Models of galactic mass distribution

Abstract

Three groups of galactic mass models, each consisting of nine inhomogeneous spheroids of two kinds are described, according to three adopted values of the total density near the Sun: 0.10, 0.15 and 0.20 M⊙ pc−3. Approximately 20% of the total mass of each model is in the halo, constructed to adequately fit recent RR Lyrae star observations. It is shown that the maxima found in the RR Lyrae star densities towards the galactic axis (Plaut, 1970) should not be interpreted as being associated with the galactic nucleus, but as the result of the greater decrease in density with increasingz over the increase in density as the galactic axis is approached. Even at the low galactic latitude of 5° (l=0°), this effect causes a 0.5 kpc correction to the distance to the galactic centre. A ‘basic’ model for\(\bar \omega _ \odot = 10.0\) kpc,\(\theta _ \odot = 252.5\) km s−1,\(\varrho _ \odot = 0.15\)M⊙ pc−3 is first constructed, mainly to satisfy structural conditions near the sun and in the halo. An attempt to optimize the basic model is made by scaling it so as to retain constant density and angular velocity near the sun, and to best fit kinematic data, including the recent re-examination of the 21-cm data of Simonson and Mader (1972). No unknown matter is required in the models, in accordance with the results of Weistrop (1972b), and, as pointed out earlier (Innanen, 1966b) the faintM-stars must be in a highly flattened spheroid. The optimizing indicates that an adequate fit to kinematics can be achieved for\(0.15< \varrho _ \odot<< 0.20{\text{ }}M_ \odot {\text{ pc}}^{{\text{ - 3}}} ,{\text{ }}8.5< \bar \omega _ \odot< 9.5{\text{ kpc, and }}215< \theta _ \odot< 250\) km s−1. More detailed results are tabulated for a ‘representative’ model for which\(\bar \omega _ \odot = 9.0{\text{ kpc, }}\theta _ \odot = 229{\text{ km s}}^{{\text{ - 1}}} {\text{ and }}\varrho _ \odot = 0.15{\text{ }}M_ \odot {\text{ pc}}^{ - 3} \). Two new galactic density functions are discussed in the Appendix.