Comparison of bar strengths in optical and near-infrared for the OSUBSGS sample

We use a gravitational bar torque method to compare bar strengths (the maximum tangential force normalized by radial force) in B and H-band images of 152 galaxies from the Ohio State University Bright Spiral Galaxy Survey. Our main motivation is to check how much the difference in the rest-frame wavelength could affect the comparisons of bar strengths in low- and high-redshift observations. Between these two bands, we find an average bar strength ratio Q(B/H) = 1.25 which factor is nearly independent of the morphological type. We show that Q(B/H) > 1 is mostly due to reduced bulge dilution of radial forces in the B band. The bar torque method needs an estimate for the vertical scaleheight of the galaxy, based on the radial scalelength of the disc and the galaxy's morphological type. Since these two might not always be possible to determine at high redshifts in a reliable manner, we also checked that similar results are obtained with vertical scaleheights estimated from the radii corresponding to the K-band surface brightness of 20 mag arcsec-2. Also, we made a simple test of the usability of the bar torque method at high redshifts by checking the effects of image degradation (nearest neighbour sampling without any adjustment of noise levels): we found that the estimated bar strengths varied by +/- 10 per cent at most as long as the total extent of the bar was at least 10 pixels. Overall, we show that the gravitational bar torque method should provide a proficient tool for quantifying bar strengths also at high redshifts.