Spin-up of classical bulges by bars

Secular evolution and cylindrical rotation in Boxy/Peanut bulges: impact of initially rotating classical bulges

<div style="text-align: justify;">Snapshot of the edge-on surface density (above) and velocity map (below) for the one of the simulated galaxies. Boxy/peanut (BP) bulged typically display cylindrical rotation. However, this model, containing an initial classical bulge, displays departures from cylindrical rotation beyond ≈ 0.5R<sub>d</sub>.</div> Zoom Image
Snapshot of the edge-on surface density (above) and velocity map (below) for the one of the simulated galaxies. Boxy/peanut (BP) bulged typically display cylindrical rotation. However, this model, containing an initial classical bulge, displays departures from cylindrical rotation beyond ≈ 0.5Rd.
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Kanak Saha & Ortwin Gerhard, 2013, MNRAS, 430, 2039

We have explored the consequences of our previous work where we showed that a initial classical bulge (ICB) may be spun-up and "hidden" within a barred galaxies similar to the Milky Way. In this work we investigated the impact of an rotating ICB on the formation and secular evolution of the bar. We use a series of models containing both a spinning classical bulge, and a disk, which from a Boxy/peanut (BP) bulge. In all the models we show that a strong bar forms and transfers angular momentum to the ICB. However, rotation in the ICB limits the emission of the bar's angular momentum, which in turn changes the size and growth of the bar, and of the BP bulge formed from the disc. We also find that deviations from the cylindrical rotation, which is typically characteristic of BP bulges, during the early phases of secular evolution. These may correspond to similar deviations observed in some bulges. We provide a simple criterion to quantify deviations from pure cylindrical rotation, apply it to all our model bulges, and also illustrate its use for two galaxies: NGC 7332 and NGC 4570.

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Spin-up of low mass classical bulges in barred galaxies

<div style="text-align: justify;">Edge-on surface density and velocity maps for the bulge particles alone at four different epochs during the secular evolution. Initially the bulge is non-rotating and flattened by the disc potential later on the classical bulge aquires the characteristic cylindrical rotation.</div> Zoom Image
Edge-on surface density and velocity maps for the bulge particles alone at four different epochs during the secular evolution. Initially the bulge is non-rotating and flattened by the disc potential later on the classical bulge aquires the characteristic cylindrical rotation.
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Kanak Saha, Inma Martinez-Valpuesta & Ortwin Gerhard, 2012, MNRAS, 421, 333

The secular processes driven by the bar may cause dramatic changes in the dynamical structure of a preexisting low-mass classical bulge, such as might be present in galaxies like the Milky Way. Such a bulge absorbs angular momentum emitted by the bar, mostly through resonances, particularly Lagrange point (-1:1) and ILR (2:1) orbits, but also retrograde non-resonant orbits absorb angular momentum while the bar  grows rapidly. Thus an initially non-rotating low-mass classical bulge transforms into a fast rotating, radially anisotropic and triaxial object, embedded in the similarly fast rotating boxy bulge formed from the disk. Towards the  end of the evolution, the classical bulge develops cylindrical rotation.


Full paper on ADS

 
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