The gravitational torque of bars in optically unbarred and barred galaxies^*

¹ Department of Computational and Applied Mathematics, University of the Witwatersrand, 1 Jan Smuts Avenue, Johannesburg 2001, South Africa
² Instituto Nacional de Astrofísica, Optica y Electrónica, Calle Luis Enrique Erro 1, 72840 Tonantzintla, Puebla, México e-mail: puerari@inaoep.mx
³ Isaac Newton Group of Telescopes, Apartado 321, 38700 Santa Cruz de La Palma, Spain e-mail: knapen@ing.iac.es
⁴ University of Hertfordshire, Dept. of Physical Sciences, Hatfield, Herts. AL10 9AB, UK e-mail: knapen@star.herts.ac.uk; stedman@star.herts.ac.uk
⁵ IBM Research Division, TJ Watson Research Center, Box 218, Yorktown Heights NY 10598, USA e-mail: bge@watson.ibm.com
⁶ Dept. of Physics and Astronomy, University of Alabama, Box 870324, Tuscaloosa, Alabama 35487, USA e-mail: buta@sarah.astr.ua.edu
⁷ Vassar College, Dept. of Physics and Astronomy, Poughkeepsie, NY 12604-0278, USA e-mail: elmegreen@vaxsar.vassar.edu

Corresponding author: D. L. Block, igalaxy@iafrica.com

Received: 4 March 2001
Accepted: 28 May 2001

Abstract

The relative bar torques for 45 galaxies observed at K-band with the 4.2 m William Herschel Telescope are determined by transforming the light distributions into potentials and deriving the maximum ratios of the tangential forces relative to the radial forces. The results are combined with the bar torques for 30 other galaxies determined from our previous K-band survey (Buta & Block [CITE]). Relative bar torques determine the degree of spiral arm forcing, gas accretion, and bar evolution. They differ from other measures of bar strength, such as the relative amplitude of the bar determined photometrically, because they include the bulge and other disk light that contributes to the radial component of the total force. If the bulge is strong and the radial forcing large, then even a prominent bar can have a relatively weak influence on the azimuthal motions in the disk. Here we find that the relative bar torque correlates only weakly with the optical bar type listed in the Revised Shapley-Ames and de Vaucouleurs systems. In fact, some classically barred galaxies have weaker relative bar torques than classically unbarred galaxies. The optical class is a poor measure of azimuthal disk forcing for two reasons: some infrared bars are not seen optically, and some bars with strong bulges have their azimuthal forces so strongly diluted by the average radial force that they exert only small torques on their disks. The Hubble classification scheme poorly recognizes the gravitational influence of bars. Applications of our bar torque method to the high-redshift universe are briefly discussed.