Dr. Gene Byrd
University of Alabama, Tuscaloosa

Special University of Florida Astronomy Colloquium - 3pm Oct. 12th, 2006

Resonance Rings in Early SB Galaxies: Estimating Pattern Speed, Disk Star Formation, Mass Density and M/L

One place where galaxy disk surface mass density (mu) and mass-to-light ratio (M/L) is known with any certainty is at the solar orbital radius for the Milky Way Galaxy (MW)(Flynn et al 2006). To get mu's for other disk galaxies, we show how to avoid the mutually contradictory needs for accurate velocity dispersion and thickness normal to the disk by using inner (r) and outer (R1) resonance rings. We use our HST observations of the r and R1 rings of the galaxy NGC3081 (Buta, Byrd, & Freeman 2003) and n-body simulations to confirm a new analytical formulation of these rings which occur inside and outside co-rotation (CR) of the orbital motion and bar perturbation (Byrd, Freeman & Buta 2006). We also discuss in-preparation results for NGC6782. The analytic formulation takes into account the strong response near CR. We show how r and R1 sizes and shapes (plus the rotation curve, V) give the strength and pattern speed of the bar potential perturbation. Using Fourier surface brightness components, we obtain disk mu's from the r through R1 ring radii. As with the MW, we also find the disk dynamics of both galaxies to be halo-dominated. Disk M/L's are also calculated over this range. Contrary to common assumptions, we find that the disk M/L is not constant for either NGC 3081 or 6782 but in general increases with radius. Having a variety of M/L's at different radii shows promise for verifying secondary M/L indicators, such as disk color, and studying of disk star formation history.