Photometric redshift
Encyclopedia
A photometric redshift is an estimate for the distance of an astronomical object, such as a galaxy
or quasar
. The technique uses photometry
(that is, the brightness of the object viewed through various standard filters
, each of which lets through a relatively broad spectrum
of colours, such as red light, green light, or blue light) to determine the redshift
, and hence, through Hubble's law
, the distance, of the observed object. The technique relies upon the spectrum of radiation being emitted by the object having strong features that can be detected by the relatively crude filters.
The technique was developed in the 1960s, but was largely replaced in the 1970s and 1980s by spectroscopic redshifts, using spectroscopy
to observe the frequency
(or wavelength
) of characteristic spectral line
s to see how far the lines were shifted from their usual position. The technique has come back into vogue as a result of large sky surveys conducted in the late 1990s and early 2000s which have detected a large number of high-redshift objects.
Photometric redshifts were originally determined by calculating the expected observed data from a known emission spectrum at a range of redshifts. In recent years, Bayesian statistical methods and artificial neural network
s have been used to estimate redshifts from photometric data.
As photometric filters are sensitive to a range of wavelengths, and the technique relies on making many assumptions about the nature of the spectrum at the light-source, errors
for these sorts of measurements can range up to δz = 0.5, and are much less reliable than spectroscopic determinations. In the absence of sufficient telescope time to determine a spectroscopic redshift for each object, the technique of photometric spectroscopy provides a method to determine an at least qualitative characterization of a redshift.
For example, if a Sun-like spectrum had a redshift of z = 1, it would be brightest in the infrared
rather than at the yellow-green color associated with the peak of its blackbody spectrum, and the light intensity will be reduced in the filter by a factor of two (i.e. 1+z) (see K correction
for more details on the photometric consequences of redshift).
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...
or quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
. The technique uses photometry
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...
(that is, the brightness of the object viewed through various standard 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....
, each of which lets through a relatively broad spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
of colours, such as red light, green light, or blue light) to determine the 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...
, and hence, through Hubble's law
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...
, the distance, of the observed object. The technique relies upon the spectrum of radiation being emitted by the object having strong features that can be detected by the relatively crude filters.
The technique was developed in the 1960s, but was largely replaced in the 1970s and 1980s by spectroscopic redshifts, using spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
to observe the frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
(or wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
) of characteristic spectral line
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
s to see how far the lines were shifted from their usual position. The technique has come back into vogue as a result of large sky surveys conducted in the late 1990s and early 2000s which have detected a large number of high-redshift objects.
Photometric redshifts were originally determined by calculating the expected observed data from a known emission spectrum at a range of redshifts. In recent years, Bayesian statistical methods and artificial neural network
Artificial neural network
An artificial neural network , usually called neural network , is a mathematical model or computational model that is inspired by the structure and/or functional aspects of biological neural networks. A neural network consists of an interconnected group of artificial neurons, and it processes...
s have been used to estimate redshifts from photometric data.
As photometric filters are sensitive to a range of wavelengths, and the technique relies on making many assumptions about the nature of the spectrum at the light-source, errors
Observational error
Observational error is the difference between a measured value of quantity and its true value. In statistics, an error is not a "mistake". Variability is an inherent part of things being measured and of the measurement process.-Science and experiments:...
for these sorts of measurements can range up to δz = 0.5, and are much less reliable than spectroscopic determinations. In the absence of sufficient telescope time to determine a spectroscopic redshift for each object, the technique of photometric spectroscopy provides a method to determine an at least qualitative characterization of a redshift.
For example, if a Sun-like spectrum had a redshift of z = 1, it would be brightest in the infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
rather than at the yellow-green color associated with the peak of its blackbody spectrum, and the light intensity will be reduced in the filter by a factor of two (i.e. 1+z) (see K correction
K correction
K correction is a correction to an astronomical object's magnitude that allows a measurement of a quantity of light from an object at a redshift z to be converted to an equivalent measurement in the rest frame of the object. If one could measure all the light from an object at all wavelengths , a...
for more details on the photometric consequences of redshift).