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
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
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
- 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
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
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
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).