Bar strengths in spiral galaxies estimated from 2MASS images

Non-axisymmetric forces are presented for a sample of 107 spiral galaxies, of which 31 are barred (SB) and 53 show nuclear activity. As a data base we use JHK images from the 2 Micron All-sky Survey, and the non-axisymmetries are characterized by the ratio of the tangential force to the mean axisymmetric radial force field, following Buta & Block. Bar strengths have an important role in many extragalactic problems and therefore it is important to verify that the different numerical methods applied for calculating the forces give mutually consistent results. We apply both direct Cartesian integration and a polar grid integration utilizing a limited number of azimuthal Fourier components of density. We find that the bar strength is independent of the method used to evaluate the gravitational potential. However, because of the distance-dependent smoothing by Fourier decomposition, the polar method is more suitable for weak and noisy images. The largest source of uncertainty in the derived bar strength appears to be the uncertainty in the vertical scaleheight, which is difficult to measure directly for most galaxies. On the other hand, the derived bar strength is rather insensitive to the possible gradient in the vertical scaleheight of the disc or to the exact model of the vertical density distribution, provided that the same effective vertical dispersion is assumed in all models. In comparison with the pioneering study by Buta & Block, the bar strength estimate is improved here by taking into account the dependence of the vertical scaleheight on the Hubble type: we find that for thin discs bar strengths are stronger than for thick discs by an amount that may correspond to as much as one bar strength class. We confirm the previous result by Buta and co-workers showing that the dispersion in bar strength is large among all the de Vaucouleurs optical bar classes. In the near-infrared 40 per cent of the galaxies in our sample have bars (showing constant phases in the m = 2 Fourier amplitudes in the bar region), while in the optical band one-third of these bars are obscured by dust. Significant non-axisymmetric forces can also be induced by the spiral arms, generally in the outer parts of the galactic discs, which may have important implications on galaxy evolution. Possible biases of the selected sample are also studied: we find that the number of bars identified drops rapidly when the inclination of the galactic disc is larger than 50degrees. A similar bias is found in the Third Reference Catalogue of Bright Galaxies, which might be of interest when comparing bar frequencies at high and low redshifts.