Previous methods for measuring distances to galaxies include parallax angles and standard candles. However, accurate parallax angles are limited to stars within about 500 pc. Beyond that distance, the angles become too small to measure. Standard candles are based on astronomical objects of known luminosity. By measuring the brightness of standard candles, observers can calculate the object’s distance using the inverse-square law. However, a problem with standard candles is in determining the absolute magnitude of the object. The challenge is finding objects that are good candidates for standard candles – that is objects that are luminous, easily identifiable, and relatively common. The more confident we know a star’s luminosity, the more certainty in determining its distance.
An alternative to the aforementioned techniques is the Tully-Fisher Relation. The attached paper published in Astronomy and Astrophysics in 1977 describes a method for determining the distances to galaxies. R. Brent Tully and J. Richard Fisher determined that the variability of light emitted from netral hydrogen in a galaxy is related to the luminosity of the galaxy. Also, the more variability in the light emitted by netral hydrogen, the more luminous the galaxy. Radiation from the approaching side of a rotating galaxy is changes to shorter wavelengths of light while the light from the receding side of the galaxy changes to longer wavelengths. The variability in light increases by an amount proportional to the rotating speed of the galaxy.
I found a visual to illustrate this point at http://lifeng.lamost.org/courses/astrotoday/CHAISSON/AT324/HTML/AT32402.HTM
Figure 24.11 A galaxy's rotation causes some of the radiation it emits to be blueshifted and some to be redshifted (relative to what the emission would be from a nonrotating source). From a distance, when the radiation from the galaxy is combined into a single beam and analyzed spectroscopically, the redshifted and blueshifted components combine to produce a broadening of the galaxy's spectral lines. The amount of broadening is a direct measure of the rotation speed of the galaxy.
The more massive the galaxy, the faster the stars move in their orbits at the galaxy’s edge. The paper summarizes the relationship between gravitational and centrifugal forces: v2 = GM/R and that luminosity is proportional to mass as well as the area of the disk.
Recent Studies: Since its introduction, there have been continued research in this area. Another paper, Evolution of the Tully-Fisher Relation up to z=1.4 published last year in Astronomy and Astrophysics, is a study that focuses on Tully Fisher relation at high redshift in the B,V, R, I, and K5 . I will review this paper and submit a follow-up to this blog.
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