An individual team, including an astronomer of Observatoire de Paris, has recently observed for the first time individual stars in a very dense -- but very interesting -- zone of an external galaxy, enabling for the first time an eagerly awaited comparison with the corresponding zone (bulge) of our Galaxy (Milky Way).

Hubble observations. Photo: Observatoire de Paris

Astronomers are always anxious to push their observations to the limit. With the advent of the refurbished Hubble Space Telescope (HST), it has become possible to study individual stars at the distance of the Andromeda galaxy (Messier 31). Resolving stars at still greater distance will require the next generation of ground or space based telescopes.

A team of astronomers (including Pascale Jablonka, from Paris-Meudon Observatory and UMR 8631 of CNRS) took this opportunity to study with the Hubble Space Telescope at visible (WFPC2) and infrared (NICMOS) wavelengths, metal-rich stellar populations in globular clusters and field stars close to or in the bulge of M31. They obtained images of the central part (bulge) of M 31 in two colors, and have analyzed these images, which include two globular clusters. These analyses pioneered the field of studies in extremely crowded environments, a difficult observation. They give the first precise picture of a galaxy bulge besides the one of our own Galaxy.  With these images, they are able to:

• Derive estimates for metallicities and ages for stellar clusters close to the bulge.
• Compare the properties of stars in cluster with those of the stars   surrounding the cluster (field stars).
• Deduce some features of the spiral galaxy formation scenario of the   clusters and of their host galaxy (M31 is a spiral galaxy similar to our Galaxy).

Composite image. Photo: Observatoire de Paris

The figure above shows the visible (V) and infrared (I) images of two bright globular clusters, G170 and G177, projected on the bulge of M31. The field of view of WFPC2 and NICMOS cameras enable to see a number of bulge stars around the clusters. By comparing the images obtained in two diferent colors V and I, it is possible to build a color-magnitude diagram (an example is shown), and several conclusions can be deduced from the shape of this diagram. An example of the colour-magnitude diagrams derived for the field stars is presented. The result of a new image deconvolution technique on G170 is also shown.

Several conclusions could be made, in particular that the globular clusters seen from the ground in projection on M31 bulge are genuine bulge clusters which formed from the same gaseous material as the bulge field stars. No intermediate age population has been detected. The clusters' very high chemical enrichment and old age indicate that they were formed, as was the bulge, in the very early stages of the galaxy history. The bulge field stellar population presents a large dispersion in abundances and an apparent sharp cut at low metallicity, very similar to what is seen in the bulge of our own Galaxy.