What is the origin of the bulge of the Milky Way? The answer to this question is a crucial step towards identifying the history of events that took place during the formation and evolution of the Galaxy as a whole. As a matter of fact, the answer to this question has changed dramatically, since the early times of Galactic bulge archaeology until today, between the two main ideas behind bulge formation: the merger-driven bulge scenario, where a bulge is formed violently and quickly during the early stages of the Galaxy dominated by the gravitational collapse or hierarchical merging of sub-clumps of dark matter carrying baryons and gas (e.g. Abadi et al 2003, Elmegreen 1999), and the secular evolution scenario where the bulge structure is naturally born from the dynamical evolution of the stellar Galactic disc (Combes and Sanders 1981, Raha et al 1991, Norman et al 1996, Athanassoula 2005). Until this day, we cannot say with absolute certainty which one of these scenarios, or if perhaps both, played a major role during the formation of the bulge of our Galaxy 1.
The reason for this long term debate might lie, ironically, in our greatest strength. The Galactic bulge allows us to investigate its properties by taking advantage of the fully resolved stellar populations - a unique strength that can be understood as the ability to see its properties in a unique level of detail with respect to what we can learn from the observation of external bulges. However, such an advantage also means that nearly 500 sq. deg. of sky must be homogeneously covered in order to obtain the most global picture of the Bulge. To obtain a general characterisation of these properties is a crucial step in order to answer the question of the origin of our Galactic bulge.
As a consequence, observational efforts during the last decade have been focused on solving this limitation. As a result, our knowledge regarding the global properties of the Galactic bulge has increased considerably thanks to the advent of dedicated spectroscopic and photometric surveys. We are currently witnessing a revolution in the field of Milky Way bulge research that will also find its place within the bulges of other disc galaxies. It is thus a moment in which the communities of Galactic and Extragalactic research are approaching each other. From this, one cannot expect anything but a fruitful exchange of ideas that will certainly push both fields forward. However, it is not straightforward for members of each community to study the other field, for at least two reasons. The first obstacle is the vast amount of work, rich in details, that one has to become familiarised with. The second obstacle is the jargon employed independently by each group which hampers understanding. This review is a modest first attempt to overcome these obstacles.
1 To be complete, we must mention the pioneering work of Eggen et al (1962), who suggested that the first stage in the formation of the Milky Way was a fast (< 108 yr) monolithic collapse of a single massive gas cloud, which could have formed both the Galaxy stellar halo and the bulge. This scenario was later replaced with the merger-driven scenario mostly due to the widely varying ages of different components of the Galaxy and the hierarchical nature of LCDM theory. We refer the reader to Brooks & Christensen (this volume) for a review on merger-driven bulge formation. In the last decade or two, the secular evolution scenario has slowly, but unequivocally, gained terrain over the merger-driven scenario. Another bulge building scenario we do not discuss here has recently been put forward by e.g. Elmegreen et al. (2008, see review by Gadotti 2012). In this scenario, bulges form by the coalescence of giant clumps in primordial discs. This scenario can explain the formation of spheroids but does not account for box/peanuts. Back.