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 fifteen 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 [ OII ] \lambda 3727 emission line .
The mean star formation rate of cluster galaxies brighter than M _ { r } = -17.5 + 5 \log { h } is found to vary from 0.17 \pm 0.02 h ^ { -2 } M _ { \odot } \mbox { yr } ^ { -1 } at R _ { 200 } ( 1.5–2 h ^ { -1 } Mpc ) to zero in the cluster center , and is always less than the mean star formation rate of field galaxies , which is 0.39 \pm 0.01 h ^ { -2 } M _ { \odot } \mbox { yr } ^ { -1 } .
It is demonstrated that this significant difference is not due exclusively to the difference in morphological type , as parameterized by the B/T value , by correcting for the B/T–radius relation .
The distribution of [ OII ] 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 .