Gravitational Bar and Spiral Arm Torques fromKs-band Observations and Implications for the Pattern Speeds

We have obtained deep near-infrared K_s-band William Herschel Telescopeobservations of a sample of 15 nearby spiral galaxies having a range of Hubbletypes and apparent bar strengths. The near-infrared light distributions areconverted into gravitational potentials, and the maximum relative gravitationaltorques due to the bars and the spirals are estimated. We find that spiralstrength, Q_s, and bar strength, Q_b, correlate well with other measures ofspiral arm and bar amplitudes, and that spiral and bar strengths also correlatewell with each other. We also find a correlation between the position angle ofthe end of the bar and the position angle of the inner spiral. Thesecorrelations suggest that the bars and spirals grow together with the samerates and pattern speeds. We also show that the strongest bars tend to have themost open spiral patterns. Because open spirals imply high disk-to-halo massratios, bars and spirals most likely grow together as a combined diskinstability. They stop growing for different reasons, however, giving theobserved variation in bar-spiral morphologies. Bar growth stops because ofsaturation when most of the inner disk is in the bar, and spiral growth stopsbecause of increased stability as the gas leaves and the outer disk heats up.Comment: AJ in press, 18 pages, 10 figures. Seven figure pages included as jpeg files, other figures included as encapsulated postscript. Complete high-resolution version of the paper in postscript or pdf format can be found at ftp://star.herts.ac.uk/pub/Knapen/BlockAJpape