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Bars are prominent components of galaxies, produced by disk instabilities, that can pump disk material above the plane generating central structures that also bulge over the thin disk (e.g. Hasan et al., 1993). As we discuss in this section, the kinematic properties of these bars are different from those observed in common bulges. The origin of some type of bulges (e.g. pseudobulges) appears to be tightly connected to secular evolutionary processes induced by bars (see Athanassoula, 2005, for a theoretical view of bulge formation in the context of bars). Bars are active agents in the inflow of gas towards the inner regions of galaxies (e.g. Sakamoto et al., 1999). This naturally allows the formation of new structures (e.g. bulges, rings, inner disks, central mass concentration).

The vertical extent of bars is best observed in edge-on galaxies. When the long axis of the bar is perpendicular to our line-of-sight bars are usually called Boxy/Peanut (BP) bulges due to their peculiar shape. Most of the material outside the disk plane has been elevated through bar buckling episodes early in the evolution of the bar (e.g. Martinez-Valpuesta et al., 2006). Kinematically, BP bulges produce a characteristic signature (i.e. a “figure-of-eight”) in the Position–Velocity Diagram (PVD). This was first predicted by Kuijken & Merrifield (1995) (see Figure 6, top row). With the aid of analytical models, they determine the location of particles in this diagram for barred and non-barred galaxies. In their view, the gap observed in the PVD of barred galaxies is produced for a lack of available orbits near the corotation radius of the bar. This effect should affect both the stellar and gas components of galaxies. This prediction was nicely confirmed with larger samples of galaxies (e.g. Merrifield & Kuijken, 1999, Bureau & Freeman, 1999). In the case of Bureau & Freeman (1999), they produced PVDs for a sample of 30 edge-on spiral galaxies with prominent BP bulges. Figure 6, bottom row, shows the observed PVD for NGC 5746 that clearly displays the predicted gap.

Figure 6

Figure 6. Position–Velocity diagrams (PVDs) of barred galaxies. (Top) Model prediction for the observed line-of-sight velocity distribution as a function of radius for non-barred and barred galaxies (Kuijken & Merrifield, 1995). (Bottom) Observed PVD for the boxy/peanut bulge of NGC 5746 (Bureau & Freeman, 1999). The kinematic signature of a bar in the observations is very evident.

Another typical kinematic feature of BP bulges predicted by numerical simulations is cylindrical rotation (e.g. Rowley, 1988, Combes et al., 1990). The first evidence for cylindrical rotation in galaxies was revealed by (Kormendy & Illingworth, 1982) for NGC 4565 when studying the stellar kinematics of galactic bulges. References of cylindrical rotation in other galaxies are rather scarce in the literature: IC 3370 (Jarvis, 1987), NGC 1055 (Shaw, 1993), NGC 3079 (Shaw et al., 1993), NGC 5266 (Varnas et al., 1987), NGC 7332 (Fisher et al., 1994). This lack of cases is likely due to: (1) inclinations effects. Cylindrical rotation is best observed in edge-on galaxies (e.g Athanassoula & Misiriotis, 2002), (2) the fact that most observations with long-slit spectrographs targeted the major and/or minor axes of the galaxies, which makes it difficult to detect. The most recent work addressing this aspect of BP bulges is that of Williams et al. (2011). This study placed long slits parallel to the major axis of five known BP bulges. The surprising result of this study is that not all BP bulges displayed cylindrical rotation. Figure 7 shows the analysis for two distinct cases in their sample. While NGC 3390 displays clear signatures of solid-body rotation, IC 4767 presents shallower major axis velocity profiles as a we move away from the disk. This outcome requires further confirmation using larger samples of edge-on galaxies. It will also benefit from studies making use of integral-field spectrographs to map the full two-dimensional kinematics over the BP dominated region. A glimpse of what this kind of studies can bring is presented in Falcón-Barroso et al. (2004) for the known case of NGC 7332.

