| We have measured spectral indices for ∼ 2000 galaxies in the CNOM redshift survey of 15 X-ray luminous clusters at 0.2 < z < 0.55. A detailed comparison is made between the star formation histories of galaxies in these clusters with an identically selected sample of galaxies in the lower density field population, to establish the effects these cluster environments have on galaxy evolution. We find that the mean star formation rate, as determined from the [OII]lambda3727 emission line, is suppressed in all cluster galaxies, out to and even beyond the virial radius. The number of actively star forming galaxies, and the mean star formation rate among cluster galaxies, increases with increasing distance from the cluster centre. This correlation is not completely due to the morphology-radius relation, as cluster galaxies of a given physical size, fractional bulge luminosity and redshift have lower star formation rates than similar galaxies in the field environment.; We find no evidence that the cluster environment induces star formation in its constituent galaxies. Galaxies with positive Wo(OII), of any strength, are more common in the field than they are in the clusters. In particular, the A+em galaxies, which have spectra that may reflect dust obscured starburst activity, make up only 6.3 +/- 2.1% of the field population, and are twice as common there as they are in the cluster sample.; If star formation is terminated in a galaxy after a short starburst, the spectrum will show strong Balmer absorption lines without [OII] emission; we find that less than ∼ 5% of all galaxies have such a spectrum, and there is no evidence that they are preferentially found within the cluster sample. Spectrophotometric model results suggest that many of these galaxies may have had their star formation abruptly truncated without such a starburst. Alternatively, Halpha observations of Abell 2390 cluster galaxies suggest that the lack of [OII] emission in some such galaxies may be due to dust obscuration, and not necessarily indicative of the absence of star formation activity.; These results suggest that star formation is terminated in galaxies that are incorporated into these clusters. This termination need not be abrupt, and may take place over a period of several Gyr. Thus, the differential evolution of cluster galaxies may result because field galaxies are able to refuel their stellar disk with gas from an extended halo, thus perpetuating star formation, while such a halo would be disrupted within rich clusters, and star formation would gradually cease. |
