Simulations for the nearby Seyfert 2 galaxy NGC 4945 Lien-Hsuan Lin 1,2, Chi Yuan 2, C.C. D. Yen 3, and S. Muller 2 1 Department of Physics, National Taiwan University 2 Institute of Astronomy and Astrophysics, Academia Sinica 3 Department of Mathematics, Fu-Jen Catholic University Abstract NGC 4945 is a nearby disk galaxy (type SB(s)cd or SAB(s)cd) at a distance of 3.82 Mpc (Karachentsev et al. 2006), the central region of which contains an active galactic nucleus classified as Seyfert 2. Its relative proximity has permitted a detailed SMA study of the circumnuclear molecular gas in a galaxy exhibiting an AGN. Based on an analysis of the high-resolution velocity field of the central region (30”×30”, 10” = 190 pc) provided by Dr. Muller, we demonstrated that the S-shape structure of the isovelocity contours is well represented by the bending produced by a shock along the spiral density waves. In particular, using the observed rotation curve and a bar potential model in our simulations, a very good fit of the observed velocity field can be obtained. The density waves excited at the outer inner Lindblad resonance (OILR) propagate inward into the very center, causing the gas to lose their angular momentum and fall into the center to fuel the AGN. The simulated density map shows a pair of tightly wound spirals in the center which pass through most of the ring-like (claimed to be a circumnuclear starburst ring in previous papers) high intensity region in the observations. In addition, the extended spirals produced in the simulations intersect strong emission line knots located outside the ring-like region. Further observations are suggested in order to confirm our simulation results. Observations Fig. 1 is the JHK image of NGC 4945 from the 2MASS large Galaxy Atlas (Jarrett et al. 2003). Its size is 19.5’X19.5’. Fig. 2 is a high-resolution velocity field of the central region (30”X30”) of the galaxy. This is observed by S. Muller using SMA. In the central 20”X20” region, the isovelocity contours show several sets of curved lines, which indicate the locations of spiral density waves. Fig. 3 is the continuum-subtracted Paα image (Marconi et al. 2000). They claim the presence of several strong emission line knots along the galactic plane, very likely resulting from a circumnuclear ring of star formation. These images have north at top, east to left. The rotation curve derived from the velocity field of Fig. 3 is represented by the dots in Fig. 4. Computational Setup The initial conditions are a uniform surface density distribution and a circular motion corresponding to the Elmegreen rotation curve (smooth line in Fig. 4) which is fit from the observed rotation curve. The corresponding angular speeds are shown in Fig. 5. The horizontal line represents the pattern speed of the bar, Ω p, which is 78 km/s/kpc here. The intersection of Ω p with the Ω–κ/2 curve determines the location of the outer inner Lindblad resonance (OILR) to be 0.4 kpc. Simulations vs Observations Fig. 6 and Fig. 7 show respectively the isovelocity contours and the projected density distribution of the simulation result. Superimposition of Fig. 6 onto Fig. 2 is shown in Fig. 8. The S-shape around the galactic center in both observation and simulation match each other very well. The results of superimposing the density distribution (Fig. 7) onto both the simulated and observed velocity fields (Fig. 6 and Fig. 2) are shown in Fig. 10 and Fig. 11. We can see clearly that the velocity contours bend outwards along the spiral arms in both figures. This is one of the distinct features for the density waves excited at the ILR. Across the spirals, matter collides with the arm from the concave (or inner) side and is deflected by the shock, resulting in mass inflow to fuel the AGN. Fig. 9 is the superposition of Fig. 3 and Fig. 7. It shows that there are a pair of tightly wound spirals in the central region other than a circumnuclear starburst ring. It is particularly notable that the spirals in our simulation not only pass through most of the strong emission line knots surrounding the center but also intersect with two strong emission line knots located outside the ring-like region. Remarks Further observations are required in order to confirm our simulation results. Moreover, estimation of the amount of the gas from observation is needed to calculate the rate of mass inflow for fueling the AGN. Fig. 8Fig. 9 Fig. 7 Fig. 6 Fig. 10Fig. 11 Fig. 4Fig. 5 Ω Ω +κ/ 2 Ω –κ/ 2 (km/s/kpc) (kpc) Fig. 1Fig. 2Fig. 3