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 lambda 5175 Angstrom and the lambda 4000-Angstrom 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 on to cooling now 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. 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 now (5-17 per cent) 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 now galaxies flatten in the internal regions (r less than or similar to 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 now 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 flow attain extremely low values, suggesting that they could be hosting star formation with an origin that is not related to the cooling flows.