Evolution of the M31 disk population

An evolutionary model for the M31 inner disk population is described, which at age 12 billion years agrees closely with the narrow-band colors and line indices recently measured by Spinradet al. (1971), and with the broad-band colors from 0.36 to 3.4 . Assuming that gE galaxies have the same stellar population as the M31 inner disk, this model is used to derive evolutionary effects in cosmology. Interpretation of the magnitude-redshift relation is substantially affected, since the evolutionary correction to V magnitudes at redshift 0.2 may be 0.09 mag. if H₀=100 km s^–1 Mpc^–1, or 0.25 mag. ifH₀=50. Changes in some line strengths may be observable at redshift 0.46, showing the younger stellar population at earlier times. Our results differ from estimates by other authors, also based on the assumption that the gE and disk populations are identical, that the photometry of Spinradet al. (1971) precludes a significant rate of evolution. One reason for the difference is that our model has an unexpectedly shallow luminosity function just below the main sequence turnoff; another reason is that authors in the past have used the change in absolute V magnitude as the evolutionary correction, whereas the appropriate quantity is the change in absolute magnitude at theemitted wavelengths, and evolution is more rapid at these shorter wavelengths.