Star Formation and Gas Dynamics in Galactic Disks: Physical Processes and Numerical Models

Star formation depends on the available gaseous "fuel" as well as galactic environment, with higher specific star formation rates where gas is predominantly molecular and where stellar (and dark matter) densities are higher. The partition of gas into different thermal components must itself depend on the star formation rate, since a steady state distribution requires a balance between heating (largely from stellar UV for the atomic component) and cooling. In this presentation, I discuss a simple thermal and dynamical equilibrium model for the star formation rate in disk galaxies, where the basic inputs are the total surface density of gas and the volume density of stars and dark matter, averaged over ~kpc scales. Galactic environment is important because the vertical gravity of the stars and dark matter compress gas toward the midplane, helping to establish the pressure, and hence the cooling rate. In equilibrium, the star formation rate must evolve until the gas heating rate is high enough to balance this cooling rate and maintain the pressure imposed by the local gravitational field. In addition to discussing the formulation of this equilibrium model, I review the current status of numerical simulations of multiphase disks, focusing on measurements of quantities that characterize the mean properties of the diffuse ISM. Based on simulations, turbulence levels in the diffuse ISM appear relatively insensitive to local disk conditions and energetic driving rates, consistent with observations. It remains to be determined, both from observations and simulations, how mass exchange processes control the ratio of cold-to-warm gas in the atomic ISM.Comment: 8 pages, 1 figure; to appear in "IAU Symposium 270: Computational Star formation", Eds. J. Alves, B. Elmegreen, J. Girart, V. Trimbl

On the structure of the turbulent interstellar atomic hydrogen. I- Physical characteristics

{We study in some details the statistical properties of the turbulent 2-phase interstellar atomic gas.{We present high resolution bidimensional numerical simulations of the interstellar atomic hydrogen which describe it over 3 to 4 orders of magnitude in spatial scales.}{The simulations produce naturally small scale structures having either large or small column density. It is tempting to propose that the former are connected to the tiny small scale structures observed in the ISM. We compute the mass spectrum of CNM structures and find that ${\cal N}(M) dM \propto M ^{-1.7} dM$, which is remarkably similar to the mass spectrum inferred for the CO clumps. We propose a theoretical explanation based on a formalism inspired from the Press & Schecter (1974) approach and used the fact that the turbulence within WNM is subsonic. This theory predicts ${\cal N}(M) \propto M ^{-5/3}$ in 2D and ${\cal N}(M) \propto M ^{-16/9}$ in 3D. We compute the velocity and the density power-spectra and conclude that, although the latter is rather flat, as observed in supersonic isothermal simulations, the former follows the Kolmogorov prediction and is dominated by its solenoidal component. This is due to the bistable nature of the flow which produces large density fluctuations even when the rms Mach number (of WNM) is not large. We also find that, whereas the energy at large scales is mainly in the WNM, at smaller scales, it is dominated by the kinetic energy of the CNM fragments.}Comment: Accepted for publication in A&

The kinematic Sunyaev Zeldovich effect and transverse cluster velocities

The polarization of the CMBR scattered by galaxy clusters in the kinematic Sunyaev Zeldovich effect depends on the transverse velocity of the cluster. This polarizing effect is proportional to the transverse velocity squared, and so weaker that the change in intensity due to the radial motion in the kinematic effect. The value given by Sunyaev and Zeldovich, and which is frequently cited, underestimates the polarizing effect by a factor of ten. We show furthermore that the polarization has a strong frequency dependence. This means that the polarization should be detectable with the new generation of CMBR probes, at least for some clusters. Thus this effect offers, almost uniquely, a method of obtaining the vectorial velocity of clusters.Comment: Submitted to MNRAS letter. 5 pages using mnras file style. email: [email protected]

The use of light polarization for weak-lensing inversions

The measurement of the integrated optical polarization of weakly gravitationally lensed galaxies can provide considerable constraints on lens models. The method outlined depends on fact that the orientation of the direction of optical polarization is not affected by weak gravitational lensing. The angle between the semi-major axis of the imaged galaxy and the direction of integrated optical polarization thus informs one of the distortion produced by the gravitational lensing. Although the method depends on the polarimetric measurement of faint galaxies, large telescopes and improved techniques should make such measurements possible in the near future.Comment: 13 pages, 11 figures, uses mnras style file. Accepted for publication in MNRA

Virtual Compton scattering off nuclei in the Δ\Delta-resonance region

Virtual Compton scattering in the $\Delta$-resonance region is considered in the case of a target nucleus. The discussion involves generalized polarizabilities and is developed for zero-spin nuclei, focusing on the new information coming from virtual Compton scattering in comparison with real Compton scattering.Comment: 8 pages, LaTeX, 3 figures available from the author