Spectral indexes in cooling flow galaxies: evidence of star-formation

Abstract

Spectroscopic observations of central dominant cluster galaxies, with and without cooling flows, are presented. Through the analysis of absorption spectral features, namely the strength of the magnesium absorption at 5175 Å and the 4000 Å break, both in the galaxy centres and as a function of radius, we have been able to estimate the ongoing star formation induced by the large amounts of gas accreted onto cooling- flow galaxies. A correlation between the central spectral indices and the mass accretion rate is found in the sense that galaxies located in clusters with large cooling flows exhibit lower Mg_(2) and D_(4000) indices. A similar correlation with D_(4000) was previously reported by Johnstone, Fabian & Nulsen (1987). Our work, with the inclusion of the correlation in Mg_(2), adds further weight to the conclusion that these spectral anomalies are caused by recent star formation. The application of simple stellar population models reveals that the measured indices are explained if a relatively small fraction of the total mass ow (5-17%) is forming new stars with a normal initial mass function. 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. We find that spectral gradients in some cooling ow galaxies flatten in the internal regions ( r <̃ r_(e)), where emission lines are usually detected. Gradients measured in the inner galaxy regions are, in the mean, lower than those of normal ellipticals, and exhibit a hint of a correlation with M. Application of the same population models to the observed spectral gradients allows us to conclude that the ongoing star formation is concentrated towards the inner parts of the cooling ow galaxies and, therefore, the star formation does not follow the X-ray derived mass accretion profiles. Simultaneously, the spectral indices in the outer regions of some galaxies with and without cooling ow attain extremely low values, suggesting that they could be hosting star formation with an origin not related to the cooling flows.