Enhanced H alpha emission in early-type galaxies belonging to compact groups

The results of H alpha photometry of a sample of early-type galaxies belong ing to nearby compact groups are presented. Over 75% of the galaxies in the sample were detected in H alpha, a result that we have interpreted as proo f of the presence of a warm interstellar medium within them. Comparing the H alpha luminosities of the galaxies in our sample with those of a sample o f early-type galaxies from the field, we have found that field early-type g alaxies are systematically fainter in H alpha than the compact group ones f or a given B absolute magnitude by Delta log (L-H alpha/L-B) approximate to 0.5. We suggest that the excess in the H alpha emission within the groups is due to photoionization of gas by Lyman continuum photons emitted by mass ive stars. Recent massive star formation would be a consequence of the accr etion by early-type galaxies of gas from the outer envelopes of gas-rich ga laxies experiencing close passages across the group.