The dependence of cluster galaxy star formation rates on the global environment

A comparison of star formation properties as a function of environment is m ade 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 tota l 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 d etermined from the [O II] lambda 3727 emission line. The mean star formatio n rate of cluster galaxies brighter than M-r = -17.5 + 5 log h is found to vary from 0.17 +/- 0.02 h(-2) M. yr(-1) at R-200 (1.5-2 h(-1) Mpc) to 0.00 +/- 0.01 in the cluster center, and it is always less than the mean star fo rmation rate of field galaxies, which is 0.39 +/- 0.01 h(-2) M. yr(-1). It is demonstrated that this significant difference is not exclusively a resul t of the difference in morphological type, as parameterized by the B/T valu e, by correcting for the B/T-radius relation. The distribution of [O II] eq uivalent widths among cluster galaxies is skewed toward lower values relati ve to the distribution for field galaxies of comparable physical size, B/T, and redshift, with a statistical significance of more than 99%. The cluste r environment affects not only the morphological mix of the galaxy populati on but also suppresses the star formation rate within those galaxies, relat ive to morphologically similar galaxies in the field.