Next Contents Previous


In this review we have presented a summary of current progress towards characterising the properties of the Milky Way bulge. In recent years, the spectroscopic and photometric surveys of the Bulge have provided us with the necessary tools to build a bridge connecting the detailed stellar population properties with a global view of the Galactic bulge. As a consequence, it is now becoming possible to discuss the Bulge properties as seen from an extragalactic perspective. Such a comparison, powered by the increasing number of models to which observations can now be directly compared, is the only way in which we can set the history of events that led to the properties of the Bulge we see today.

The era of surveys looking towards the Galactic bulge was born not only from our intrinsic desire to explore, but also as a response to the increasing complexity in Bulge properties revealed with previous individual observations. The need to further map its morphology, to better constrain the spatial variations of its stellar populations and subsequently connect it with their kinematics required the larger spatial coverage offered by such large-scale observations.

While the dominant B/P nature of the Milky Way bulge has now been well established, it remains to be understood if the observed stellar population properties relate solely to the same structure or if they each have a different origin. Another way to phrase this would be to ask the following question: do the metal-poor, α-element enhanced, old bulge stars belong to a different structure than the B/P, which was formed somehow independently to the buckling instability process of the bar (i.e. as a classical bulge)? Currently, while we have important evidence from the connections between kinematics and metallicities of Bulge stars as well as their spatial distribution, we can only suspect about the presence of different components. However, creating the link between these components and the specific formation scenarios should be done with extreme caution, as a number of processes could have played a specific role at different stages of the assembly of the Galaxy. For example, the coalescence of disc clumps and the accretion of gas could have formed a thin disc, a thick disc, and even a spheroid in the centre during the early stages of formation of the Milky Way. The merging history could have also contributed to this assembly, which will depend on gas content, mass ratio and orbital parameters of the mergers, until the formation of a bar and the onset of the buckling instability took place to shape the dominant central component we see today – the B/P Bulge. Only by carrying out an extensive comparison of all the observational properties of Bulge stars with models and external galaxies, can we constrain the importance of all these events during the formation history of the Bulge.

Acknowledgements We are grateful to an anonymous referee for many useful comments. We warmly thank Istvan Dekany for kindly providing us the table of individual distances to the RR-lyrae that we used to produce Figure 1.

Next Contents Previous