Two of the world's largest fully steerable radio telescopes, the 100-m dish at Effelsberg/Germany and the 64-m Parkes/Australia telescope, mapped the detailed structure of neutral hydrogen across the Northern and Southern hemispheres. Today, the complete survey, HI4PI, is released to the scientific community. It discloses a wealth of fine details of the large scale structure of the Milky Way's gas distribution. HI4PI is the product of a joined effort of astronomers of many countries and will be a milestone for the decades to come. The results are published in the current issue of "Astronomy and Astrophysics."

Atomic hydrogen is the most abundant element in space. It is the prime constituent in almost all astronomical objects such as stars, galaxies, and even clusters of galaxies. Hydrogen consists of a single proton and is the simplest element in space. It was already formed during the big bang nucleosynthesis. If the proton is combined with an electron, it is called neutral atomic hydrogen, abbreviated as HI.

In addition to the well-known hydrogen spectral lines at visual wavelengths, extremely faint hydrogen line emission can be observed at radio wavelengths, the so-called 21-cm line. Even though the emitted energy is tiny, the sheer amount of hydrogen in space makes the 21-cm line emission observable in nearly all galactic environments, even far beyond the stellar population of galaxies.

In 1951 three independent research groups from the United States, the Netherlands and Australia announced the first detection of HI 21-cm line emission. Now, 65 years later, an international collaboration of scientists from all over the world announced the release of a new full-sky 21-cm spectral line survey, called HI4PI. HI4PI is an acronym for HI across the whole sky (the surface area of the full sphere corresponding to 4*PI steradian).

The HI4PI collaboration, led by a German team from Bonn University and Max Planck Institute for Radio Astronomy (MPIfR), is publishing the results in the current issue of "Astronomy and Astrophysics."

With modern radio telescopes, HI is fairly easy to detect towards any direction on the sky. Mapping the whole sky is nevertheless time consuming and costly in terms of manual labor.

To map the whole sky demanded more than a million individual observations with two of the World's largest radio telescopes, the 100-m telescope at Effelsberg, Germany, and the Parkes 64-m telescope in Australia. In total, dozens of terabytes of raw data have been recorded. The raw data sets were processed by Astronomers in Bonn, yielding the final data product.

"Besides a careful calibration of the data, we also had to remove man-made noise from the data. This so-called radio frequency interference (RFI) is, for example, produced by telecommunication and broadcast stations, or military RADAR and pollutes the faint emission of the astronomical sources," explains Benjamin Winkel from MPIfR, responsible for the data acquisition and processing in the HI4PI collaboration.

"The computational effort for the data processing was huge, adding to the thousands of hours of observations thousands of hours of computing time."

The new observations were only possible because the technical equipment at radio telescopes was hugely improved in the last decade. On the one hand, new receiving systems utilizing multi-pixel feeds increased the mapping speed by an order of magnitude. On the other hand, extremely capable spectrometers based on state-of-the-art digital processors became available.

Previously, state-of-the-art HI data came from the Leiden-Argentine-Bonn survey (LAB), which is based on observations with 30-m class telescopes. The new HI4PI survey has twice the sensitivity and four times better angular resolution compared to the LAB survey.

Because HI is ubiquitous in the universe, HI4PI will serve as a major resource for researchers working with observational data at all wavelengths. As an example, X-ray and Gamma ray photons are partly absorbed, scattered or re-emitted at other wavelengths by Milky Way hydrogen during their journey from outer space to our telescopes.

Therefore, the distribution of HI in the Milky Way significantly alters the incoming signal observed by high-energy telescopes. The HI4PI data set allows the scientists to correct for these disturbing effects, cleaning our window to the distant universe.

Also, for astrophysicists studying the Milky Way gas distribution itself, HI4PI will be a valuable new resource. Owing to the increased sensitivity and angular resolution much finer structures of the interstellar medium are now revealed.

"Many studies that use pre-release data of the HI4PI survey have already been published in the last years, providing a wealth of new insights and amazing scientific results," says Peter Kalberla from Bonn University, the leading senior scientist in the project.

"HI4PI sets a benchmark for the decades to come," concludes Jurgen Kerp, also from Bonn University, the project coordinator and principal investigator of the Effelsberg survey.

"Although upcoming new instruments such as the Square Kilometre Array (SKA) will push sensitivity and angular resolution to new realms, being radio interferometers they are by design insensitive to diffusely distributed HI gas. The HI4PI will be the prime resource to add this missing information to the SKA data."

HI4PI data will be freely accessible on request from interested persons all over the world via CDS, the Strasbourg data center.

"HI4PI: A Full-Sky HI Survey Based on EBHIS and GASS," HI4PI Collaboration, 2016, Astronomy and Astrophysics.