The Turbulent Interstellar Medium

An overview is presented of the main properties of the interstellar medium. Evidence is summarized that the interstellar medium is highly turbulent, driven on different length scales by various energetic processes. Large-scale turbulence determines the formation of structures like filaments and shells in the diffuse interstellar medium. It also regulates the formation of dense, cold molecular clouds. Molecular clouds are now believed to be transient objects that form on timescales of order 1e7 yrs in regions where HI gas is compressed and cools. Supersonic turbulence in the compressed HI slab is generated by a combination of hydrodynamical instabilities, coupled with cooling. Turbulent dissipation is compensated by the kinetic energy input of the inflow. Molecular hydrogen eventually forms when the surface density in the slab reaches a threshold value of 1e21 cm^-2 at which point further cooling triggers the onset of star formation by gravitational collapse. A few Myrs later, the newly formed stars and resulting supernovae will disperse their molecular surrounding and generate new expanding shells that drive again turbulence in the diffuse gas and trigger the formation of a next generation of cold clouds. Although a consistent scenario of interstellar medium dynamics and star formation is emerging many details are still unclear and require more detailed work on microphysical processes as well as a better understanding of supersonic, compressible turbulence.Comment: 13 pages, 4 figures, to appear in "Statistical Mechanics of Non-Extensive Systems", eds. F. Combes and R. Robert (Elsevier

Theory of interstellar medium diagnostics

The theoretical interpretation of observed interplanetary resonance luminescence patterns is used as one of the must promising methods to determine the state of the local interstellar medium (LISM). However, these methods lead to discrepant results that would be hard to understand in the framework of any physical LISM scenario. Assuming that the observational data are reliable, two possibilities which could help to resolve these discrepancies are discussed: (1) the current modeling of resonance luminescence patterns is unsatisfactory and has to be improved, and (2) the extrapolated interstellar parameters are not indicative of the unperturbed LISM state, but rather designate an intermediate state attained in the outer regions of the solar system. It is shown that a quantitative treatment of the neutral gas-plasma interaction effects in the interface between the heliospheric and the interstellar plasmas is of major importance for the correct understanding of the whole complex

Astrophysical Fractals: Interstellar Medium and Galaxies

The interstellar medium is structured as a hierachy of gas clouds, that looks self-similar over 6 orders of magnitude in scales and 9 in masses. This is one of the more extended fractal in the Universe. At even larger scales, the ensemble of galaxies looks also self-similar over a certain ranges of scales, but more limited, may be over 3-4 orders of magnitude in scales. These two fractals appear to be characterized by similar Hausdorff dimensions, between 1.6 and 2. The various interpretations of these structures are discussed, in particular formation theories based on turbulence and self-gravity. In the latter, the fractal ensembles are considered in a critical state, as in second order phase transitions, when large density fluctuations are observed, that also obey scaling laws, and look self-similar over an extended range.Comment: 30 pages, 6 figures, Proceedings of "The Chaotic Universe", Roma colloquium, 1-5 Feb 99, World Scientific Advanced Series in Astrophysics and Cosmology, ed. V. Gurzadyan, Li-Zhi Fang and Remo Ruffin

A Plasma Instability Theory of Gamma-Ray Burst Emission

A new theory for gamma-ray burst radiation is presented. In this theory, magnetic fields and relativistic electrons are created through plasma processes arising as a relativistic shell passes through the interstellar medium. The gamma-rays are produced through synchrotron self-Compton emission. It is found that shocks do not arise in this theory, and that efficient gamma-ray emission only occurs for a high Lorentz factor and a high-density interstellar medium. The former explains the absence of gamma-ray bursts with thermal spectra. The latter provides the Compton attenuation theory with an explanation of why the interstellar medium density is always high. The theory predicts the existence of a class of extragalactic optical transient that emit no gamma-rays.Comment: Presented at the 20 Texas Symposium on Relativistic Astrophysics, December 1998, Paris, France. To appear on the proceedings compact dis

Resolving the Interstellar Medium at the Peak of Cosmic Star Formation

The interstellar medium feeds both the formation of stars and the growth of black holes, making it a key ingredient in the evolution of galaxies. With the advent of the Atacama Large Millimeter/ submillimeter Array (ALMA), we can now probe the interstellar medium within high-redshift galaxies in increasingly exquisite detail. Our recent ALMA observations map the molecular gas and dust continuum emission in sub-millimetre-selected galaxies on 1-5 kpc scales, revealing significant differences in how the gas, dust continuum, and existing stellar emission are distributed within the galaxies. This study demonstrates the power of ALMA to shed new light on the structure and kinematics of the interstellar medium in the early Universe, suggesting that the interpretation of such observations is more complex than typically assumed.Comment: 4 pages, 5 figures, published on The Messenger, No. 173 (September 2018