7. CONCLUDING REMARKS AND FUTURE PROSPECTS

I present here a review of our current understanding of the intrinsic 3D shape of galaxy bulges. The approach taken in this review is largely observational and follows the historical development of the field. Thus, a journey through the past and present of our knowledge on the intrinsic shape of other galaxy ellipsoids such as elliptical galaxies or galaxy discs was needed to put the problem in context. The major conclusions of this review are:

• The observational data representing the whole population of elliptical galaxies is consistent with a mixed model, combining partly oblate and partly prolate galaxies, although a more likely alternative point towards at least some fraction of the ellipticals being triaxial ellipsoids. Triaxiality is also supported by several photometric and kinematics properties, as well as for detailed modelling of individual galaxies.
• The intrinsic shape of ellipticals shows a dependence on galaxy luminosity. Bright ellipticals are in general triaxial with a tendency to be rounder whereas faint ellipticals are more flattened with a tendency to be oblate ellipsoids.
• Even if uncertainties due to the lack of number statistics have been overcome with the advent of recent surveys, the data can still be reproduced by a wide variety of intrinsic shape distributions. Furthermore, a proper interpretation of the data is complicated by the fact that the AARD and kinematic misalignments are often a function of the radius. Therefore it is generally impossible to characterize the full shape of a single elliptical galaxy with only one or two parameters.
• Galaxy discs are, in general, well represented by nearly oblate models with Q ∼ 0.9. Their intrinsic flattening is also well constrained to values spanning 0.2 < F < 0.3.
• The population of galaxy bulges can be modelled as slightly triaxial ellipsoids with a tendency to be oblate. This population has typical intrinsic flattenings of F ∼ 0.65. However, individual galaxies can have a variety of intrinsic flattenings with some extreme cases sticking out the plane of the disc, these are called polar bulges.
• The distribution of the triaxiality parameter of galaxy bulges is strongly bimodal. This bimodality is driven by bulges with Sérsic index n > 2. According to numerical simulations they can be explained assuming a combination of major dissipational and dissipationless mergers during their formation.
• Despite previous findings showing a triaxial bulge in the Milky Way, more recent studies have found that is more likely a boxy bulge produced by the vertical instabilities of the Galactic bar. Owing to recent kinematic measurements a classical bulge with mass > 15% of the disc mass can be ruled out.

Despite the study of the intrinsic shape of elliptical galaxies has a long track record, our knowledge of the 3D shape of bulges is still in its infancy. Therefore, further work on the topic is needed to fully exploit its possibilities. A few guidelines to this future prospects are outlined in the following:

• From a photometric point of view, even if new methodologies have been developed they need to be applied to larger samples of galaxy bulges. The number of elliptical galaxies recently analysed to recover their intrinsic shape is several orders of magnitude larger than the current samples of galaxy bulges. Large number statistics have led to the discovery of important relations for ellipticals galaxies, such as the different shapes of bright and faint ellipticals, and similar studies can be crucial for galaxy bulges. This is particularly relevant in the current picture of bulge formation with a different population of classical and pseudobulges dependent of the galaxy mass (Fisher & Drory, 2011).
• An even more promising path, already explored in elliptical galaxies, is the use of combined information from photometric and kinematic data. In particular, the common use of integral field spectroscopy is now providing an exquisite detail of the stellar and gaseous kinematics on large sample of galaxies. This wealth of information together with the development of galaxy dynamical modelling can provide a proper understanding of the intrinsic shape of galaxy bulges.
• It is doubtless that the comparison of the derived intrinsic shape of bulges with the state-of-the-art numerical simulations is a promising way to gain insights on the formation and evolution of bulges. However, there is still a lack of simulations with a large variety of initial and physical conditions interested on a structural analysis of the different galaxy components, and in particular, in the intrinsic shape evolution of galaxy bulges.
• Historically, galaxy bulges were thought as single-component objects at the centre of galaxies. This picture is now questioned since different bulge types with different formation paths have been found coexisting within the same galaxy (see Méndez-Abreu et al., 2014, and references therein). A proper separation of different bulges types, as well as the identification of possible unresolved nuclear structures such as bars, rings, etc, must be accounted for to improve our knowledge on bulge formation and evolution.
• The study of the intrinsic shape of elliptical galaxies at high redshift has recently suffered a boost thanks to the arrival of high spatial resolution surveys on large fields of view (see Chang et al., 2013, and references therein). This kind of studies can provide an in-situ view of galaxy evolution and their application to the intrinsic shape of bulges will be key to further progress on this topic.

Acknowledgements I would like to thank the editors E. Laurikainen, R.F. Peletier, and D. Gadotti for their invitation to take part in this volume. I would also like to thank A. de Lorenzo-Cáceres and J. Argyle for a careful reading of this manuscript. JMA acknowledges support from the European Research Council Starting Grant (SEDmorph; P.I. V. Wild).