Evidence and Implications of Pressure Fluctuations in the ISM

Abstract

A recent survey of the fine-structure excitation of neutral carbon reveals that the interstellar medium in the Galactic plane exhibits a thermal pressure, nT/k, that ranges from about 10^3 to 10^4 cm{-3}K from one location to the next, with occasional excursions in excess of about 10^5 cm^{-3}K. The large excitations for small amounts of gas indicate that some regions are either subjected to shocks or must be pressurized within time scales much shorter than the time needed to reach thermal equilibrium. These rapid fluctuations probably arise from the cascade of macroscopic mechanical energy to small scales through a turbulent cascade. One consequence of this effect is that changes in gas temperature can arise from near adiabatic compressions and expansions, and this may explain why investigations of 21-cm emission and absorption reveal the presence of hydrogen at temperatures well below the expected values derived from the balance of various known heating and cooling processes.

10 pages, 3 figures, Invited Talk for the JENAM 2002 Workshop "From Observations to Self-Consistent Modelling of the ISM in Galaxies", Kluwer Academic Publishers (in press)

References in corpus (6)

• The Millennium Arecibo 21-CM Absorption Line Survey. II. Properties of the Warm and Cold Neutral Media
• Electron-Ion Recombination on Grains and Polycyclic Aromatic Hydrocarbons
• The Temperature Distribution in Turbulent Interstellar Gas
• New Temperatures of Diffuse Interstellar Gas: Thermally Unstable Gas
• Fitting Together the HI Absorption and Emission in the SGPS
• Small-Scale Interstellar Na I Structure Toward M92

Cited by in corpus (10)

• Interstellar Turbulence I: Observations and Processes
• A Self-Absorption Census of Cold HI Clouds in the Canadian Galactic Plane Survey
• Numerical Study of Turbulent Mixing Layers with Non-Equilibrium Ionization Calculations
• The pressure distribution in thermally bistable turbulent flows
• Spatial and Temporal Variations in Small-Scale Galactic HI Structure Toward 3C~138
• Interstellar cloud structure: The statistics of centroid velocities
• The Turbulent Interstellar Medium
• Monitoring the Variable Interstellar Absorption toward HD 219188 with HST/STIS
• Simultaneous evolution of the virial parameter and star formation rate in molecular clumps undergoing global hierarchical collapse
• Spatial Variations in Galactic H I Structure on AU-Scales Toward 3C 147 Observed with the Very Long Baseline Array