Stellar populations in nearby galaxies

Dwarf spheroidal (dSph) galaxies

Observing a collection of stars in our Galaxy as well as in neighbour galaxies offers the possiblity to recover the past evolution of the local universe, which can be considered as representative of the universe at large through the cosmological principle. The oldest stars we can observe now are kinds of fossil records of the conditions that prevailed in the early phases of the evolution of the galaxies they are members of. For instance, extremely metal poor stars retain the chemical composition of the interstellar gas from which they formed, and this gas owes its metals to very few supernovae (if not a single one) that exploded before. In this way, one can put constraints on the nucleosynthesis that occurred in these massive, first generation stars, and even on their average mass.

 

We are currently contributing to an international collaboration called DART, aiming at the systematic study of nearby dwarf spheroidal (dSph) galaxies, using spectroscopy of individual red giants in them. Measuring the abundances of various elements in the atmosphere of these stars allows to tell from which kind of supernova (SN) these elements originated when the stars formed. Spectroscopic diagnostics (with the help of photometric ones) thus give access to the star formation history of the galaxy under study. Focussing on dSph galaxies is motivated by the very small mass and hence the relative simplicity of these systems, which can be modeled more easily than a very large galaxy like ours.

 

Each chemical element has its own nucleosynthesis history and can bring some light of its own to the history of the galaxy studied. The so-called alpha elements, the nuclei of which are made of an integer number of alpha particles (like Mg, Si, Ca), are especially interesting. They are synthesized in core-collapse SNe, while iron is rather made in type Ia SNe, which explode later in the history of the galaxy. But heavier elements are also interesting. We have recently focussed on manganese, a neutron-rich element which production history is rather complex.

The elliptical galaxy Centaurus A

It is generally assumed that large elliptical galaxies were formed from the coalescence of two or more spiral galaxies, or from a few large spirals and a number of dwarf galaxies. Determining the metallicity distribution of the stars in an elliptical galaxy is one step towards understanding how such a galaxy evolved. It is very difficult, however, to study individual stars in an elliptical galaxy, because such galaxies are sparse in our vicinity and, therefore, relatively far away. Centaurus A is the most nearby large elliptical galaxy, and is well suited for this kind of study, which nevertheless necessitates very up-to-date technical facilities. Photometric data obtained from the ground in the near infrared with adaptative optics are under study, and public data taken with the HST in the visible will be analyzed as the next step.