Mathematics – Logic
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt.........4j&link_type=abstract
Thesis (PHD). CORNELL UNIVERSITY , Source DAI-B 58/06, p. 3090, Dec 1997, 292 pages.
Mathematics
Logic
8
Spirals, Stellar Disk, Dark Matter
Scientific paper
A study of the kinematics of the stellar and gaseous components of Sa galaxies was undertaken in order to understand the origins of the heterogeneity of the Sa galaxies as a class and to investigate previous observations suggesting that the fraction of the mass of Sa galaxies in the form of dark matter is smaller than that found in later type spirals. A sample of 23 nearby non-interacting morphologically normal galaxies was studied. The small-bulged Sa, NGC 4138, was found to contain two extended counter-rotating stellar disks. The primary stellar disk contains ~80% of the stars; the presence of Hα absorption suggests that star formation in this component ceased ~108 years ago. The remaining stars, along with the H I and emission line gas, are rotating counter to the primary disk with comparable observed velocity. The counter-rotating disk may either be the result of a merger of a spiral with a gas-rich dwarf, or the continual infall of material of opposite spin vector onto the galaxy. For the entire sample, comparison of the stellar and optical rotation curves reveals that only 4 of the 23 galaxies in the sample are free of kinematic peculiarities. Eighteen galaxies in the sample have non-circular motions along the minor axis. Along the major axis, 11 galaxies have kinematic peculiarities including extended counter-rotating stars and∨ gas, counter-rotating stellar or gas cores, or multiple stellar or gas components. The presence of kinematically decoupled components suggests that minor mergers may have been common events even among morphologically normal Sa's. Dynamical masses were obtained from the velocity fields mapped by optical and H I observations by adopting the maximum disk hypothesis. As in previous studies, the kinematics of most Sa galaxies can be modeled within the optical disk without invoking a significant dark matter halo component. However, in order to model the velocity fields as traced by the H I sizable dark matter halo components are required. To the limit reachable by H I mapping, the mass of Sa galaxies is dominated by a dark matter halo so that the median dark to luminous mass ratio is 2.45.
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