Spectral indices in cooling flow galaxies: evidence of star formation

Through the study of two absorption spectral features in the optical range(Mg2 and the 4000 A break), we find evidence for star formation in the innerregions of cooling-flow galaxies. The application of simple stellar populationmodels reveals that the measured indices are explained if a relatively smallfraction of the total mass flow (5-17%) is forming new stars with a normal IMF.However, we argue that this is only a lower limit, and conclude that a largefraction of the gas accreted inside the galaxy could be forming stars. Inaddition, the analysis of line-strength gradients in the inner galaxy regionsreveals that, in the mean, they are lower than those of normal ellipticals, andexhibit a hint of correlation with the mass accretion rate. Simultaneously, thespectral indices in the outer regions of some galaxies, with and withoutcooling flow, attain extremely low values, suggesting that they could behosting star formation with an origin not related to the cooling flows.Comment: PostScript file (compressed and encoded) containing 21 page