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 inner regions of cooling-flow galaxies. The application of simple stellar population models reveals that the measured indices are explained if a relatively small fraction 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 large fraction of the gas accreted inside the galaxy could be forming stars. In addition, the analysis of line-strength gradients in the inner galaxy regions reveals that, in the mean, they are lower than those of normal ellipticals, and exhibit a hint of correlation with the mass accretion rate. Simultaneously, the spectral indices in the outer regions of some galaxies, with and without cooling flow, attain extremely low values, suggesting that they could be hosting star formation with an origin not related to the cooling flows.