CFD-MHD Project Research Highlights

The structure and evolution of gas disks in galaxies

The central regions of disk galaxies contain a large amount of gas and can be approximated as either isothermal or polytropic gas disks. We apply the Antares codes to simulate the evolution of these gas disks under the influence of different external force fields. Our present focus is on the potential due to a bar and spiral potential rotating at a constant angular speed. Theories explaining the spiral structures in galaxies have been around for a few decades by now, among which the density wave theory is perhaps the best motivated and most widely accepted. The theory, however, only address the structure of galaxies and cannot answer the evolution of galaxies. With our simulations we hope to be able to lend further support to it and fill in details of the evolution of galaxies.

Fig. 1
In Fig. 1 is shown our simulation results for the 3-kpc arm in the Milky Way, which is believed to be a spiral galaxy. We conjecture that this 3-kpc arm is generated by density waves excited by a central bar potential. In our simulations we have considered two different sets of conditions, the main difference between those being the presence or absence of the self-gravity of the gas. The evolution of the resulting surface density distributions are shown from left to right. The upper panel corresponds to the case in which self-gravity is absent, while the opposite is true for the lower panel. The spiral structures for the two cases are clearly different. In particular, nearly chaotic behavior ensues only when self-gravity is considered.

Similar simulations of a gas disk driven by a fast bar in a galaxy with nearly flat roatation curve gives rise to the starburst ring at the outer Lindblad resonance (OLR) outside, a stable circumnuclear dense molecular disk in the center (ILR, inner Lindblad resonance), and a general gas depletion region in between. All these are commonly seen in the nearby galaxies. In addition, a diamond shape feature is formed, resulting from the interaction between waves excited at OLR and ILR. A comparison with NGC6782 is shown in Figure 2.

Fig. 2

In the major-bar galaxies NGC1097 and NGC 1300 (Figure 3), there are a pair of very straight dust lanes in the major bar which end up to form a starburst ring in the galactic center. Similar morphology is observed in many not so overwhelming bar galaxies as well. We find that this type of structure can be reproduced in our numerical simulations. Preliminary results are shown in Fig. 4

Fig. 3 Two bar-spiral galaxies: NGC1097(left) and NGC1300 (right)		Fig. 4