The intrinsic shape of bulges keeps important information about their formation history, with different merger, accretion and assembly scenarios resulting in different shapes. Hence, the comparison of measured intrinsic shapes with the output from numerical simulations represents an intrinsic way to gain insights on their formation. However, numerical resolution problems have often hampered these studies and our interpretation of the shapes of bulges is usually restricted to the analysis of simulated elliptical galaxies.
Cox et al. (2006) studied the structure of ellipsoidal remnants formed by either major (equal-mass) dissipationless or dissipational mergers of disc galaxies. They found a bimodal distribution of the triaxiality parameter in their remnant ellipticals (see right panel in Figure 4). Thus, dissipationless remnants are triaxial with a tendency to be more prolate and with a mean triaxiality parameter T = 0.55, whereas dissipational remnants are triaxial and tend to be much closer to oblate with triaxiality T = 0.28. This simulated bimodal distribution was compared by Méndez-Abreu et al. (2010) to the triaxiality measured in their sample of 115 galaxy bulges (Figure 4). They concluded that both major dissipational and dissipationless mergers are required to explain the variety of shapes found for bulges. The detailed study presented by Cox et al. (2006) is consistent with previous studies of dissipationless and dissipational mergers (e.g., Barnes, 1992, Hernquist, 1992, Springel, 2000). However, the study of González-García & Balcells (2005) they found how the degree of triaxiality of the elliptical remnants in dissipationless mergers also depends on the morphology of the progenitor spirals. The presence of central bulges on the progenitor galaxies produce remnants which tend to be more oblate whereas bulgeless progenitors lead to highly triaxial remnants which seems inconsistent with observations. Therefore, the comparison between simulations and observations are still subject to the range of initial conditions explored by numerical simulations.
On the other hand, even if the similarities between bulges and ellipticals have prompted observers to compare the measured properties of bulges to the properties of simulated elliptical galaxies, the formation path of bulges is likely a more complex process involving the interaction with other galaxy structural components (Kormendy & Kennicutt, 2004, Athanassoula, 2005). The recent work by Tapia et al. (2014) has started to fill the gap on studies about the intrinsic shape of galaxy bulges from numerical simulations. They analysed a set of N−body simulations of intermediate and minor dry mergers onto S0s to understand the structural and kinematic evolution induced by the encounters. In their experiments, the progenitor bulges are nearly spherical. The remnant bulges remain spherical as well (Q ∼ F > 0.9), but exhibiting a wide range of triaxialities (0.20 < T < 1.00), remarking how the definition of this shape parameter is too sensitive to nearly spherical systems. Figure 7 (second panel) shows how the axis ratios derived from these simulations (open stars) are hardly reconcilable with the observations (black diamonds) by Méndez-Abreu et al. (2010). Still, the strong triaxiality agrees with the structure of elliptical remnants resulting from major-to-intermediate mergers (Cox et al., 2006).
Figure 7. Intrinsic shape of bulges and elliptical galaxies obtained from numerical simulations. A comparison with observed bulges is shown in the second panel. The blue and green stars in all panels represent the bulge remnants after suffering intermediate/minor mergers. The location of the progenitor bulges is shown with orange stars. The elliptical remnants of major mergers with pure exponential stellar discs (black circles) and containing 40% of gas (red circles) are also shown. First panel: intrinsic ellipticity b (Q in this chapter) versus the intrinsic flattening c (F in this chapter) Second panel: as panel 1 but adding the observed distribution of bulges in Méndez-Abreu et al. (2010) (black diamonds). Third and fourth panels: triaxiality parameter as a function of the intrinsic ellipticity and flattening. Extracted from Tapia et al. (2014). Reproduced with permission from Astronomy & Astrophysics, ⓒ ESO.