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| 21 cm line | |
|---|---|
| Name | 21 cm line |
| Frequency | 1420.40575177 MHz |
| Wavelength | 21 cm |
| Species | neutral hydrogen |
| Transition | hyperfine |
| Discovered | 1951 |
| Discoverer | Ewen and Purcell |
21 cm line.
The 21 cm line is a spectral radio emission produced by the hyperfine transition of neutral hydrogen atoms in the interstellar medium, first detected by Ewen and Purcell in 1951. It plays a pivotal role in studies by observatories such as Arecibo Observatory, Very Large Array, Parkes Observatory, Green Bank Telescope, and surveys like the HI Parkes All Sky Survey and the Sloan Digital Sky Survey. The line enables mapping of structures from nearby objects like Orion Nebula and Andromeda Galaxy to large-scale features studied by collaborations including the Planck Collaboration and projects such as LOFAR, SKA Project, and CHIME.
The hyperfine splitting responsible for the line arises from the interaction between proton and electron magnetic moments in the ground state of neutral hydrogen, a process described by quantum mechanics developed by figures including Heisenberg, Dirac, Schrödinger, and formalism used by researchers at institutions like Harvard University and California Institute of Technology. The transition has an extremely low Einstein A coefficient, yielding a long spontaneous decay time, a fact exploited in studies by teams at Max Planck Institute for Radio Astronomy and National Radio Astronomy Observatory. Radiative transfer and collisionally driven excitation in environments such as Orion Nebula, Taurus Molecular Cloud, and Magellanic Clouds involve rates influenced by cosmic-ray backgrounds measured by instruments aboard Voyager 1 and missions like WMAP and Planck Collaboration.
Detection and mapping of the line use single-dish telescopes like Arecibo Observatory, Effelsberg 100-m Radio Telescope, and Parkes Observatory, and interferometers such as Very Large Array, MeerKAT, and Atacama Large Millimeter Array. Digital backends, low-noise amplifiers developed at MIT, and correlators designed at Jodrell Bank Observatory enable spectral resolution sufficient to separate galactic rotation signatures traced against standards from Hipparcos and catalogs compiled by IAU committees. Techniques like aperture synthesis pioneered by Martin Ryle and syntheses used in surveys by Australian Square Kilometre Array Pathfinder and LOFAR permit imaging of neutral hydrogen across redshifts probed by experiments including CHIME and prototypes for the SKA Project.
The 21 cm signal is used to probe galactic rotation curves in studies involving Vera C. Rubin Observatory data, measure baryon acoustic oscillations targeted by experiments like BOSS and eBOSS, and to trace reionization epochs investigated alongside Planck Collaboration and WMAP results. It informs models of structure formation developed by theorists from Princeton University and Cambridge University, and contributes to constraints on dark matter and dark energy parameters explored by collaborations including DESI and Euclid (spacecraft). Observations of nearby systems such as Andromeda Galaxy, Milky Way, and Triangulum Galaxy integrate with multiwavelength campaigns by Hubble Space Telescope, Chandra X-ray Observatory, and Spitzer Space Telescope.
Analysis pipelines built by teams at National Radio Astronomy Observatory, CERN, and universities like University of Cambridge employ calibration strategies tied to standards from National Institute of Standards and Technology and techniques such as principal component analysis, matched filtering, and Bayesian inference used in workshops at Perimeter Institute and Kavli Institute for Cosmological Physics. Cross-correlation methods with datasets from Sloan Digital Sky Survey, DES (Dark Energy Survey), and Planck Collaboration assist in isolating cosmological signals. Instrument teams from SKA Project and MeerKAT implement RFI mitigation inspired by protocols from ITU and observatory policies at Arecibo Observatory.
Foreground contamination from galactic synchrotron emission tied to regions like the Galactic Center, extragalactic point sources cataloged by NVSS, and terrestrial radio-frequency interference regulated by International Telecommunication Union pose major hurdles. Systematic errors due to beam chromaticity, polarization leakage characterized in tests at Jodrell Bank Observatory, and calibration stability informed by standards at National Radio Astronomy Observatory complicate recovery of faint signals comparable to constraints on reionization reported by Planck Collaboration. Simulations developed at Lawrence Berkeley National Laboratory and Los Alamos National Laboratory help assess bias introduced by ionospheric variability monitored by networks coordinated with NOAA and observatories such as LOFAR.
Next-generation facilities like the Square Kilometre Array and upgrades to MeerKAT and SKA Project prototypes aim to map 21 cm emission across cosmic time, complementing missions like Euclid (spacecraft), Nancy Grace Roman Space Telescope, and surveys by Vera C. Rubin Observatory. Planned experiments including expanded arrays modeled after HERA and next phases of CHIME and LOFAR seek to improve sensitivity and systematics control, informed by collaborations with institutions such as Cambridge University, Caltech, MIT, and agencies like NASA and European Space Agency. Advances in computing from centers like Argonne National Laboratory and algorithmic innovations led by researchers at Google DeepMind and OpenAI are expected to accelerate analysis and foster discoveries about the epochs probed by the line.