Section of the 'Maggie' filament.
(T. Müller/J. Syed/MPIA)
About 370,000 years later, hydrogen had formed, the building block of stars, which fuse hydrogen and helium in their interiors to create all the heavier elements. While hydrogen remains the most pervasive 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 research the early phases of star formation, which would offer clues about the evolution of galaxies and the cosmos.
An international team led by astronomers from the Max Planck Institute of 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.
(ESA/Gaia/DPAC/T. Müller/J. Syed/MPIA)
Above: The section of the Milky Way, as measured by ESA's Gaia satellite (top). The box marks the location of the 'Maggie' filament and the false-color image of atomic hydrogen distribution (bottom), the red line indicating the 'Maggie' filament.The study that describes their findings, which recently appeared 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), the Max Planck Institute for Radio Astronomy (MPIFR), the University of Calgary, the Universität Heidelberg, the Centre for Astrophysics and Planetary Science, the Argelander-Institute for Astronomy, the Indian Institute of Science, and NASA's Jet Propulsion Laboratory (JPL).
https://youtu.be/sLX7HNclaFA
The process of how atomic hydrogen transitions to molecular hydrogen is still largely unknown, which made this extraordinarily long filament an especially exciting find.
Whereas the largest known clouds of molecular gas typically measure around 800 light-years in length, Maggie measures 3,900 light-years long and 130 light-years wide. As Syed explained in a recent MPIA press release:
"The location of this filament has contributed to this success. We don't yet know exactly how it got there. But the filament extends about 1600 light-years below the Milky Way plane. The observations also allowed us to determine the velocity of the hydrogen gas. This allowed us to show that the velocities along the filament barely differ."
The team's analysis showed that matter in the filament had a mean velocity of 54 km/s-1, which they determined mainly by measuring it against the rotation of the Milky Way 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 the hydrogen gas," said Henrik Beuther, the head of THOR and a co-author on the study. "This allowed us to show that the velocities along the filament barely differ."
From this, the researchers concluded that Maggie is a coherent structure. These findings confirmed observations made a year before by Juan D. Soler, an astrophysicist with the University of Vienna and co-author on the paper.
When he observed the filament, he named it after the longest river in his native Colombia: the Río Magdalena (Anglicized: Margaret, or "Maggie"). While Maggie was recognizable in Soler's earlier evaluation of the THOR data, only the current study proves beyond a doubt that it is a coherent structure.
Based on previously published data, the team also estimated that Maggie contains 8 percent molecular hydrogen by a mass fraction.
On closer inspection, the team noticed that the gas converges at various points along the filament, which led them to conclude that the hydrogen gas accumulates into large clouds at those locations. They further speculate that atomic gas will gradually condense into a molecular form in those environments.
"However, many questions remain unanswered," Syed added. "Additional data, which we hope will give us more clues about the fraction of molecular gas, are already waiting to be analyzed."
Fortunately, several space-based and ground-based observatories will become operational soon, telescopes that will be equipped to study these filaments in the future. These include the James Webb Space Telescope (JWST) and radio surveys like the Square Kilometer Array (SKA), which will allow us to view the very earliest period of the Universe ("Cosmic Dawn") and the first stars in our Universe.
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JANUARY 5, 2022
Coherent interstellar magnetic field detected
by Chinese Academy of Sciences
The Taurus molecular cloud (grey scale), of which L1544 is a part, is superimposed onto the 2MASS sky image and the field orientation based on Planck data (thin white lines). The HINSA Zeeman spectrum (thick white line) is shown with the fitted Zeeman signature (blue).
Credit: NAOC
Magnetic fields are the essential, but often "secret" ingredients of the interstellar medium and the process of making stars. The secrecy shrouding interstellar magnetic fields can be attributed to the lack of experimental probes.
While Michael Faraday was probing the link between magnetism and electricity with coils in the early 19th century in the basement of the Royal Institution, astronomers today still cannot deploy coils light-years away.
Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), an international team led by Dr. LI Di from National Astronomical Observatories of Chinese Academy of Sciences (NAOC) has obtained accurate magnetic field strength in molecular cloud L1544—a region of the interstellar medium that seems ready to form stars.
The team employed the so-called HI Narrow Self-Absorption (HINSA) technique, first conceived by LI Di and Paul Goldsmith based on Arecibo data in 2003. FAST's sensitivity facilitated a clear detection of the HINSA's Zeeman effect. The results suggest that such clouds achieve a supercritical state, i.e., are primed for collapse, earlier than standard models suggest.
"FAST's design of focusing radio waves on a cable-driven cabin results in clean optics, which has been vital to the success of the HINSA Zeeman experiment," said Dr. LI.
The study was published in Nature on Jan. 5.
The Zeeman effect—the splitting of a spectral line into several components of frequency in the presence of a magnetic field—is the only direct probe of interstellar magnetic field strength. The interstellar Zeeman effect is small. The frequency shift originating in the relevant clouds is only a few billionths of the intrinsic frequencies of the emitting lines.
In 2003, the spectra of molecular clouds were found to contain an atomic-hydrogen feature called HINSA, which is produced by hydrogen atoms cooled through collisions with hydrogen molecules. Since this detection was made by the Arecibo telescope, the Zeeman effect for HINSA has been deemed a promising probe of the magnetic field in molecular clouds.
HINSA has a line strength 5–10 times higher than that of molecular tracers. HINSA also has a relatively strong response to magnetic fields and, unlike most molecular tracers, is robust against astrochemical variations.
FAST's HINSA measurements put the magnetic field strength in L1544 at about 4 µGauss, i.e., 6 million times weaker than that of Earth. A combined analysis with quasar (active supermassive blackhole) absorption and hydroxyl emission also revealed a coherent magnetic field structure throughout the cold neutral medium, the molecular envelope, and the dense core, with similar orientation and magnitude.
Therefore, the transition from magnetic subcriticality to supercriticality—i.e., when the field can and cannot support the cloud against gravity, respectively—occurs in the envelope instead of the core, in contrast with the conventional picture.
How the interstellar magnetic field dissipates to enable cloud collapse remains an unsolved problem in star formation. The main proposed solution has long been ambipolar diffusion—the decoupling of neutral particles from plasma—in cloud cores.
The coherence of the magnetic field revealed by the HINSA Zeeman effect means that dissipation of the field occurs during the formation of the molecular envelope, possibly through a different mechanism than ambipolar diffusion.
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