TWO DECOUPLED BARS IN THE SPIRAL GALAXY NGC 3359

MARÍA SEMPEREInstituto de Estructura de la Materia (CSIC) – C. Serrano 121 – 28006 Madrid, Spain

Abstract. We present self-consistent numerical simulations of the molecular gas in the spiral galaxyNGC 3359. Using the real potential derived from an I-band image, we have tried to find the dynamicalparameters able to account for the observed gas kinematics and morphology (in both Hα and HI). Thebest result is obtained for a two pattern speeds model: the central region (up to∼ 15′′), correspondingto a nuclear bar, rotates with an�p = 100 km s−1 kpc−1. The outer bar + spiral, decoupled fromthe nuclear region, has a slower angular velocity�p = 27 km s−1 kpc−1. The model reproduces thecomplex Hα structure in the inner 20′′ radius, which is certainly not fitted with the one mode pattern.

1. Has NGC 3359 a Decoupled Nuclear Bar? – Numerical Simulations

NGC 3359 is a well studied galaxy in a wide range of wavelengths. High resolutionand sensitivity maps of the gas distribution have been obtained in Hα (Rozasetal., 1999) and HI (Ball, 1986). A detailed study of its kinematics and dynamicsin HI was undertaken by Ball (1992), whereas Rozaset al. (1999) analysed thedistribution and kinematics of the HII regions.

The hydrodynamical model of Ball (1992) was a simplified approximation tothe real potential and roughly reproduced the main features seen in the HI map. Heused a one pattern speed model and needed to add an oval component to obtain theextended outer spiral structure of the galaxy.

Through new self-consistent hydrodynamical simulations of the molecular gasin NGC 3359, we show that to obtain the complex inner Hα morphology and theouter HI spiral structure seen in the real galaxy, two different pattern speeds areneeded.

We have performed simulations of the molecular gas hydrodynamics in thepotential derived from a I-band image. A constant M/L ratio was assumed to derivethe rotation curve. The code was formerly developed by Combes and Gerin (1985)(see, for a detailed description) and later improved to add the self-gravity of the gasand to be applied to real galaxies (García-Burilloet al., 1998).

We have carried out several runs with different�p values. At first, we have trieda unique pattern speed, though the inner morphology is not matched by the model.The best fit is obtained for two decoupled bars with different angular velocities:the main bar + outer spiral rotates with a�s ∼ 27 km s−1 kpc−1, and the nuclearbar with�f ∼ 100 km s−1 kpc−1. The corotation of the fast pattern falls in the

Astrophysics and Space Scienceis the original source of publication of this article. It is recom-mended that this article is cited as:Astrophysics and Space Science269–270:665–666, 1999.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

666 MARIA SEMPERE

Figure 1.a) Best fit of the numerical simulations overlaid with the HI gas distribution. b) The centralregion of the model superposed to the Hα map. The ellipse traces the corotation resonance of the fastpattern speed.

ILR region of the slow one (Taggeret al., 1987). Figure 1 shows an overlay of themodel with the HI gas distribution and with the 50′′ Hα region. Being HI anaemicin the center of the galaxy, CO observations would be of invaluable help to confirmour results.

Acknowledgements

This work has been partially supported by the Spanish DGES under grants PB96–0883 and by PNIE grant ESP97–1618–E. We thank Dr Maite Rozas for providingthe Hα, HI, and I-band image used in this paper.

References

Ball, R.: 1986,Astrophys. J.307, 453; Ball, R.: 1992,Astrophys. J.395, 418.Combes, F. and Gerin, M.: 1985,Astron. Astrophys.150, 327.García-Burillo, S., Sempere, M.J., Combes, F. and Neri, R.: 1998,Astron. Astrophys.333, 864.Rozas, M., Zurita, A., Beckman, J.E. and Pérez, D.: 1999,Astron. Astrophys., in press.Tagger, M., Sygnet, J.F., Athannassoula, E. and Pellat, R.: 1987,Astrophys. J.318, L43.