Mass distributions and stellar populations in spiral galaxies
Davis, Joseph C.
Talbot, Raymond J.
Master of Science
The techniques of obtaining mass models for spiral galaxies from their observed rotation curves are reviewed. A fast method for fitting the Toomre type disk mass model together with a simple prescription for the distribution of central bulge material is developed based on observed rotation curves. Several well observed galaxies are then modeled in this fashion. These mass models of real galaxies are then used as input to an extension of an ongoing program of galactic evolution modeling. Multizone galactic models are constructed assuming each radial zone evolves independently with a rate of star formation proportional to the frequency of spiral arm passage as derived from density wave theory. The proportionality factor, called the efficiency of star formation, is the only free parameter of the model. An initial era of star formation is assumed to deplete an amount of gas into stars equal to: the amount of bulge mass at each radius. A constant initial mass function equal to the Salpeter IMF is employed throughout. An evolutionary model of the nearby spiral galaxy M33 shows that a variable efficiency factor is required to reproduce both the observed integrated colors and luminosities. It is also shown that this model for the star formation rate coupled with the initial mass function determined from the stars in the local solar neighborhood will not produce model galaxies as blue as the bluest real galaxies. Therefore, either the IMF cannot be a universal constant or the star formation rate is a more complex function of time.