EDP Sciences
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Volume 371, Number 2, May IV 2001
Page(s) 538 - 559
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20010392

A&A 371, 538-559 (2001)
DOI: 10.1051/0004-6361:20010392

Restoring the full velocity field in the gaseous disk of the spiral galaxy NGC 157

A. M. Fridman1, 2, O. V. Khoruzhii1, 3, V. V. Lyakhovich1, O. K. Sil'chenko2, A. V. Zasov2, V. L. Afanasiev4, S. N. Dodonov4 and J. Boulesteix5

1  Institute of Astronomy of the Russian Academy of Science, 48, Pyatnitskaya St., Moscow 109017, Russia
2  Sternberg Astronomical Institute, Moscow State University, University prospect, 13, Moscow 119899, Russia
3  National Research Center "Troitsk Institute for Innovation and Thermonuclear Researches", Troitsk, Moscow reg. 142092, Russia
4  Special Astrophysical Observatory of the Russian Academy of Sciences, Zelenchukskaya 377140, Russia
5  Observatoire de Marseille, Place le Verrier, 13248 Marseille Cedex 04, France

(Received 24 August 1999 / Accepted 9 March 2001 )

This paper is the first in a series of articles devoted to the construction and analysis of three-component vector velocity fields in the gaseous disks of spiral galaxies, and to the discovery of giant anticyclones near corotation which were predicted earlier. We analyse the line-of-sight velocity field of ionized gas in the spiral galaxy NGC 157 which has been obtained in the H$\alpha$ emission line at the 6 m telescope of SAO RAS. The field contains more than 11 000 velocity estimates. The existence of systematic deviations of the observed gas velocities from pure circular motion is shown. A detailed investigation of these deviations is undertaken by applying a recently formulated method based on Fourier analysis of the azimuthal distributions of the line-of-sight velocities at different distances from the galactic center. To restore the three-component vector velocity field from the observed line-of-sight velocity field, two assumptions were made: 1) the perturbed surface density and residual velocity components can be approximated to the formulae $C_i \cos(2\varphi+F_i)$, where Ci and Fi are respectively the amplitude and the phase; 2) the perturbed surface density and residual velocity components satisfy the Euler equations. The correctness of both assumptions is proven on the basis of the observational data. As a result of the analysis, all the main parameters of the wave spiral pattern are determined: the corotation radius, the amplitudes and phases of the gas velocity perturbations at different radii, as well as the velocity of circular rotation of the disk corrected for the influence of the velocity perturbations connected with the spiral arms. Finally, a restoration of the vector velocity field in the gaseous disk of NGC 157 is performed. At a high confidence level, the presence of the two giant anticyclones in the reference frame rotating with the spiral pattern is shown; their sizes and the localization of their centers are consistent with the results of the analytic theory and of numerical simulations. Besides the anticyclones, the existence of cyclones in any residual velocity field is predicted. In the reference frame of the spiral pattern, detection of such cyclones is possible in the galaxies with a radial gradient of azimuthal residual velocity steeper than that of the rotation velocity.

Key words: methods: data analysis -- galaxies: individual: NGC 157 -- galaxies: ISM -- galaxies: kinematics and dynamics -- galaxies: spiral -- galaxies: structure

Offprint request: A. M. Fridman, afridman@inasan.rssi.ru

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