Hydrogen filament 3,900 light-years long

About 13.8 billion years ago, our Universe was born from a massive explosion that gave rise to the first subatomic particles and the laws of physics as we know them. About 370,000 years later hydrogen had formed, the building block of stars, which fuse the hydrogen and helium within them to create all of the heavier elements. Although hydrogen remains the most common element in the Universe, it can be difficult to detect individual clouds of hydrogen gas in the interstellar medium (ISM).

This makes it difficult to find the earliest phases of star formation, which would offer clues to the evolution of galaxies and the cosmos. An international team led by astronomers from the Max Planck Institute for Astronomy (MPIA) recently noticed a massive filament of atomic hydrogen gas in our galaxy. This structure, named “Maggie”, is located about 55,000 light-years away (on the other side of the Milky Way) and is one of the longest structures ever observed in our galaxy.

The study that describes their findings, recently published in the journal Astronomy & Astrophysics, was led by Jonas Syed, a Ph.D. student at the MPIA. He was joined by researchers from the University of Vienna, the Harvard-Smithsonian Center for Astrophysics (CFA), Max Planck Institute for Radio Astronomy (MPIFR), University of Calgary, Universität Heidelberg, Center for Astrophysics and Planetary Science, Argelander-Institute for Astronomy, Indian Institute of Science, and ">Nasa‘s Jet Propulsion Laboratory (JPL).

The research is based on data obtained from the Milky Way HI/OH/Recombination Line Study (THOR), an observing program that relies on the Karl G. Jansky Very Large Array (VLA) at the New Mexico. Using the VLA’s centimetre-wave radio dishes, this project studies molecular cloud formation, the conversion of atomic hydrogen to molecular hydrogen, the magnetic field of the galaxy, and other questions related to ISM and to star formation.

The ultimate goal is to determine how the two most common hydrogen isotopes converge to create dense clouds that rise to new stars. Isotopes include atomic hydrogen (H), consisting of one proton, one electron, and no neutrons, and molecular hydrogen (H₂) is composed of two hydrogen atoms linked by a covalent bond. Only the latter condenses into relatively compact clouds that will develop frosty regions where new stars eventually emerge.

The transition process from atomic hydrogen to molecular hydrogen is still largely unknown, which made this extraordinarily long filament a particularly exciting discovery. While the largest known clouds of molecular gas are typically around 800 light-years in length, Maggie is 3,900 light-years long and 130 light-years wide. As Syed explained in a recent MPIA press release:

“The location of this filament contributed to this success. We don’t yet know exactly how it got there. But the filament extends about 1600 light-years below the plane of the Milky Way. The observations also allowed us to determine the velocity of hydrogen gas. This allowed us to show that the velocities along the filament hardly differ.”

The team’s analysis showed that the material in the filament had an average speed of 54 km/s^-1, which they determined primarily by measuring it relative to the rotation of the Milky Way’s disk. This meant that radiation at a wavelength of 21 cm (aka the “hydrogen line”) was visible against the cosmic background, making the structure discernible. “The observations also allowed us to determine the velocity of hydrogen gas,” said Henrik Beuther, director of THOR and co-author of the study. “This allowed us to show that the velocities along the filament hardly differ.”

From there, the researchers concluded that Maggie is a coherent structure. These findings confirmed observations made a year earlier by Juan D. Soler, an astrophysicist at the University of Vienna and co-author of the paper. When he observed the filament, he gave it the name of the longest river in his native Colombia: the Río Magdalena (anglicized: Margaret, or “Maggie”). While Maggie was recognizable in Soler’s earlier assessment of THOR data, only the current study proves beyond doubt that it is a consistent structure.

Based on previously published data, the team also estimated that Maggie contains 8% molecular hydrogen by mass fraction. Upon closer inspection, the team noticed that the gas converges at various points along the filament, leading them to conclude that the hydrogen gas collects in large clouds at these locations. They further speculate that atomic gas will gradually condense into a molecular form in these environments.

“However, many questions remain unanswered,” Syed added. “Additional data, which we hope will give us more clues about the molecular gas fraction, is already waiting to be analyzed.” Fortunately, several space and ground observatories will soon be operational, telescopes that will be equipped to study these filaments in the future. These include the James Webb Space Telescope (JWST) and radio soundings like the Square Kilometer Array (SKA), which will allow us to visualize the very first period of the Universe (“Cosmic Dawn”) and the first stars of our Universe.

Originally published on Universe Today.

For more on this research, see Massive Filament Structure – 3900 Light-Years Long – Discovered in the Milky Way.

Reference: “The “Maggie” filament: physical properties of a giant atomic cloud” by J. Syed, JD Soler, H. Beuther, Y. Wang, S. Suri, JD Henshaw, M. Riener, S. Bialy, S 20 December 2021, Astronomy & Astrophysics.
DOI: 10.1051/0004-6361/202141265