Lyman-break galaxy
Encyclopedia
Lyman-break galaxies are star-forming
galaxies
at high redshift
that are selected using the differing appearance of the galaxy in several imaging filters
due to the position of the Lyman limit
. The technique has primarily been used to select galaxies at redshifts of z = 3–4 using ultraviolet
and optical
filters, but progress in infrared astronomy
has allowed the use of this technique at higher redshifts using infrared filters.
The Lyman-break galaxy selection technique relies on the fact that radiation at higher energies than the Lyman limit
at 912 Å
is almost completely absorbed by neutral gas around star-forming
regions of galaxies. In the rest frame
of the emitting galaxy, the emitted spectrum
is bright at wavelengths longer than 912 Å, but very dim or imperceptible at shorter wavelengths—this sharp drop (or "break") can be used to find the position of the Lyman limit. Light with a wavelength shorter than 912 Å is in the far-ultraviolet range and is blocked by the Earth's atmosphere, but for very distant galaxies the wavelengths of light are stretched
considerably because of the expansion of the Universe
. For a galaxy at redshift z = 3, the Lyman break will appear to be at wavelengths of about 3600 Å, which is long enough to be detected by ground- or space-based telescopes.
Candidate galaxies around redshift z = 3 can then be selected by looking for galaxies which appear in optical images (which are sensitive to wavelengths greater than 3600 Å), but do not appear in ultraviolet images (which are sensitive to light at wavelengths shorter than 3600 Å). The technique may be adapted to look for galaxies at other redshifts by choosing different sets of filters—the method works as long as images may be taken through at least one filter above and below the wavelength of the redshifted .In order to confirm the redshift estimated by the color selection, follow-up spectroscopy
is performed. Although spectroscopic measurements are necessary to obtain a high-precision redshift, spectroscopy is typically much more time-consuming than imaging, so the selection of candidate galaxies via the Lyman-break technique greatly improves the efficiency of high-redshift galaxy surveys.
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
at high redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
that are selected using the differing appearance of the galaxy in several imaging filters
Filter (optics)
Optical filters are devices which selectively transmit light of different wavelengths, usually implemented as plane glass or plastic devices in the optical path which are either dyed in the mass or have interference coatings....
due to the position of the Lyman limit
Lyman limit
The Lyman limit is the short-wavelength end of the Hydrogen Lyman series, at 91.2 nm. It corresponds to the energy required for an electron in the Hydrogen ground state to escape from the electric potential barrier that originally confined it, thus creating a Hydrogen ion...
. The technique has primarily been used to select galaxies at redshifts of z = 3–4 using ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
and optical
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
filters, but progress in infrared astronomy
Infrared astronomy
Infrared astronomy is the branch of astronomy and astrophysics that studies astronomical objects visible in infrared radiation. The wavelength of infrared light ranges from 0.75 to 300 micrometers...
has allowed the use of this technique at higher redshifts using infrared filters.
The Lyman-break galaxy selection technique relies on the fact that radiation at higher energies than the Lyman limit
Lyman limit
The Lyman limit is the short-wavelength end of the Hydrogen Lyman series, at 91.2 nm. It corresponds to the energy required for an electron in the Hydrogen ground state to escape from the electric potential barrier that originally confined it, thus creating a Hydrogen ion...
at 912 Å
Ångström
The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....
is almost completely absorbed by neutral gas around star-forming
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
regions of galaxies. In the rest frame
Rest frame
In special relativity the rest frame of a particle is the coordinate system in which the particle is at rest.The rest frame of compound objects is taken to be the frame of reference in which the average momentum of the particles which make up the substance is zero In special relativity the rest...
of the emitting galaxy, the emitted spectrum
Emission spectrum
The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted by the element's atoms or the compound's molecules when they are returned to a lower energy state....
is bright at wavelengths longer than 912 Å, but very dim or imperceptible at shorter wavelengths—this sharp drop (or "break") can be used to find the position of the Lyman limit. Light with a wavelength shorter than 912 Å is in the far-ultraviolet range and is blocked by the Earth's atmosphere, but for very distant galaxies the wavelengths of light are stretched
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
considerably because of the expansion of the Universe
Hubble's law
Hubble's law is the name for the astronomical observation in physical cosmology that: all objects observed in deep space are found to have a doppler shift observable relative velocity to Earth, and to each other; and that this doppler-shift-measured velocity, of various galaxies receding from...
. For a galaxy at redshift z = 3, the Lyman break will appear to be at wavelengths of about 3600 Å, which is long enough to be detected by ground- or space-based telescopes.
Candidate galaxies around redshift z = 3 can then be selected by looking for galaxies which appear in optical images (which are sensitive to wavelengths greater than 3600 Å), but do not appear in ultraviolet images (which are sensitive to light at wavelengths shorter than 3600 Å). The technique may be adapted to look for galaxies at other redshifts by choosing different sets of filters—the method works as long as images may be taken through at least one filter above and below the wavelength of the redshifted .In order to confirm the redshift estimated by the color selection, follow-up spectroscopy
Astronomical spectroscopy
Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects...
is performed. Although spectroscopic measurements are necessary to obtain a high-precision redshift, spectroscopy is typically much more time-consuming than imaging, so the selection of candidate galaxies via the Lyman-break technique greatly improves the efficiency of high-redshift galaxy surveys.
See also
- Lyman seriesLyman seriesIn physics and chemistry, the Lyman series is the series of transitions and resulting ultraviolet emission lines of the hydrogen atom as an electron goes from n ≥ 2 to n = 1...
- Lyman-alpha forestLyman-alpha forestIn astronomical spectroscopy, the Lyman-alpha forest is the sum of absorption lines arising from the Lyman-alpha transition of the neutral hydrogen in the spectra of distant galaxies and quasars....
- Lyman alpha emitterLyman alpha emitterLyman alpha emitters are a type of distant galaxy that emits Lyman-alpha radiation. They are extremely distant and because of the finite travel time of light they are glimpses of the past history of the universe. They are thought to be the progenitors of most modern Milky Way type galaxies...
- Lyman alpha blob
- Damped Lyman-alpha system
- Lyman limitLyman limitThe Lyman limit is the short-wavelength end of the Hydrogen Lyman series, at 91.2 nm. It corresponds to the energy required for an electron in the Hydrogen ground state to escape from the electric potential barrier that originally confined it, thus creating a Hydrogen ion...