Usually the redshift of a galaxy is measured spectroscopically. The spectrum of the galaxy is observed: Emission or absorption lines are identified and their wavelengths are measured. These measured wavelengths are then compared with the rest wavelengths to determine the redshift. Photometric redshifts (also known as "color redshifts" or "photo-z") utilize broad-band photometry to measure the redshifts of galaxies rather than spectroscopy. With photometric redshifts broad-band photometry is compared to predictions from galaxy spectral energy distributions (SED's) for a range of types of galaxies to determine the redshift. Rather than observing narrow spectral features of galaxy spectra the photometric redshift technique uses broad features, such as the 4000≈ break and the overall shape of a spectrum. View a real elliptical galaxy [gif] being passed though different redshifts in our filter system.
Photometric redshifts can be measured much faster and in larger quantities than their spectroscopic counterparts. In spectroscopy, the light from the galaxy is separated into narrow wavelength bins of a few angstroms width. Each bin then receives only a small fraction of the total light from the galaxy. So to achieve a sufficiently high signal-to-noise ratio in each bin, very long integration times are required. For photometry, the bins are much larger: typically 1000 ≈ wide. This requires only a short exposure time to reach the same signal-to-noise ratio. In addition, imaging detectors generally cover a greater area of the sky than multi-object spectrographs. Thus the redshifts of more objects can be measured simultaneously by using photometry than by spectroscopy.
How do we know that the redshifts estimated by the photo-z technique are correct? Any multi-band photometric system has errors: bias and scatter. One needs to know what these errors are as a function of redshift in order to carry out the weak lens and baryon acoustic oscillation probes of dark energy. So a key requirement is the spectroscopic calibration of the photometric redshift system. This calibration plan is presented in the link to the pdf file "A Plan for Photo-z Calibrations." It makes use of both spectral information and angular correlations. Please view the following document: A Plan for Photo-z Calibrations [PDF]
References: Connolly et al. (1997, ApJ, 486, 11)
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