AAS Meeting #193 - Austin, Texas, January 1999
Session 105. Galaxy Kinematics and Dynamics
Display, Saturday, January 9, 1999, 9:20am-4:00pm, Exhibit Hall 1

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[105.05] Star Formation Color-Time Sequence and ``Anti-Spiral" Wagon-Spoke Perturbation in Resonance Ring Galaxies: NGC3081

D. Ousley, G. G. Byrd (U. Alabama, Tuscaloosa)

Byrd, Ousley, and Dalla Piazza(1998a, MNRAS, 298, 78) described a hybrid analytic/computer formulation of periodic interstellar cloud orbits which explains morphologies of resonance rings in SB galaxies. Byrd, Ousley, Dalla Piazza, and Domingue (1998b, Dynamics of Galaxies conf.) applied the formulation to NGC 3081 to show how ring morphology and rotation curve can give the two-fold perturbation strength and pattern speed plus the disk inclination and line of nodes. Using the perturbation in Poisson's equation with the outer disk isophote ellipticity gives the disk surface mass density as a function of radius. Within errors, the density is sufficient to explain the NGC 3081 rotation curve i.e. no halo is required. Disk density/surface brightness at each radius gives the NGC 3081 M/L, indicating a large amount of dark matter in its outer disk plane possibly eliminating certain galaxy dark matter candidates.

In this poster, we study how star formation in inner resonance rings occurs where gas clouds are crowded near the end of the bar of NGC 3081. As these orbit in position angle away from the end, they age and their B-I colors change. Using the above formulation, color indexes of stellar associations can thus be empirically calibrated in years, to serve in age estimates of associations in other galaxies and as an observational test of association models. Our morphological match for NGC 3081 is excellent from the nuclear ring through the outer rings. The perturbation potential must also be valid indicating it is the radial ``wagon-spoke" type descried by Shu (1970) with two spokes. Thus NGC 3081's disk must be non-dissipational (stellar) with a stabilizing velocity dispersion. Globally, gas must be gravitationally unimportant. We explore the possibility that the elongated ``bar" of NGC3081 may only be a minority of luminous resonance ring stars superposed on a gentler global old disk star perturbation.

Ousley was supported by a McWane Undergraduate Research Fellowship. We thank R. Buta and G. Purcell for use of figures.

The author(s) of this abstract have provided an email address for comments about the abstract: byrd@possum.astr.ua.edu

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