Comparison of bar strengths in active and non-active galaxies

From an original sample of 107 spiral galaxies, the bar strengths of 21 active galaxies are compared with those of 22 non-active galaxies. Our identifications of bars are inferred from near-IR images using Fourier methods. The bar torques are determined using a new technique due to Buta & Block, in which tangential forces are calculated in the bar region normalized to the axisymmetric radial force field. As a data base we use the JHK images of the 2 Micron All Sky Survey. The ellipticities epsilon of the bars are also estimated with an isophotal fitting algorithm and the bar lengths from the phases of m = 2 and m = 4 Fourier components of density. We show a first clear indication that the ellipticity of a bar, generally used as a measure of the bar strength, is quite well correlated with the maximum relative tangential force, Q(b), in the bar region. Most surprisingly, the galaxies with the strongest bars are non-active. A possible understanding of this unusual result is that previous gaseous inflow in such cases may have been so efficient that fuelling of the active nuclei may simply have ceased. We find that nuclear activity occurs preferentially in those barred early-type galaxies in which the maximal bar torques are weak ([Q(b)] = 0.21) and appear at quite large distances from the galactic centre when scaled with the radial scalelength of the disc ([r(Qb)/h] = 1.24). For comparison, for the non-active galaxies [Q(b)] = 0.37 and [r(Qb)/h] = 0.59. The force maximum in the active late-type galaxies also appears at rather large distances, but the difference from the non-active galaxies is smaller. These results imply that the bulges may be important for the onset of nuclear activity, but it is not clear why nuclear activity appears in some early-type galaxies but is missing from some others. We also find that for active early-type galaxies, bar length is not correlated with bar strength, although a weak correlation appears for the other barred galaxies studied.