As a prediction of the Λ cold dark matter (ΛCDM) cosmological model (Springel, Frenk & White 2006), hierarchical clustering (White 1997; Steinmetz 2001) and major mergers (Toomre 1977) build elliptical galaxies and classical (elliptical-like) bulges of disc galaxies on a short time-scale through the complete and violent collapse of the protogalactic clouds. In this schema, mergers get less common through the expansion of the Universe and consequently the evolution of galaxies, which are getting now more and more isolated, gradually changing to a more secularly driven one under the influence of internal rather than external actors (Toomre 1977; Le Fèvre et al. 2000; Conselice, Chapman & Windhorst 2003; Kormendy et al. 2010; Athanassoula, Machado & Rodionov 2013; Knapen 2013). This slow evolution of galaxies has been introduced as ‘secular evolution’ (Kormendy & Illingworth 1982; Kormendy & Kennicutt 2004; Kormendy 2008). However, mergers and accretion must also occur along the life of galaxies, even in low-density environments, and such external processes are known to push disc galaxies along the Hubble sequence towards Sa/S0 (Aguerri, Balcells & Peletier 2001; Eliche-Moral et al. 2006, 2011). Much observational and numerical work has been devoted to discriminating between internal and external evolutionary mechanisms (for a review see Kormendy 2013). Bars in disc galaxies are considered as key drivers of the secular evolution by redistribution of the angular momentum, triggering of star formation, and the morphological transformation of galaxies. During the secular evolution phase, bars can significantly alter the structure and kinematics of disc galaxies and develop a complex rotation pattern within the central parts of galaxies, called ‘cylindrical rotation’ (Kormendy & Illingworth 1982). This feature is defined as almost constant stellar rotation speed within the bulge of a galaxy in the direction perpendicular to the disc plane, such that δ$v$/δ|$z$| ∼ 0, where $v$ is the line-of-sight velocity and $z$ is the distance from the disc plane in edge-on projection. These feature has been confirmed in several observational studies of nearby barred galaxies observed in the edge-on view (e.g. Pence 1981; Kormendy 1983; Bettoni & Galletta 1997; Emsellem et al. 2001; Márquez et al. 2003; Pérez, Sánchez-Blázquez & Zurita 2009; Williams et al. 2011; Molaeinezhad et al. 2016, 2017), including our Milky Way (see Barbuy, Chiappini & Gerhard 2018, and references therein). N-body simulations of barred galaxies confirm the link between the presence of a bar and a tendency to rotate on cylinders for stars within the bar, as a natural consequence of the bar buckling processes (see Combes & Sanders 1981; Athanassoula 2003, 2005; Martinez-Valpuesta & Shlosman 2004; Bureau & Athanassoula 2005). In this scenario, after the bar develops, it experiences a period of buckling instability that thickens it in the axial direction (e.g. Combes & Sanders 1981), giving birth to a boxy or peanut-shape bulgy structure in the central parts of the bar, termed boxy/peanut bulge (B/P, hereafter). From the galactic-dynamics point of view, bar formation makes the orbits in the bar more eccentric and more boxy and aligns their principal axes without greatly affecting the motion perpendicular to the plane; consequently, the cylindrical rotation follows naturally (Bureau & Freeman 1997). As demonstrated in numerous N-body simulation studies of barred galaxies, the resulting velocity field due to this orbital reconfiguration is compatible with the observed cylindrical rotation in B/P bulges (see Molaeinezhad et al. 2016, and references therein).
Citation: Eliche-Moral et al.; 2011
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