HI line
The HI line (H-one line), also known as the hydrogen line , is the name in astronomy for the characteristic radio emission of neutral hydrogen . The term 21 cm line also used comes from the corresponding wavelength in a vacuum . In radio astronomy , this radiation plays an important role, because their investigation information about the density distribution, velocity and temperature of hydrogen atoms in the universe are.
Emergence
The emission or absorption line is created by the hyperfine structure transition of the neutral hydrogen atom in the 1s ground state . That is the energy difference between the parallel and antiparallel spin orientation of the electron relative to the spin of the proton . The energy difference is about 5.9 · 10 −6 eV , corresponding to a radio frequency of 1,420.40575177 MHz and a wavelength of about 21 cm.
meaning
Since the HI line is only slightly attenuated by interstellar matter because of the very low probability of transition , it is a preferred observation object in radio astronomy . It can be used to determine the distribution of hydrogen, which makes up around 90 percent of interstellar matter. Doppler shifts of the line provide information about the movements of astronomical objects .
In 1944, Hendrik Christoffel van de Hulst calculated the 21 cm line. The astronomers Colin Stanley Gum , Frank John Kerr and Gart Westerhout recognized the importance of the HI line in 1951.
The Pioneer badge , attached to the space probes Pioneer 10 and Pioneer 11 , shows the hyperfine structure transition of the neutral hydrogen atom. The wavelength was used as the standard unit of length and the period as the standard unit of time.
By observing the 21 cm signal, one hopes to gain new insights into the end of the dark age and the subsequent epochs ( reionization epoch and beyond ). The signal is strongly redshifted for these epochs today . In March 2018 was EDGES - collaboration ( Experiment to Detect the Global Epoch of Reionization Signature ) observing an absorption profile posted at 78 MHz, which points to the reionization. There are also indications from it that may point to dark matter . The signal was very difficult to observe as it is heavily obscured by terrestrial sources, galactic radiation and other cosmic sources . It is hoped that the planned Square Kilometer Array (SKA) will provide better data . 21 cm wavelength astronomy can also be used to study the further development of the early universe in other epochs.
See also
Web links
- Prediction of 21cm Line Radiation & The Discovery of Hydrogen Radio emission by Ewen and Purcell National Radio Astronomy Observatory (accessed June 30, 2010)
- A Sky Full Of Hydrogen Astronomy Picture of the Day January 13, 2001
- The HI 21 cm line
Individual evidence
- ↑ EDGES , MIT Haystack Observatory
- ^ Judd Bowman, Alan Rogers, Raul Monsalve, Thomas Mozdzen, Nivedita Mahesh: An absorption profile centered at 78 megahertz in the sky-averaged spectrum, Nature, Volume 555, 2018, pp. 67-70, abstract
- ↑ Joshua Kerrigan, First Detection of the 21cm Cosmic Dawn Signal , Astrobites, March 14, 2018