The spectral evolution of galaxies. I. an observational approach.

The author has conducted a major new spectrophotometric survey of giant elliptical galaxies in order to measure changes due to spectral evolution. The data set consists of good spectrophotometry on 33 galaxies to a redshift of z = 0.8. The look-back time for this redshift, H₀ ≈ 50 km s^-1Mpc^-1, q₀ ≈ 0, is 7 - 8 Gyr. The objects were chosen from a new multicolor, multifield survey using the prime focus cameras of the KPNO and CTIO 4 m telescopes. The amplitude of the 4000 Å break was used as the indicator of spectral evolution. The data of the evolution diagram (redshift vs. break amplitude) indicate that a change of less than 7% in the break amplitude is evident at the high-redshift end, z ≈ 0.8. These data also indicate that giant elliptical galaxies are much older than 8 Gyr. The observed width of the break amplitude distribution for all program galaxies indicates the validity of the assumption of coevality to within ±2 Gyr to a redshift of z ≈ 0.8.