830 research outputs found
Optical Color Gradients in Star-Forming Ring Galaxies
We compute radial color gradients produced by an outwardly propagating
circular wave of star formation and compare our results with color gradients
observed in the classical ring galaxy, the ``Cartwheel''. We invoke two
independent models of star formation in the ring galaxies. The first one is the
conventional density wave scenario, in which an intruder galaxy creates a
radially propagating density wave accompanied by an enhanced star formation
following the Schmidt law. The second scenario is a pure self-propagating star
formation model, in which the intruder only sets off the first burst of stars
at the point of impact. Both models give essentially the same results.
Systematic reddening of B-V, V-K colors towards the center, such as that
observed in the Cartwheel, can be obtained only if the abundance of heavy
elements in the star-forming gas is a few times below solar. The B-V and V-K
color gradients observed in the Cartwheel can be explained as a result of
mixing of stellar populations born in a star-forming wave propagating through a
low-metallicity gaseous disk, and a pre-existing stellar disk of the size of
the gaseous disk with color properties typical to those observed in nearby disk
galaxies.Comment: 16 pages, 12 figures; accepted for publication in the Astrophysical
Journa
The Futile Search for Galactic Disk Dark Matter
Several approaches have been used to search for dark matter in our galactic
disk, but with mixed results: {\em maybe yes and maybe no}. The prevailing
approach, integrating the Poisson-Boltzmann equation for tracer stars, has led
to more definitive results: {\em yes and no}. The touchstone {\em yes} analysis
of Bahcall et al. (1992) has subsequently been confirmed or refuted by various
other investigators. This has been our motivation for approaching the search
from a different direction: applying the Virial Theorem to extant data. We
conclude that the vertical density profile of the disk is not in a state of
equilbrium and, therefore, that the Poisson-Boltzmann approach is inappropriate
and it thereby leads to indefensible conclusions.Comment: Accepted for publication in ApJ Letter
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