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The interstellar medium (ISM) reveals strongly inhomogeneous structures on every scale. These structures do not seem completely random since they obey certain power laws. Larson's (1981)) law σ R^sup δ^ and the plausible assumption of virial equilibrium justify the consideration of fractals as a possible description of the ISM. In the following we investigate how self-gravitation, differential rotation and dissipation affect the matter distribution in galaxies. To this end we have performed 3D simulations for self-gravitating local boxes embedded in a larger disc, extending the 2D method of Toomre and Kalnajs (1991) and Wisdom and Tremaine (1988). Our simulations lead to the conclusion that gravitation, shearing and dissipation can be dominantly responsible for maintaining an inhomogeneous and eventually a fractal distribution of the matter.[PUBLICATION ABSTRACT]

The properties of chaos in 2D self-consistent models of barred galaxies are investigated using Kolmogorov-Sinai entropy h^sub KS^. These models are constructed with Schwarzschild's method which combines orbits as elementary building blocks. Most models are dominated by chaos near the 2/3 of the length of the bar and close to corotation. These locations correspond to regions where star forming HII regions are observed because gas clouds could shock, shrink and fragment such that star formation could be ignited. The model the most similar to N-body models shows a peak of h^sub KS^ between the corners of the rectangular-like x^sub 1^ orbits and the maximum extension points of the Lagrangian orbits. This emphasizes the role of Lagrangian orbits in the morphology of bars. Most models essentially contain 'semi-chaotic' orbits confined inside the corotation.[PUBLICATION ABSTRACT]

To understand galaxies and their evolution, it is necessary to describe how the different scales interact: how the microscopic physics, such as star formation, or the large scale physics, such as galaxy interactions may modify the galaxy global shapes. The purpose of this review is to point out some general or recent topics related to such scale interactions, both observational and theoretical, which are relevant in the present understanding of galaxies.

The confrontation of theoretical arguments and observational facts leads to view the Hubble sequence as an aging sequence from late to early type galaxies. To illustrate this, a few N-body simulations of disk galaxies are described showing a series of dynamical events such as bulge building must occur in the life of a single isolated galaxy over time-scales much shorter than the Hubble time. We concentrate here on the basic physics ruling galaxies, collisionless gravitational dynamics, because any major of these global morphology modifications must trigger further important consequences in the subsidiary physics associated with gas and star formation. A new kind of instability in 3D models is reported, analogous to the vertical 2/1 instability leading to the formation of peanut-shaped bars: a 1/1 vertical instability in which a secondary bar embedded in a larger one spontaneously inclines its rotation axis and warps the surrounding disk. The role of an outer disk is particularly important to maintain the spiral evolution over several Gyr by allowing a prolonged exchange of angular momentum with the inner disk.[PUBLICATION ABSTRACT]

An elementary review about stellar and galactic dynamics is presented. Despite involving extremely classical Newtonian physics, stellar dynamics presents some fundamental difficulties rarely discussed in the literature, such as why the phase space distribution is assumed to be a smooth function of coordinates. Many systems are found to be unstable over intermediate time-scales, as more instabilities have been discovered over the years, so the old aim of describing equilibrium stable systems shifts presently toward understanding evolutive systems. From the linearized variational Boltzmann equation a distinction can be made between instabilities triggered by the chaotic part of phase space, and instabilities caused by steep gradients in the velocity part of the distribution function. The new challenges to include evolutive systems can presently only be studied efficiently with computer techniques. Future studies are likely to involve orders of magnitude more advanced computers in which parallelism will play a major role.

The properties of chaos in 2D self-consistent models of barred galaxies are investigated using Kolmogorov-Sinai entropy h sub(KS). These models are constructed with Schwarzschild's method which combines orbits as elementary building blocks. Most models are dominated by chaos near the 2/3 of the length of the bar and close to corotation. These locations correspond to regions where star forming HII regions are observed because gas clouds could shock, shrink and fragment such that star formation could be ignited. The model the most similar to N-body models shows a peak of h sub(KS) between the corners of the rectangular-like x sub(1) orbits and the maximum extension points of the Lagrangian orbits. This emphasizes the role of Lagrangian orbits in the morphology of bars. Most models essentially contain 'semi-chaotic' orbits confined inside the corotation.

Results from the UV satellite GALEX revealed large extensions of disks in some nearby spiral galaxies, extending out to 3 to 4 times the isophotal radius, r25. M63 is a remarkable example of a spiral galaxy with one of the most extended UV disks, so it offers the opportunity to search for the molecular gas and characterize the star formation in outer disk regions as revealed by the UV emission. We obtained deep CO(1-0) and CO(2-1) observations on the IRAM 30 m telescope along the major axis of the M63 disk from the center out to the galactocentric radius rgal = 1.6 r25 and over a bright UV region at rgal = 1.36 r25. CO(1-0) is detected all along the M63 major axis out to r25, and CO(2-1) is confined to rgal = 0.68 r25, which may betray lower excitation temperatures in the outer disk. CO(1-0) is also detected in the external bright UV region of M63. The radial profiles of the CO emission and of the Halpha, 24 micron, NUV and FUV star formation tracers and HI taken from the literature show a severe drop with the galactocentric radius, such that beyond r25 they are all absent with the exception of a faint UV emission and HI. The CO emission detection in the external UV region, where the UV flux is higher than the UV flux observed beyond r25, highlights a tight correlation between the CO and UV fluxes, namely the amount of molecular gas and the intensity of star formation. This external UV region is dominated by the atomic gas, suggesting that HI is more likely the precursor of H2 rather than the product of UV photodissociation. A broken power law needs to be invoked to describe the Kennicutt-Schmidt (K-S) relation of M63 from the center of the galaxy out to rgal = 1.36 r25. While all along the major axis out to r25 the K-S relation is almost linear, in the external UV region the SFR regime is highly nonlinear and characterized by a steep K-S relation and very low star formation efficiency.

A single merger scenario for making galaxies such as NGC 4550 possessing equal coplanar counter-rotating stellar disks is investigated by collisionless N-body technique. The scenario is successful in producing an axisymmetric disk made of two almost equal counter-rotating populations. The final disk shows a clear bimodal line profile in the outer part, which demonstrates that disk-disk mergers do not always produce ellipticals.