Free Access
Volume 567, July 2014
Article Number A122
Number of page(s) 21
Section Galactic structure, stellar clusters and populations
Published online 24 July 2014

Online material

Appendix A: Exploring different galaxy models

The galaxy simulations presented in Sects. 2 and 3 develop a bar whose length (rbar = 7 kpc) is about half of the initial disk size (13 kpc). In these simulations, the corotation radius and the OLR are located at 7 − 8 kpc, and 13 kpc, respectively. Since the initial disk does not extend further, it is natural to investigate what is the response and redistribution of stars to bar formation and evolution that are initially at even larger radii than in our original simulations. In this Appendix, we have analyzed two additional simulations which are part of the sample of simulations described in Hallé & Combes (2013). These simulations include gas physics, implementing a cooling prescription which allows the gas to cool low temperatures and the formation of a molecular hydrogen component, together with star formation and several different stellar feedback efficiencies (see Hallé & Combes 2013, for details). For the purpose of the comparison to the models presented in this paper, we have chosen two simulations from this suite without molecular hydrogen and alternatively, with and without including stellar feedback. We have chosen these two simulations because they develop stellar bars with different characteristics, thus allowing us to investigate the impact of those characteristics, and specifically, the impact of the location of the associated resonances, on the overall disk evolution. We summarize the main features of the simulated galaxies for these two simulations from Hallé & Combes (2013): they consist of a gSb-like galaxy, composed of a stellar disk whose mass is Md = 4.5 × 1010M, and whose scale length is ad = 5 kpc; a classical bulge whose mass is 0.25 Md and characteristic radius ab = 1 kpc; and a gaseous disk whose mass is 0.2 Md, and whose scale length is 11.8 kpc. The gaseous and stellar disk components follow a Miyamoto-Nagai density profile while the stellar bulge and dark halo components have Plummer density profiles (see Hallé & Combes 2013, for a complete description of the models). In particular, in these models the initial stellar disk extends to 36 kpc from the galaxy center, thus allowing us to trace

the evolution of stars up to distances of ~8 disk scale lengths. The total number of particles employed is 1 200 000, equally distributed in number as gas, stars and dark matter particles.

Both models develop prominent stellar bars, whose length is about 7 kpc, at 1 Gyr after the start of the simulation. At about t = 5 Gyr, the corotation and OLR are located at r = 10 kpc and r = 17 kpc respectively for the simulation with stellar feedback (Fig. A.1), while they are located slightly further out in the simulation without stellar feedback (at r = 12 kpc and r = 20 kpc; Fig. A.2). This difference is a result of the fact that the simulation without stellar feedback has a lower bar pattern speed than the simulation with stellar feedback. As was the case in our original simulations (Sect. 3), the formation of the bar is accompanied by a significant redistribution of the stars in the disk, with stars initially as far as 1520 kpc, reaching the inner regions in less than 2 Gyr. These outside-in migrators become part of the stellar bar, as is evident in their asymmetric distribution in the inner 510 kpc which tracks the orientation of the bar. However, it is also interesting to note that the outermost regions of the simulated disks, those at distances greater than 15 kpc (Fig. A.1) and 20 kpc (Fig. A.2) in the two simulated galaxies, stay mostly confined to the outer disk, with only less than 1/1000 of those stars reaching the bar region, after 9 Gyr in the simulations. The transition radius where the different responses of the disk to the spatial redistribution seems to occur is approximately the radius of the OLR. In particular, comparing Figs. A.1 and A.2, one see that the greater the radius of the OLR, the larger the disk region involved in contributing stars to the outside-in migration. As a corollary to this, it also means that the regions where stars do not migrate inward also occurs beyond a larger radius. While the study of this behavior will be the subject of subsequent studies, the point we wish to emphasize here is that in the MW all kinematically cold populations inside the solar circle, where the MW bar OLR is probably located (see, for example, Gerhard 2010), may have contributed to make up the Galactic boxy/peanut-shaped bulge.

thumbnail Fig. A.1

From top to bottom: face-on density distribution of stars with different birth radii: rini 5 kpc; 5 kpc rini 10 kpc; 10 kpc rini 15 kpc; 15 kpc rini 20 kpc; 25 kpc rini. Different columns correspond to different times, as indicated. The total stellar density distribution is given in the top row, at different times.

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thumbnail Fig. A.2

Same as Fig. A.1, but for the model without stellar feedback.

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© ESO, 2014

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