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 database 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 (2001). Bar strengths have an important role in many extra-galactic 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 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 scale-height, which is difficult to measure directly for most galaxies. On the other hand, the derived bar stength is rather insensitive to the possible gradient in the vertical scale-height of the disk or to the exact model of the vertical density distribution, provided that a same effective vertical dispersion is assumed in all models. In comparison to the pioneering study by Buta & Block (2001), bar strength estimate is here improved by taking into account the dependence of the vertical scale-height on the Hubble type: we find that for thin disks bar strengths are stronger than for thick disks by an amount which may correspond to even one bar strength class. We confirm the previous result by Buta & Block (2001) and Block et al. (2001) showing that the dispersion in bar strength is large among all the de Vaucouleurs’s optical bar classes. In the near-IR 40 % 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 1/3 of these bars are obscured by dust. Significant non-axisymmetric forces can be induced also by the spiral arms, generally in the outer parts of the galactic disks, which may have important implications on galaxy evolution. Possible biases of the selected sample are also studied: we find that the number of identified bars rapidly drops when the inclination of the galactic disk is larger than 50. 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.