. Those are important information in various researches in the field of astrophysics and will help understand even better the structures of other galaxies. The global map also shows the Magellanic Clouds in the lower right corner.

The HI4PI survey will be important for years, even in the next decade, after the next-generation radio telescope, SKA (Square Kilometre Array), will enter service. In fact, it will not have a large neutral atomic hydrogen sensitivity so the HI4PI map will also be useful in research conducted with SKA completing the data collected by the new radio telescope.

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