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 research program of the Liquid Scintillation Detector (LSD) in the Mont Blanc Laboratory

A massive (90 tons) liquid scintillation detector (LSD) has been running since October 1984 in the Mont Blanc Laboratory at a depth of 5,200 hg/sq cm of standard rock. The research program of the experiment covers a variety of topics in particle physics and astrophysics. The performance of the detector, the main fields of research are presented and the preliminary results are discussed

Global spiral modes in multi-component disks

We performed two-dimensional non-linear hydrodynamical simulations of two-component gravitating disks aimed at studying stability properties of these systems. In agreement with previous analytical and numerical simulations, we find that the cold gas component strongly affects the growth rates of the unstable global spiral modes in the disk. Already a five percent admixture of cold gas increases approximately two-fold the growth rate of the most unstable global mode while a twenty percent gas admixture enhances the modal growth rate by a factor of four. The local stability properties of a two-component disk coupled by self-gravity are governed by a stability criterion similar to Toomre's Q-parameter derived for one-component systems. Using numerical simulations, we analyse the applicability of a two-component local stability criterion for the analysis of the stability properties of global modes. The comparison of non-linear simulations with the analytical stability criterion shows that the two-component disks can be globally unstable while being stable to the local perturbations. The minimum value of the local stability criterion provides, however, a rough estimate of the global stability properties of two-component systems. We find that two-component systems with a content of cold gas of ten percent or less are globally stable, if the minimum value of the stability parameter exceeds about 2.5.Comment: 7 pages, with 8 postscript figures, accepted for publication by Astronomy & Astrophysic