Spiral Structure and Dynamics of NGC 5248

1. Spiral Structure and Dynamics of NGC 5248 D.C.C. Yena, C.C. Yangb and C. Yuan a,b a Institute of Astronomy & Astrophysics, Academia Sinica, Taipei, Taiwan, R.O.C b Physics Department, National Taiwan University, Taipei, Taiwan, R.O.C. Abstract :Recent high resolution radio (OVRO) and optical (HST) observations have revealed the inner most spiral structure of spiral-bar galaxy NGC5248 (Jogee et al 2002a, ApJ, 570,L55 ; Jogee et al 2002b, ApJ, 575, 156). The spiral structure of that galaxy has been interpreted as waves driven by its major bar at the outer inner Lindblad resonance (OILR) with galactic-centric distance equal to 2.0 kpc in the 2002 paper quoted above. Associated with the OILR, there is an inner inner Lindblad resonance (IILR) at .375 kpc.In another case, a second OILR at .2 kpc is invoked. We use both the analytic method and numerical simulations to study the dynamics of the same galaxy. Although our result supports the general conclusion of Jogee et al’s, i.e., the spiral structure is excited by the major bar of the system, there are significant differences between the two studies. The rotation rate of the bar is lower in our case, such that the OILR is located at 3.0 kpc and unlike theirs. The role of the IILR is not effective. We also find that the spiral pattern fits best with the observations if the the self-gravitation of the gas disk initially is not important. It becomes locally important when the gas mass accumulates toward the center as the disk evolves. We believe some of the difference result from the way that the boundary conditions are treated. The work is in part supported by a grant from NSC (NSC-90-2112-M-001-018) The Rotation Curve of NGC5248 Introduction :NGC5248 is a nearby grand-design galaxy, classified as SAB(rs)bc, with a bright nucleus. The spiral arms can be traced from 7 kpc all the way almost to the center, with a double circumnuclear rings, which are suspected to power the nuclear activities (Perez-Ramirez et al 2000). Recent high resolution radio (OVRO) observations reveal the inner most spiral structure and kinematics of the molecular gas disk (Jogee et al 2002a). They have further analyzed the results by numerical simulations and concluded that a large scale oval distortion or bar of the system is responsible for overall spiral structure including the CO spirals at the center (Jogee et al 2002b). In this paper, we report our study of this galaxy, using both non-linear asymptotic theory and numerical simulations. Although we support the idea that a large-scale bar in the system can drive these observed spirals, there are significant differences between our results and theirs. Non-linear Asymptotic Theory : Based on the theory developed by Yuan and Kuo (1997), we analyze the observation data and search through the parameter space of the rotation speed of the bar (or OILR), self-gravitation, viscosity, strength of the bar field and effective sound speed, for a solution which can best match the observations. For observations, we also use the wavelet method to analyze the HST data and obtain a better view of the inner spirals within 10 arcsec, for comparison with our theory. The best results correspond to OILR at 3.0 kpc, bar field equal to about 5% of the mean field at OILR, viscosity equal to 0.3 km kpc/s, and sound speed put at 10 km/s. The calculated pattern agrees well with the observed spirals from 5 kpc to 100 pc. We found the self-gravitation of the gas disk and the IILR do not play an important role there, contrary to Jogee et al's results. Left is our theory (middle column) compared with the observations (left column), with the two superposed (right column). Below is the isovelocity contours from observation and our theory. Numerical dynamical simulations : We use the high order Godunov code in the polar coordinates, which we develop to study the evolution of gas disks driven by a rotating bar potential, to study the structure and evolution of the disk of NGC5248. The evolution of our numerical experiment demonstrates that the excitations of both leading and trailing waves are almost simultaneous. Unfortunately, the leading wave does not exist permanently. The interaction of both waves shows that the leading wave will prevent trailing wave from going into the center and forming a spiral-ring structure. Our numerical results differ from Jogee's and also differ somewhat from our analytical results. It demonstrates the uncertainty of the CFD computation as far as the details are concerned. There are too many possible approximations one can choose in the computation.The evolution of our numerical simulation is shown in the figures below from left to right. References : Jogee et al 2002a, ApJ, 570, L55 Jogee et al 2002b, ApJ, 575, 156 D. Perez-Ramirez et al 2000, MNRAS, 317, 234 Yuan, C. 1984, ApJ, 281, 600 Yuan, C. & Cheng, Y. 1989, ApJ, 340, 216 Yuan, C. & Kuo, C.L., 1997, ApJ, 486, 750 Conclusion : We find that the spiral structure of NGC5248 can be explained by a resonance excitation mechanism. The spirals are density waves excited by a slow rotating bar potential of the system at the OILR, which is located at 3.0 kpc. We first use the analytic approach to obtain our results and then numerical simulations to confirm our calculations. The results support the general conclusion of Jogee et al (2002b), but there exist significant differences in detail. We believe the differences probably come from the way the boundary conditions are treated and the self-gravitation near the center is included between the two studies. Other factors, such as the role of IILR and the bar forcing may also contribute to the differences.