Anomalous star formation activity of less-luminous galaxies in a cluster environment

We discuss a correlation between the star formation activity (SFA) and the luminosity of star-forming galaxies at intermediate redshifts of 0.2 less t han or equal to z less than or equal to 0.6 in both cluster and field envir onments. The equivalent width (EW) of [O II] is used for the measurement of SFA, and the R-band absolute magnitude, M-R, is used for the measurement o f luminosity. In less-luminous (M-R greater than or similar to -20.7) galax ies, we find (1) the mean EW ([O II]) of cluster galaxies is smaller than t hat of field galaxies but that (2) some cluster galaxies have as large EWs ([ O II]) as that of actively star-forming field galaxies. Based on both of these results, we discuss a new possible mechanism for the Butcher-Oemler (BO) effect, assuming that the luminosity of a galaxy is proportional to it s dynamical mass. Our proposal is that BO galaxies are less-massive cluster galaxies with smaller peculiar velocities. They are then stable against Ke lvin-Helmholtz instability (KHI) and are not affected by the tidal interact ion between clusters and themselves. Their interstellar medium (ISM) would hardly be stripped, and their SFA would be little suppressed. Hence, as lon g as such galaxies keep up their SFA, the fraction of blue galaxies in a cl uster does not decrease. As a cluster becomes virialized, however, such gal axies become more accelerated, the ISM available for SFA is stripped by KHI , and their color evolves redward, which produces the BO effect.