Is this the largest gas cloud in the known Universe?


A group of astronomers have identified one of the longest known structures in the Milky Way – a cloud of atomic hydrogen stretching some 3,900 light-years, approximately 40,000,000,000,000,000 km across.

This filament, called “Maggie”, is five times the length of the largest molecular gas cloud known and could represent a link in the matter cycle of the stars.

This is because “Maggie’s” atomic gas converges locally to form molecular hydrogen, which when compressed in large clouds, is the material from which stars form.

“Despite hydrogen being the most widespread element known, detecting individual clouds of hydrogen gas is a demanding task, which makes research into the early phases of star formation challenging,” explained Professor Steven Longmore, at LJMU’s Astrophysics Research Institute.

Steven is part of a team led by the Max Planck Institute for Astronomy (MPIA) in Germany which is first to measure the filament and identify its potential in star formation.

How they found it...

Jonas Syed, a PhD student at MPIA and first author of the paper published today in the journal Astronomy & Astrophysics, explained that radiation from the hydrogen, which is at a wavelength of 21 centimetres, stands out clearly against the background, making the filament visible.

The team took measurements at different points of the hydrogen cloud and comparing their velocity were able to deduce that the filament was indeed a single, coherent structure.

Professor Longmore, said the discovery could be key in our understanding of the origins of stars:  “These clouds are the cradles of stars and planets, so our observations of this filament should tell us something about how molecular gas clouds and celestial bodies might form.”


Sümeyye Suri, co-author at the University of Vienna, explained the cloud’s mind-boggling dimensions: “The cloud is around 55,000 light-years away on the far side of the Milky Way. It is about 3,900 light-years long and 130 light-years wide. In contrast, the largest known clouds of molecular gas typically extend “only” about 800 light-years across.”

Hydrogen occurs in the Universe in various states but molecular hydrogen condenses to relatively compact clouds, which develop frosty regions where new stars finally emerge.

“Exactly how the transition from atomic to molecular hydrogen happens is still largely unknown. That makes the opportunity to study this extraordinarily long filament all the more exciting,” added Jonas Syed.

Another co-author Juan D. Soler, of Istituto Nazionale di Astrofisica, Rome, already found the first clue to this object a year ago when he named the filament “Maggie” but says this new study proves beyond doubt that it is a coherent structure,”.

'Cradle' of Milky Way

On closer inspection, the team noticed that the gas converges at some points along the filament. They conclude that the hydrogen gas accumulates at those locations and condenses into large clouds. The researchers also suspect that those are the environments where the atomic gas gradually changes into a molecular form.

In previously published data, they indeed found evidence of Maggie containing molecular hydrogen at a mass fraction of about 8%.

“We may be looking at a region in the Milky Way where the immediate raw material for new stars is being produced. Hence, new stars could form here in the distant future,” added Syed.

“However, many questions remain unanswered. More data, which we hope will give us more clues about the fraction of molecular gas, are already waiting to be analysed.”

Additional information

The team consists of Jonas Syed (Max Planck Institute for Astronomy, Heidelberg, Germany [MPIA]), Juan D. Soler (MPIA; Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale di Astrofisica, Rome, Italy), Henrik Beuther (MPIA), Yuan Wang (MPIA), Sümeyye Suri (MPIA; Astrophysical Institute, University of Vienna, Austria), Jonathan D. Henshaw (MPIA), Manuel Riener (MPIA), Shmuel Bialy (Harvard Smithsonian Center, Cambridge, USA), Sara Rezaei Khoshbakht (MPIA; Chalmers tekniska högskola, Gothenburg, Sweden), Jeroen M. Stil (Department of Physics and Astronomy, The University of Calgary, Canada), Paul F. Goldsmith (Jet Propulsion Laboratory, California Institute of Technology, Pasadena, USA), Michael R. Rugel (Max Planck Institute for Radio Astronomy, Bonn, Germany), Simon C. O. Glover (Centre for Astronomy, Institute for Theoretical Astrophysics, University of Heidelberg, Germany [ZAH/ITA]), Ralf S. Klessen (ZAH/ITA; Interdisciplinary Centre for Scientific Computing, University of Heidelberg, Germany), Jürgen Kerp (Argelander Institute for Astronomy, University of Bonn, Germany), James S. Urquhart (Centre for Astrophysics and Planetary Science, University of Kent, UK), Jürgen Ott (National Radio Astronomy Observatory, Socorro, USA), Nirupam Roy (Department of Physics, Indian Institute of Science, Begaluru, India), Nicola Schneider (I. Phyikalisches Institut, University of Cologne, Germany), Rowan J. Smith (Jodrell Bank Centre for Astrophysics, University of Manchester, UK), Steven N. Longmore (Astrophysics Research Institute, Liverpool John Moores University, Liverpool, UK), Hendrik Linz (MPIA)


This image shows a section of the side view of the Milky Way as measured by ESA’s Gaia satellite. The dark band consists of gas and dust, which dims the light from the embedded stars. The Galactic Centre of the Milky Way is indicated on the right of the image, shining brightly below the dark zone. The box to the left of the middle marks the location of the “Maggie” filament. It shows the distribution of atomic hydrogen. The colours indicate different velocities of the gas. Image: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO & T. Müller/J. Syed/MPIA

This false-colour image shows the distribution of atomic hydrogen measured at a wavelength of 21 cm. The red dashed line traces the “Maggie” filament. Image: J. Syed/MPIA


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