Astrophysics (Index)About

21-cm line

(hydrogen line, HI line)
(hydrogen line useful for determining redshift)

The well-known 21-cm line (hydrogen line or HI line) is hydrogen's 21.1 cm (microwave) spectral line, which is emitted during a transition between two neutral hydrogen ground states that are within its hyperfine structure. The type of transition is termed a spin-flip transition. The line is a useful sign of the presence of neutral atomic hydrogen (HI). It is a forbidden line, i.e., virtually never created under everyday Earth-bound conditions.

The line is recognizable at a lot of redshifts, and passes through dust clouds better than much electromagnetic radiation. HI regions in galaxies produce 21-cm emission lines used to estimate galaxy mass, and map out velocities through analysis of Doppler shifts. Knowledge of the Milky Way's structure owes much to such analysis. Regions of HI in front of hot objects (i.e., objects emitting a continuous spectrum) yield 21-cm absorption lines, also useful for analysis.

The neutral hydrogen existing between recombination and the epoch of reionization is presumed to produce 21-cm emission lines, which could reveal much about reionization and times since, including analysis of gravitational lensing to reveal galaxy clusters and Doppler shifts that would reveal voids, and for these reasons it is a target of cosmological intensity mapping. Much effort has been put into detecting and mapping it (21-cm experiments), such as PAPER and HERA, but they have not yet detected it: the 21-cm signal is very weak and as of 2024, the only detection claim I'm aware of (by EDGES in 2018) has been challenged.

The analogous line for deuterium is 91.6 cm (aka 92-cm line). In sources sufficiently bright to measure it, a D/1H ratio can be determined.


(emission,hydrogen,redshift,line)
Further reading:
https://en.wikipedia.org/wiki/Hydrogen_line
http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/h21.html
https://astronomy.swin.edu.au/cosmos/s/Spin-flip+Transition
http://www.physics.smu.edu/scalise/P3368fa12/PHYSS3368-HI.pdf
http://egg.astro.cornell.edu/alfalfa/ugradteam/uat16talks/uat16_introRadAstro.pdf
https://ui.adsabs.harvard.edu/abs/2012RPPh...75h6901P/abstract
http://www.astronomy.ohio-state.edu/~pogge/Ast871/Notes/Neutral.pdf
https://ui.adsabs.harvard.edu/abs/2023PASJ...75S...1S/abstract
WaveLFreqPhoton
Energy
  
21.1cm1.5GHz5.9μeV21-cm line

Referenced by pages:
21-cm experiment
Allen Telescope Array (ATA)
Anticenter Shell
Arecibo Observatory (NAIC)
atmospheric window
Australian Square Kilometre Array Pathfinder (ASKAP)
BINGO
blind survey
Canadian Hydrogen Intensity Mapping Experiment (CHIME)
cold gas
COLD GASS
degeneracy
deuterium (D)
Dwingeloo Obscured Galaxy Survey (DOGS)
EDGES
electron orbital
epoch of reionization (EOR)
Fast Fourier Transform Telescope
fine structure
forbidden line
Galactic All-sky Survey (GASS)
galactic worm (GW)
GALEX Arecibo SDSS Survey (GASS)
Giant Metrewave Radio Telescope (GMRT)
Green Bank Telescope (GBT)
HERA
HI 4-Pi Survey (HI4PI)
HI Jodrell All-Sky Survey (HIJASS)
HI region (HI)
HIPASS
HIRAX
hydrogen (H)
hyperfine structure
intensity mapping (IM)
intensity mapping surveys
isotope
Low-frequency Array (LOFAR)
MeerKLASS
metastable
microwave
Murchison Widefield Array (MWA)
neutral atomic hydrogen (HI)
OVRO-LWA
PAPER
Parkes HI Zone of Avoidance Survey (HIZOA)
Parkes Multibeam Pulsar Survey (PMPS)
RESOLVE­
SARAS
self-absorption
spectral line designation
spin (ms)
star formation (SF)
Tianlai Project
Westerhout Radio Survey (W)
zone of avoidance (ZOA)

Index