The filter in which a galaxy vanishes from view gives a pretty strong clue as to the galaxy's photometric redshift
, even without a spectrum; spectroscopy can later provide unambiguous confirmation.
By combining these color data, it is possible to make a crude estimate of the distances to the faint background galaxies (called photometric redshift
That is, until Chuck Steidel (Caltech), Piero Madau (University of California, Santa Cruz), and others perfected the photometric redshift technique, in which a galaxy's redshift can be estimated from its relative brightness in various passbands.
"To me, the validation of the concept of photometric redshift is the single most important result that emerged from the Hubble Deep Field and its successors."
Team member Michael Perryman (who with Clare Foden and Tone Peacock first proposed the detection method) concedes that the spectral resolution is relatively low--in fact, not very much better than the photometric redshift
technique, which simply relies on color filters.
Appendices cover photometric redshifts
and K-corrections, broadband photometry, and physical and astronomical constants and unit conversions.
Observations must also be conducted at multiple wavelengths to enable measurements of photometric redshifts, which provide distance estimates to faint galaxies based on their brightness levels at different wavelengths (see box on page 40).
"In the end, the Dark Energy Survey will probably have better statistics," says Hoekstra, "but KIDS provides better photometric redshifts, because observations are carried out both at optical and infrared wavelengths." Both surveys released preliminary maps mid-year in 2015.
While LSST has multiple wavelength filters, good for estimating photometric redshifts and hence distances, Euclid will instead need supplemental observations from ground-based telescopes.
11.8 billion years HSC 8.4 billion years KIDS 8 billion years DES Look-back time (billions of years) to a Milky Way-like galaxy SURVEY COLORS While the DES and HSC surveys use five "color" filters each, the KIDS survey looks farther into both the ultraviolet and infrared, using nine filters total to get a better handle on the photometric redshifts, and hence distances, of its galaxies, (filter colors are representative.)
are less accurate than spectroscopic ones, which measure narrow spectral lines.