Stars, neutral hydrogen and ionised gas in early-type galaxies

The discovery that early-type galaxies can host large amounts of neutral hydrogen gas is relatively recent. Many questions regarding the typical properties of this gas and its role in the evolution of early-type galaxies are still unanswered. In this thesis, we attempt to answer some of these questions by investigating the relation between neutral hydrogen, stellar populations, stellar kinematics and ionised-gas content of a sample of 39 nearby early-type galaxies. We find that the global stellar age and chemical composition of early-type galaxies do not depend on their neutral-hydrogen content. However, about half of the objects with little neutral gas have younger stars in their centre than at large radius. This is never observed in galaxies with large masses of neutral gas. A possible explanation of this result is that a large fraction of early-type galaxies formed via merging of gas-rich disc galaxies. During this process the survival of neutral hydrogen may be inversely proportional to the efficiency of gas infall and therefore central star formation. We also find that the stellar kinematics does not depend on the neutral-hydrogen properties. On the contrary, ionised and neutral-hydrogen gas are strongly related to each other. A large mass of ionised gas is only found in galaxies with a large mass of neutral hydrogen, as long as the latter is distributed all the way down to the inner region. In all such systems, the kinematics of the ionised-gas is consistent with that of the neutral-hydrogen. Therefore, the two gas phases seem to belong to a same structure.