Formation of gaseous arms in barred galaxies with dynamically important magnetic field: 3D MHD simulations

Abstract

Aims. We present results of three-dimensional nonlinear MHD simulations of a large-scale magnetic field and its evolution inside a barred galaxy with the back reaction of the magnetic field on the gas. The model does not consider the dynamo process. To compare our modeling results with observations, we construct maps of the high-frequency (Faraday-rotation-free) polarized radio emission on the basis of simulated magnetic fields. The model accounts for the effects of projection and the limited resolution of real observations. Methods. We performed 3D MHD numerical simulations of barred galaxies and polarization maps. Results. The main result is that the modeled magnetic field configurations resemble maps of the polarized intensity observed in barred galaxies. They exhibit polarization vectors along the bar and arms forming coherent structures similar to the observed ones. In the paper, we also explain the previously unsolved issue of discrepancy between the velocity and magnetic field configurations in this type of galaxies. The dynamical influence of the bar causes gas to form spiral waves that travel outwards. Each gaseous spiral arm is accompanied by a magnetic counterpart, which separates and survives in the inter-arm region. Because of a strong compression, shear of non-axisymmetric bar flows and differential rotation, the total energy of modeled magnetic field grows constantly, while the azimuthal flux grows slightly until 0.05 Gyr and then saturates.

3D MHD simulations of magnetic field evolution and radio polarization of barred galaxies

“…The effect described above repeats and at the time step t = 1.68 Gyr magnetic arms again drift into the inter-arm region and form the doublering structure. The drift of the magnetic arms into the inter-arm area was described in our previous paper (Kulesza-Żydzik et al 2009), where we concluded that this behaviour is caused by the difference in the angular velocity of the magnetic arms and the gaseous spirals. Namely, the magnetic arms do not corotate with gaseous spiral structure, but follow the general gas motion in the disk, which has a slightly lower angular velocity.…”

“…However, they did not find the mechanism which is responsible for shifting of the magnetic arms from the gasoues ones to the inter-arm space. As we mentioned in the intorduction, the concept of the magnetic field drift into the inter-arm region was described earlier only in two papers (Otmianowska-Mazur et al 2002;Kulesza-Żydzik et al 2009). In the aforesaid paper the authours found this mechanism for the first time, but in those calculation they did not take into account the back-reaction of magnetic field to the gas, which could be crucial in these studies.…”

“…A number of works (Otmianowska-Mazur et al 1997Moss et al 1998Moss et al , 1999Moss et al , 2001Moss et al , 2007Kulesza-Żydzik et al 2009) A&A 522, A61 (2010) have been done to study numerically the magnetic field evolution in barred galaxies. The results of 3D numerical simulations of magnetohydrodynamic (MHD) evolution of one example, NGC 1365 (Moss et al 2007), show that even after checking the whole range of input parameters there is no agreement between the topology of the polarization vectors observed in this galaxy and given by computations.…”

“…We present the results of the new calculations made under the assumption applied earlier in Kulesza-Żydzik et al (2009), where we computed only one set of the initial parameters. Similarly, we solve the 3D MHD equations including the backreaction of the magnetic field on the gas, using the analytically prescribed gravitational potential of the disk, halo, bulge, and bar.…”

3D numerical simulations of magnetic field evolution in barred galaxies and in spiral galaxies under influence of tidal forces

“…The development of the magnetic field in barred galaxies was studied numerically by many authors (e.g. Otmianowska-Mazur et al 2002, Moss et al 2007, Kulesza-Żydzik et al 2009-Paper I, Kulesza-Żydzik et al 2010, however, the models including the classical dynamo theory in this calculations did not show the promising results (Moss et al 2007). In opposition to them in our two papers (Paper I & II) we presented numerical models which are capable to explain the drift of magnetic field arms into the interarm region.…”