Redshift and Measuring Distance to Remote Galaxies
Galaxies emit light across the entire electromagnetic spectrum. Star-forming galaxies have areas of intense activity, but the light in the ultraviolet can be blocked by clouds surrounding the star-formation region. This causes a significant and identifiable drop in the light emitted at higher energies as in the upper spectrum in the figure labeled "Emitted (rest)." If the light of the galaxy is measured at its location (like galaxies near us), the drop occurs in the ultraviolet as expected (shortward of the Lyman break feature, at 912 Angstroms, due to hydrogen) as shown. The drop is the dark area to the left of the region labeled "UV." When astronomers observe distant galaxies, the entire spectrum is redshifted, so all features appear at a wavelength redder than would be expected. The ultraviolet drop is such a dramatic feature that it can be readily identified and used to obtain the redshift of a galaxy, and therefore its distance.
The galaxy GN-z11 is such a galaxy. Its spectrum apparently contains the ultraviolet drop, but it is shifted due to its redshift of z~11. Therefore, the drop in intensity is shifted to the red and can be observed with infrared observations, as shown in the middle of the illustration. (The break is the dark area just above the "IR" label in the middle of the illustration.) The line graph depicts the shape of the emission from the galaxy, and the colorful spectrum shows the effect on an observation taken with a spectrograph. Spectroscopy allows an accurate measurement of the redshift (and thus the distance).
The lower portion of the figure illustrates how the redshift manifests when imaging observations are obtained. Many times, galaxies are too faint to be observed with spectroscopy. Astronomers use an imaging camera, with several filters, and may use several telescopes to determine where the light from the galaxy is emitted. For images, the galaxy is not detected at energies higher than the ultraviolet drop (towards the left in this illustration). If the galaxy is redshifted, the drop appears in the visual data, or even the infrared, depending upon the redshift. An estimate of the redshift can be obtained from comparing the data from the ultraviolet, the visual, the near infrared, and the infrared. This is less accurate than spectroscopy, but does give a rough indication of the redshift and thus the distance to the object.