1995A&A...298..361J

We discuss the nature of blue galaxies in three clusters, AC 103, AC 114 and AC 118, at z=0.3. The galaxy properties are approached from the integrated stellar population found in these blue objects as compared to those in a set of nearby spiral galaxies. The stellar population is analyzed through a population synthesis technique which takes into account both age and metallicity effects. Regarding the integrated properties of nearby spirals, we find that early-type and late-type spiral galaxies form two distinct groups. From Sa to Sb, the Hubble sequence is illustrated through monotonic spectral changes. This translates into a smooth decrease of the mean metallicity in the old population as well as an increase of the flux contribution from the intermediate age and young stars. Around 65% to 75% of the light arises from old stellar population, with 20% to 40% being metal-rich ([Z/Z_☉]>0). In Sa-Sb -type spirals, the stellar components younger than 7.5Gyr have [Z/Z_☉]=0. For late-type spirals, Sbc-Sc-types, the large dispersion of their spectrophotometric properties is due to a dispersion both in the contributions from young stellar components and in the metallicity distribution of the old population. About half of their flux comes from stellar components younger than 7.5Gyr and with [Z/Z_☉]=-0.5. No dispersion is observed in the spectrophotometric properties of Sd galaxies. Their stellar content appears to be in continuity with that of Sc galaxies. Turning to the stellar population analysis of galaxies at z=0.3, we have first built a few groups in each galaxy cluster, according to the colours and spectral similarities of the objects. The spectra of three groups of red galaxies, five groups of blue galaxies and a few individual blue objects in AC 118 have then been analyzed by population synthesis. The red galaxies at z≃0.3 are confirmed as elliptical-like galaxies with a noticeable intermediate age population (1-5Gyr). Most of the blue galaxies at z≃0.3 appear to be compatible with early-type spirals. A third of our sample of blue galaxies experienced an enhanced star formation about 0.1Gyr ago.