The Dependence of Cluster Galaxy Star Formation Rates on the Global Environment

A comparison of star formation properties as a function of environment is made from the spectra of identically selected cluster and field galaxies in the CNOC 1 redshift survey of over 2000 galaxies in the fields of 15 X-ray luminous clusters at 0.18<z<0.55. The ratio of bulge luminosity to total galaxy luminosity (B/T) is computed for galaxies in this sample, and this measure of morphology is compared with the galaxy star formation rate as determined from the [O II] λ3727 emission line. The mean star formation rate of cluster galaxies brighter than M_r=-17.5+5logh is found to vary from 0.17+/-0.02 h^-2 M_solar yr^-1 at R₂₀₀ (1.5-2 h^-1 Mpc) to 0.00+/-0.01 in the cluster center, and it is always less than the mean star formation rate of field galaxies, which is 0.39+/-0.01 h^-2 M_solar yr^-1. It is demonstrated that this significant difference is not exclusively a result of the difference in morphological type, as parameterized by the B/T value, by correcting for the B/T-radius relation. The distribution of [O II] equivalent widths among cluster galaxies is skewed toward lower values relative to the distribution for field galaxies of comparable physical size, B/T, and redshift, with a statistical significance of more than 99%. The cluster environment affects not only the morphological mix of the galaxy population but also suppresses the star formation rate within those galaxies, relative to morphologically similar galaxies in the field.