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| 21 cm | |
|---|---|
| Name | Hydrogen line |
| Wavelength | 21 cm |
| Frequency | 1,420.40575177 MHz |
| Transition | Hyperfine transition of neutral hydrogen |
| Discovered | 1951 |
| Discoverers | Harold E. Davis; Edward M. Purcell; Hendrik C. van de Hulst (prediction) |
| Significance | Tracer of interstellar medium, cosmic structure, epoch of reionization probe |
21 cm
The 21 cm signal arises from the hyperfine transition in neutral hydrogen and serves as a fundamental probe across radio astronomy, cosmology, and astrophysics. It enables mapping of the Milky Way, detection of structures in the Local Group, and investigations of the early Universe including the Cosmic microwave background era and the Epoch of Reionization. Instruments such as the Very Large Array, Arecibo Observatory, Square Kilometre Array, Giant Metrewave Radio Telescope, and Low-Frequency Array exploit this line for studies spanning from the Interstellar medium to large-scale structure surveys like those pursued by the Westerbork Synthesis Radio Telescope and planned by the Hydrogen Epoch of Reionization Array.
The transition involves a spin flip between parallel and antiparallel configurations of the proton and electron in neutral hydrogen atoms produced in primordial nucleosynthesis during the Big Bang and later in stellar nucleosynthesis within galaxies such as Andromeda, Triangulum Galaxy, and dwarf systems like Sculptor Dwarf Galaxy. The emitted photon has a rest-frame frequency of 1,420.40575177 MHz and a wavelength near 21.1 centimetres, determined by quantum electrodynamics and hyperfine splitting calculations linked to principles developed by contributors like Enrico Fermi and Werner Heisenberg. The transition’s Einstein A coefficient is extremely low, yielding long radiative lifetimes and making collisional and radiative coupling—via mechanisms described by Lyman-alpha pumping and the Wouthuysen–Field effect—critical in setting the spin temperature relative to the Cosmic microwave background.
Observations use single-dish telescopes and interferometers to detect emission, absorption, and redshifted signals. Facilities such as Green Bank Telescope, Effelsberg 100-m Radio Telescope, Parkes Observatory, and arrays including MeerKAT and the Australian Square Kilometre Array Pathfinder employ techniques like frequency switching, beamforming, and aperture synthesis to measure brightness temperature, column density, and kinematics. Spectrometers and digital backends developed at institutions like National Radio Astronomy Observatory and CSIRO implement high-resolution channelization, while calibration strategies draw on standards associated with sources such as Cassiopeia A, Cygnus A, Calibrator 3C286, and pulsar timing methods refined by teams at Jodrell Bank Observatory and Arecibo Observatory.
The line traces atomic hydrogen reservoirs, enabling rotation curve measurements in galaxies like NGC 2403 and M33, mass modeling invoked by results from Vera C. Rubin Observatory era studies, and dark matter distribution constraints originally motivated by work following Fritz Zwicky and Vera Rubin. In cosmology, redshifted signals target the Cosmic Dawn, the Epoch of Reionization, and the post-reionization large-scale structure exploited by baryon acoustic oscillation surveys connected to analyses by the Baryon Oscillation Spectroscopic Survey and future Euclid-era synergies. Absorption studies against background radiosources like 3C273 and PKS 1934-638 probe cold neutral medium phases and link to chemical enrichment traced by Sloan Digital Sky Survey galaxy catalogs and Damped Lyman-alpha systems analyzed with data from the Hubble Space Telescope and Keck Observatory.
Extraction of faint cosmological signals requires mitigation of strong foregrounds from the Milky Way, bright extragalactic sources such as Cygnus A and Centaurus A, and radio-frequency interference regulated by agencies including International Telecommunication Union. Techniques include foreground subtraction, component separation methods inspired by cosmic microwave background pipelines developed for Planck and WMAP, and statistical estimators like the power spectrum and bispectrum used in analyses led by collaborations such as PAPER and LOFAR. Instrumental systematics—beam chromaticity, polarization leakage, and gain calibration errors—are addressed through end-to-end simulations, pipeline validation methods adopted by SKAO working groups, and software frameworks stemming from efforts at National Centre for Radio Astrophysics and CITA.
The 21 cm line was predicted in the 1940s by theoretical work associated with astronomers such as Hendrik C. van de Hulst and first detected in the 1950s by observers including Harold E. Davis and teams at Harvard College Observatory and National Radio Astronomy Observatory predecessors. Early surveys by Gordon Pettengill and I. S. Shklovsky established Galactic structure models later refined by comprehensive mapping from Leiden/Argentine/Bonn survey and results from the Canadian Galactic Plane Survey. Key discoveries include insights into Galactic rotation first argued by Jan Oort and observational confirmation of spiral arm structure noted by William Wilson Morgan and contemporaries, plus constraints on the neutral fraction during reionization advanced by recent LOFAR and MWA results.
Upcoming and planned projects—most prominently the Square Kilometre Array and staged efforts like SKA1-MID, SKA1-LOW, Hydrogen Epoch of Reionization Array, and expansions of MeerKAT into SKA precursor networks—aim to deliver transformative sensitivity for tomography of the Cosmic Dawn and precision cosmology. Synergies with facilities such as James Webb Space Telescope, Atacama Large Millimeter/submillimeter Array, Euclid, and surveys by the Vera C. Rubin Observatory will link 21 cm tomography to galaxy formation, reionization histories, and tests of fundamental physics including neutrino mass constraints pursued alongside analyses by Planck and future CMB-S4 collaborations.
Category:Radio astronomy Category:Cosmology Category:Interstellar medium