Figure 7

Figure 7. Stellar line-of-sight rotation curves and velocity dispersion profiles for two Boxy/Peanut, edge-on galaxies in the Williams et al. (2011) sample. NGC 3390 shows clear signatures of cylindrical rotation, while IC 4767 does not (i.e. kinematics at increasing distance from the main disk shows different behaviour). The shaded regions mark the disk dominated regions.

Bars are also capable of producing other distinct features in the stellar kinematics of galaxies, which are often related to resonances induced by the bar itself in the host galaxy. Bureau & Athanassoula (2005) established, using N-body simulations, a series of kinematic diagnostics for bars of different strength and orientations in highly-inclined galaxies (see Figure 8): (1) a “double-hump” rotation curves, (2) velocity dispersion profiles with a plateau at moderate radii, and often displaying a σ-drop in the centre, (3) a positive correlation between the velocity and the h3 Gauss-Hermite moment over the length of the bar. Some of these features have been recognised observationally in several studies (e.g. Pence, 1981, Kormendy, 1983, Bettoni & Galletta, 1997, Emsellem et al., 2001, Márquez et al., 2003, Pérez et al., 2009). While having the most potential to unravel the presence of bars, the V–h3 correlation has been hardly studied observationally (e.g. Chung & Bureau, 2004). These diagnostics work best for edge-on galaxies. The kinematic tracer of BP bulges in face-on systems is the h4 Gauss-Hermite moment. Simulations carried out by Debattista et al. (2005) predict that a negative double minima around the centre of the galaxy is an excellent indicator of a BP bulge for a wide range of bar strengths and inclinations. Although the observational requirements to measure this parameter are very demanding, this feature has been nicely confirmed observationally by Méndez-Abreu et al. (2008). Interestingly, Laurikainen et al. (2014) suggest that the barlenses observed in the face-on view of many disk galaxies (e.g. Laurikainen et al., 2011) are effectively the thick part of the BP bulge when seen face-on. See also Athanassoula et al. (2014) for a theoretical interpretation.

Figure 8

Figure 8. Stellar kinematic diagnostics for barred galaxies in N-body simulations from (Bureau & Athanassoula, 2005). (Left to right) No-bar, weak-bar, intermediate-bar, and strong-bar case. (Top to bottom) image, PVD, surface brightness, and kinematic parameters (velocity, velocity dispersion, h3 and h4 Gauss-Hermite moments) as a function of bar orientation, from end-on to side-on.

There are strong indications that large bulges can have an effect in the strength of a bar. Stronger bars appear in galaxies with low bulge-to-total ratios and central velocity dispersions (Das et al., 2008, Aguerri et al., 2009, Laurikainen et al., 2009). What it is not well established yet, observationally, is the effect a bar would have on the dynamics of a pre-existing bulge. Numerical simulations by Saha & Gerhard (2013) suggest that a pressure supported bulge would gain net rotation as a result of angular momentum exchange with the bar. Rotation of the final composite classical and BP bulge would be close to cylindrical, with small deviations in the early phases of the secular evolution. Therefore, untangling the intrinsic properties of bulges in barred galaxies is a very difficult task that will require detailed dynamical modelling of high quality observations. Numerical tools like the NMAGIC code (de Lorenzi et al., 2007) applied to high-quality, integral-field data (e.g. De Lorenzi et al., 2013) seems the way forward.

The Milky Way bulge is the most vivid example of a complex system. Besides cylindrical rotation, it displays many of the other kinematic signatures of bars summarised above. The origin of the multiple substructures present at the centre of our Galaxy (possibly including other types of bulges, e.g. Ness et al. (2014)) cannot be solved by inspecting the kinematics alone, as angular momentum transfer is expected between them. Most of the efforts today to solve this puzzle come from relating the observed kinematics to the distinct stellar populations present in those regions. We refer the reader to Oscar González and Dimitri Gadotti's review in this volume for a comprehensive summary of the properties observed in the Galactic bulge, but also Juntai Shen's chapter for a theoretical view on the possible paths for its formation and evolution.

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