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 (within the hyperfine structure). The transition is termed a spin-flip transition. The line is a useful sign of the presence of neutral atomic hydrogen. 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 this analysis. Regions in front of hot objects, i.e., with 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 is one target of cosmological intensity mapping. The signal is weak and has yet to be detected (as of 2017) but much effort has been put into detecting it (21-cm experiments), such as PAPER and HERA.

The analogous line for deuterium is 91.6 cm (aka 92-cm line). In sources sufficiently bright to see it, this allows determination of a D/1H ratio.

Further reading:
21.1cm1.5GHz5.9μeV21-cm line

Referenced by pages:
Anticenter Shell
Arecibo Observatory (NAIC)
Allen Telescope Array (ATA)
blind survey
Canadian Hydrogen Intensity Mapping Experiment (CHIME)
cold gas
Dwingeloo Obscured Galaxy Survey (DOGS)
electron orbital
epoch of reionization (EOR)
Fast Fourier Transform Telescope
fine structure
forbidden line
galactic worm (GW)
GALEX Arecibo SDSS Survey (GASS)
Galactic All-sky Survey (GASS)
Green Bank Telescope (GBT)
Giant Metrewave Radio Telescope (GMRT)
neutral atomic hydrogen (HI)
HI 4-Pi Survey (HI4PI)
HI region (HI)
Parkes HI Zone of Avoidance Survey (HIZOA)
hydrogen (H)
hyperfine structure
intensity mapping
Low-frequency Array (LOFAR)
Murchison Widefield Array (MWA)
Parkes Multibeam Pulsar Survey (PMPS)
spectral line designation
spin (ms)
star formation (SF)
Tianlai Project
21-cm experiment
atmospheric window
zone of avoidance (ZOA)