Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:


HI4PI: A new all-sky survey of neutral hydrogen


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 mile stone for the decades to come.

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.

The entire sky in the light of neutral atomic hydrogen (HI) as seen by the Parkes and Effelsberg radio telescope with the Milky Way in the middle. .

HI4PI Collaboration

Two of the largest radio telescopes in the world, utilized for HI4PI, the high-resolution map of the full sky in the light of neutral hydrogen: Effelsberg (100-m, left) and Parkes (64-m, right).

Norbert Junkes (left image), Shaun Amy (right image).

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 Tera-bytes 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 Jürgen Kerp, also from Bonn University, the project coordinator and principal investigator of the Effelsberg survey. “Although upcoming new instruments such as the Square Kilometer 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 (after the embargo expires).

The team comprises the HI4PI Collaboration: N. Ben Bekhti, L. Flöer, R. Keller, J. Kerp, D. Lenz, and B. Winkel, moreover J. Bailin, M. R. Calabretta, L. Dedes, H. A. Ford, B. K. Gibson, U. Haud, S. Janowiecki, P. M.W. Kalberla, F. J. Lockman, N. M. McClure-Griffiths, T. Murphy, H. Nakanishi, D. J. Pisano, and L. Staveley-Smith.

Original Paper:

HI4PI collaboration: HI4PI: A full-sky HI survey based on EBHIS and GASS, 2016, Astronomy & Astrophysics, Online Publication October 20 (DOI: 10.1051/0004-6361/201629178).
URL: (after the embargo expires)


Priv.-Doz. Dr. Jürgen Kerp,
Argelander-Institut für Astronomie,
Universität Bonn.
Fon: +49 228 73-3667

Dr. Benjamin Winkel,
Radio-Observatorium Effelsberg,
Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49 2257 301-167

Dr. Norbert Junkes,
Press and Public Outreach
Max-Planck-Institut für Radioastronomie, Bonn.
Fon: +49 228 525-399

Weitere Informationen:

Norbert Junkes | Max-Planck-Institut für Radioastronomie

More articles from Physics and Astronomy:

nachricht FAST detects neutral hydrogen emission from extragalactic galaxies for the first time
02.07.2020 | Chinese Academy of Sciences Headquarters

nachricht First exposed planetary core discovered
01.07.2020 | Universität Bern

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Electrons in the fast lane

Solar cells based on perovskite compounds could soon make electricity generation from sunlight even more efficient and cheaper. The laboratory efficiency of these perovskite solar cells already exceeds that of the well-known silicon solar cells. An international team led by Stefan Weber from the Max Planck Institute for Polymer Research (MPI-P) in Mainz has found microscopic structures in perovskite crystals that can guide the charge transport in the solar cell. Clever alignment of these "electron highways" could make perovskite solar cells even more powerful.

Solar cells convert sunlight into electricity. During this process, the electrons of the material inside the cell absorb the energy of the light....

Im Focus: The lightest electromagnetic shielding material in the world

Empa researchers have succeeded in applying aerogels to microelectronics: Aerogels based on cellulose nanofibers can effectively shield electromagnetic radiation over a wide frequency range – and they are unrivalled in terms of weight.

Electric motors and electronic devices generate electromagnetic fields that sometimes have to be shielded in order not to affect neighboring electronic...

Im Focus: Gentle wall contact – the right scenario for a fusion power plant

Quasi-continuous power exhaust developed as a wall-friendly method on ASDEX Upgrade

A promising operating mode for the plasma of a future power plant has been developed at the ASDEX Upgrade fusion device at Max Planck Institute for Plasma...

Im Focus: ILA Goes Digital – Automation & Production Technology for Adaptable Aircraft Production

Live event – July 1, 2020 - 11:00 to 11:45 (CET)
"Automation in Aerospace Industry @ Fraunhofer IFAM"

The Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM l Stade is presenting its forward-looking R&D portfolio for the first time at...

Im Focus: AI monitoring of laser welding processes - X-ray vision and eavesdropping ensure quality

With an X-ray experiment at the European Synchrotron ESRF in Grenoble (France), Empa researchers were able to demonstrate how well their real-time acoustic monitoring of laser weld seams works. With almost 90 percent reliability, they detected the formation of unwanted pores that impair the quality of weld seams. Thanks to a special evaluation method based on artificial intelligence (AI), the detection process is completed in just 70 milliseconds.

Laser welding is a process suitable for joining metals and thermoplastics. It has become particularly well established in highly automated production, for...

All Focus news of the innovation-report >>>



Industry & Economy
Event News

International conference QuApps shows status quo of quantum technology

02.07.2020 | Event News

Dresden Nexus Conference 2020: Same Time, Virtual Format, Registration Opened

19.05.2020 | Event News

Aachen Machine Tool Colloquium AWK'21 will take place on June 10 and 11, 2021

07.04.2020 | Event News

Latest News

Rising water temperatures could endanger the mating of many fish species

03.07.2020 | Life Sciences

Risk of infection with COVID-19 from singing: First results of aerosol study with the Bavarian Radio Chorus

03.07.2020 | Studies and Analyses

Efficient, Economical and Aesthetic: Researchers Build Electrodes from Leaves

03.07.2020 | Power and Electrical Engineering

Science & Research
Overview of more VideoLinks >>>