ar X iv : a st ro - p h / 98 11 02 4 v 1 2 N ov 1 99 8 Heavy Bars and Light Spirals – Taking Advantage of Asymmetries in Galaxies

I summarize a series of studies where the underlying theme is to exploit non-axisymmetry disk structures such as bars and spiral arms. By measuring the strength of gravitational forces from these structures we can measure the mass in them. The advantage of this approach is that mass to light ratios can be constrained in a way independent of the dark matter profile. By examining a variety of types of galaxies it appears that a correlation between morphology and dark matter fraction emerges. Axisymmetric fits to rotation curves in disk galaxies and velocity profiles in elliptical galaxies allow a large range of freedom in choosing the mass to light (M/L) ratio of the stellar population. This is because there is little observational constraint on the actual form of the dark matter profile. Here we introduce a different approach. By measuring the strength of non-axisymmetric structures we can constrain M/L in disk galaxies in a way that is independent of any assumptions about the dark matter profile. 1.1. Resonant Orbits Outside a Bar In Quillen & Frogel 1997 we studied the affect of the mass of the bar on the shape of the resonance R1 orbits at the Outer Lindblad resonance. By modeling the shape and velocity field of the outer ring in NGC 6782 we found that the mass of the bar must be quite massive, nearly that of a maximal disk (implying little dark matter within the bar). A strong spiral gas response results from a gravitational perturbation strong enough to produce shocks. When strong spiral structure is observed in HI we can then place a lower limit on M/L. If velocity perturbations are small then there the mass in spiral structure cannot be large. This lets us place an upper limit on M/L. In Quillen & Pickering 1997 we place upper and lower limits on M/L of the spiral arms in two low surface brightness galaxies. We find that they have normal stellar populations. Since the current star formation rates of these galaxies is very low they must have had much higher star formation rates previously. These galaxies are therefore good examples of old but faded