A Radio Recombination Line (RRL) in

astrophysics Astrophysics is a science that employs the methods and principles of physics and chemistry in the study of astronomical objects and phenomena. As one of the founders of the discipline, James Keeler, said, astrophysics "seeks to ascertain the ...

is a spectral

emission line A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used ...

produced by the transition of

electrons The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

between two

energy levels A quantum mechanics, quantum mechanical system or particle that is bound state, bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels. This contrasts with classical mechanics, classical pa ...

in an

atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...

, and with a rest frequency in the

radio Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connec ...

band of the

electromagnetic spectrum The electromagnetic spectrum is the full range of electromagnetic radiation, organized by frequency or wavelength. The spectrum is divided into separate bands, with different names for the electromagnetic waves within each band. From low to high ...

. This effect is believed to be produced in ionized gas, and driven by the recombination of

ions An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

with

, when the energy of the electron falls enough to be captured by the ion. Following this capture, the electron cascades down through the energy levels to a stable state, emitting a

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...

at each transition. In low density regions of the Interstellar Medium (ISM), these atoms can remain stable up to very high

quantum number In quantum physics and chemistry, quantum numbers are quantities that characterize the possible states of the system. To fully specify the state of the electron in a hydrogen atom, four quantum numbers are needed. The traditional set of quantu ...

(n~1000), and have a diameter up to 1 micron across, similar to the size of

human hair Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and f ...

. The

Bohr model In atomic physics, the Bohr model or Rutherford–Bohr model was a model of the atom that incorporated some early quantum concepts. Developed from 1911 to 1918 by Niels Bohr and building on Ernest Rutherford's nuclear Rutherford model, model, i ...

predicting the strength of emission lines at different quantum numbers allows for the existence of RRLs, but they were not studied until several decades later in 1945 by Dutch astronomer, Hendrik van de Hulst. In his work, van de Hulst proposed the existence of observable radio line emission from highly excited atoms in the ISM, as well as predicted the observable 21cm emission in neutral hydrogen regions. For

Hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

, this effect occurs in

HII regions An H II region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light year ...

and can be thought of as a rarer example of the

Hydrogen spectral series The emission spectrum of atomic hydrogen has been divided into a number of ''spectral series'', with wavelengths given by the Rydberg formula. These observed spectral lines are due to the electron making transitions between two energy levels i ...

, only occurring for electron transitions beginning above

n>27

. The first search for astronomical RRLs began in 1958 by T.M. Egorova and N.F. Ryzhkov, focusing on the H271α line in the

galactic plane The galactic plane is the plane (geometry), plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms ''galactic plane'' and ''galac ...

, resulting in no clear detection of the line. Six years after the first searches began, a group at

Lebedev Physical Institute The Lebedev Physical Institute of the Russian Academy of Sciences (LPI RAS or just LPI) (in ), situated in Moscow, is a Russian research institute specializing in physics. The institute was established in its present shape in 1934 by academician ...

Moscow, Russia Moscow is the Capital city, capital and List of cities and towns in Russia by population, largest city of Russia, standing on the Moskva (river), Moskva River in Central Russia. It has a population estimated at over 13 million residents with ...

detected the first RRLs using a 22-m radio telescope on 27 April 1964. The group detected the H90α transition, at a rest frequency of 8872.5 MHz and produced in the

Omega Nebula The Omega Nebula is an H II region in the constellation Sagittarius. It was discovered by Philippe Loys de Chéseaux in 1745. Charles Messier catalogued it in 1764. It is by some of the richest starfields of the Milky Way, figuring in the north ...

. A separate Russian group detected the H104α line one month later in May 1964, again in the Omega nebula. RRLs were first observed at cosmological distances ( z = 1.124) in 2019, and are suggested to have originated from a warm Hydrogen or cold neutral

Carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

region within a dwarf-like galaxy that is being energized by a background

quasar A quasar ( ) is an extremely Luminosity, luminous active galactic nucleus (AGN). It is sometimes known as a quasi-stellar object, abbreviated QSO. The emission from an AGN is powered by accretion onto a supermassive black hole with a mass rangi ...

.{{Cite journal , last1=Emig , first1=K. L. , last2=Salas , first2=P. , last3=de Gasperin , first3=F. , last4=Oonk , first4=J. B. R. , last5=Toribio , first5=M. C. , last6=Röttgering , first6=H. J. A. , last7=Tielens , first7=A. G. G. M. , date=February 2019 , title=The first detection of radio recombination lines at cosmological distances , url=https://www.aanda.org/10.1051/0004-6361/201834052 , journal=Astronomy & Astrophysics , volume=622 , pages=A7 , doi=10.1051/0004-6361/201834052 , arxiv=1811.08104 , bibcode=2019A&A...622A...7E , issn=0004-6361

References

Emission-line stars Astrophysics Emission spectroscopy Electron states