SPIRAL STRUCTURE-BASED LIMITS ON THE DISK MASS OF THE LOW SURFACE BRIGHTNESS GALAXIES UGC-6614 AND F568-6

The spiral structure of the low surface brightness galaxies F568-6 (Ma lin 2) and UGC 6614 is large scale, with arms that wrap more than half a revolution, and extend out to 50 and 80 kpc in UGC 6614 and F568-6, respectively. The density contrasts observed in the H I maps are high , with arm/interarm contrasts of similar to 2:1, whereas the velocity perturbations due to spiral structure are low, in the range 10-20 km/s and 10-30 km/s in UGC 6614 and F568-6, respectively. Upper limits for the disk mass-to-light ratios are estimated by considering the minimu m velocity perturbations in the H I velocity field that should result from the spiral structure observed in the R band images. The weak obse rved response in the phi velocity component limits the mass-to-light r atios of the disk inside a scale length to M/L less than or similar to 3 and 6 for UGC 6614 for F568-6, respectively (in solar units), based upon azimuthal variations observed in the R band images. These limits are sufficiently strong to require a significant dark matter componen t even in the central regions of these galaxies. Our limits furthermor e imply that this dark matter component cannot be in the form of a col d disk since a cold disk would necessarily be involved in the spiral s tructure. However, a more massive disk could be consistent with the ob servations because of a non-linear gas response or if the gas is drive n by bar-like distortions instead of spiral structure. To produce the large observed arm/interarm H I density variations it is likely that t he spiral arm potential perturbation is sufficiently strong to produce shocks in the gas. For a forcing that is greater than 2% of the axisy mmetric force, M/L greater than or similar to 1 is required in both ga laxies in the outer regions. This is equivalent to a disk surface dens ity between r=60 ''-120 '' in UGC 6614 of 2.6-1.0 M./pc(2) and between r=40 ''-90 '' in F568-6 of 6.6-1.0 M./pc(2) assuming that the amplitu de of the variations in the disk mass is the same as that observed in the R band. These lower limits imply that the stellar surface density is at least of the same order as the gas surface density. This is cons istent with the large scale morphology of the spiral structure, and th e stability of the gas disk, both which suggest that a moderate stella r component is required to produce the observed spiral structure. (C) 1997 American Astronomical Society.