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Paper: Resonance Rings in Early SB Galaxies: Estimating Bar Strength, Disk Star-Formation, Mass Density and M/L
Volume: 395, Frontiers of Astrophysics: A Celebration of NRAO's 50th Anniversary
Page: 363
Authors: Byrd, G.; Freeman, T.; Buta, R.
Abstract: We use our HST observations of the inner (r) and outer (R1) rings of the galaxy NGC3081 [Buta, Byrd, & Freeman (2004: AJ, 127, 1982)] with n-body simulations to confirm a new analytical ring formulation [Byrd, Freeman, & Buta (2006: AJ, 131, 1377)]. The formulation takes into account the strong response for these rings which are just inside and outside co-rotation of the bar and orbital motion. We show how r and R1 sizes and shapes (plus the rotation curve) give the strength and pattern speed of the bar potential perturbation. The NGC3081 HST observations show individual clusters in the inner ring, some of which appear to be as old as 400 Myr. We see evidence of aging in the cluster luminosity function and colors in orbital angle from the inner ring tips. Disk surface mass density (μ) and mass-to-light ratio (M/L) is known with real certainty only at the solar orbital radius for the Milky Way galaxy (MW)—Flynn et al. (2006: MNRAS, 372, 1149). For other disk galaxies, there are mutually contradictory needs for accurate velocity-dispersion and thickness measurements normal to the disk. We derive how to use r and outer R1 rings to get disk μs. Using Fourier surface-brightness components, we obtained disk μs from the r through R1 ring radii for NGC3081 and new results for NGC6782. As with the MW, we also find the disk dynamics of both galaxies to be halo-dominated. Disk M/L’s are also calculated. Contrary to common assumptions, we find that the disk M/L is not constant for either NGC3081 or NGC6782 but increases with radius. Having a variety of M/Ls at different radii shows promise for studying secondary M/L indicators and disk star-formation history. We acknowledge grant support NASA/STScI GO 8707 and NSF AST-0206177.
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