Dynamical evolution of a bulge in an N-body model of the Milky Way
aa r X i v : . [ a s t r o - ph . GA ] O c t Dynamical evolution of a bulge in an N-body model of theMilky Way
Kanak Saha , a , Inma Martinez-Valpuesta , and Ortwin Gerhard MPE, Garching, Germany
Abstract.
The detailed dynamical structure of the bulge in the Milky Way is currentlyunder debate. Although kinematics of the bulge stars can be well reproduced by a boxy-bulge, the possible existence of a small embedded classical bulge can not be ruled out.We study the dynamical evolution of a small classical bulge in a model of the MilkyWay using a self-consistent high resolution N-body simulation. Detailed kinematics anddynamical properties of such a bulge are presented.
In the standard Λ CDM cosmology, nearly non-rotating classical bulges which are the central buildingblocks in spiral galaxies, are generally formed in dry major mergers [1]. As mergers were nearlyinescapable in the past and ∼ / ∼
8% of the disk mass) has been setby modelling the kinematics from the Bulge Radial Velocity Assay (BRAVA) data [3]. But there isevidence for a metallicity gradient above the Galactic plane [4] which is taken as an indication forthe existence of a classical bulge in our Galaxy. Hence, it is important to understand the dynamicalinteraction between a preexisting classical bulge and the bar in the Galaxy.
Fig. 1.
Left: Evolution of the change in the specific angular momentum normalized to disk angular momentum atT =
0. Right: Spectral analysis showing 2:1 resonance mainly responsible for transferring angular momentum tothe classical bulge from the bar rotating with a pattern speed Ω b . Other notations bear usual meaning. a e-mail: [email protected] PJ Web of Conferences
Fig. 2.
Surface density and velocity maps of the classical bulge alone. From left to right, panels are taken at T = In order to follow the dynamical evolution of a small, initially isotropic, non-rotating classical bulge,we construct an equilibrium model of a live disk galaxy consisting of 10 million particles using themethod of [5]. The disk density follows an exponential profile with a scale length of 4 kpc, total mass M d = . × M ⊙ and Q = . / D) is 0.067. Otherdetails of the galaxy model can be found in [6].A strong bar forms in the disk within 0.5 Gyr and is transformed into a boxy bulge via the well-known buckling instability. During the secular evolution that bar drives a substantial fraction of theenergy and angular momentum from the disk are being transferred to the surrounding dark matter haloand the preexisting classical bulge. Based on the work of Lynden-Bell & Kalnajs [7], several authorshave emphasized that resonant interaction plays a significant role in the angular momentum transfer.The left panel of Fig. 1 shows the evolution of specific angular momentum gained by the embeddedclassical bulge. Orbital spectral analysis reveals that the 2:1 resonance (right panel of Fig. 1) plays thedominant role in the transfer of angular momentum from the bar to the classical bulge [6]. As a resultof the angular momentum gain, the initially non-rotating isotropic low mass classical bulge transformsinto a highly rotating triaxial and anisotropic object.The angular momentum gained by the low mass classical bulge has a profound e ff ect on its struc-ture, kinematics and dynamics. In the upper panel of Fig. 2, we show edge-on surface density maps atfour di ff erent epochs during the secular evolution in the galaxy. The normalized fourth-order Fouriercosine coe ffi cient obtained by analyzing the density field indicates the presence of non-axisymmetricfeatures developing inside the classical bulge at around T = .
56 Gyr which also marks the bar buck-ling instability in the disk. At later phases of evolution, the inner regions of the classical bulge becomerounder and the outer parts become disky. The corresponding velocity maps on the lower panel ofFig. 2 show that as the bar evolves, the bulge spins up and shows prominent signatures of cylindricalrotation in the inner regions (at X / R d < . References
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