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 lines 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 networks 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).
- The technique was first described by Baum, W. A.: 1962, in G. C. McVittie (ed.), Problems of extra-galactic research, p. 390, IAU Symposium No. 15
- Bolzonella, M.; Miralles, J.-M.; Pelló, R., Photometric redshifts based on standard SED fitting procedures, Astronomy and Astrophysics, 363, p.476-492 (2000).
- A pedagogical overview of the K-correction by David Hogg and other members of the SDSS collaboration can be found at astro-ph.
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