Energy in the ISM

As discussed on this page, the ISM has multiple components, all of which have different temperatures, densities, and ionization states. As the ISM is a very dynamic environment, clearly it contains nonzero amounts of energy. We are interested in the types of energy stored in the ISM and the relative importance of each component. This is outlined in the pie chart below, adapted from Bruce Draine’s book Physics of the Interstellar and Intergalactic Medium (2011).

ISM energy
Figure showing the distribution of energy in the ISM. Adapted by Alyssa Goodman from Table 1.5 of Draine 2011.

A brief description of each component is as follows (ranked in decreasing order of importance to the total energy balance of the ISM):

  • Cosmic Rays (light yellow slice): Cosmic rays are very high-energy particles that have been accelerated to relativistic speeds. The origin of these particles is still under debate, although a significant fraction are believed to have been accelerated by supernovae.
  • Magnetic Fields (orange slice): Magnetic fields play an important role in shaping the ISM. Developing accurate models of interstellar magnetic fields is an area of ongoing research.
  • Starlight (purple slice): This includes light from stars at all wavelengths.
  • Thermal Energy, or “nkT” (red slice): This is the energy from the random thermal motions of individual particles in the ISM due to their temperature. The hotter the gas, the more thermal energy it has.
  • Far Infrared (FIR) Emission from Dust (blue slice): Dust particles can be modeled as blackbodies, which absorb energy from starlight and other sources and re-emit it in the far infrared.
  • CMB (dark yellow slice): Energy in light from the Cosmic Microwave Background, which is the afterglow of the Big Bang. The CMB is nearly perfect blackbody radiation with a temperature of 3K and contributes a small amount of energy to the ISM. CMB photons were emitted by the hot, dense, glowing plasma that filled the early Universe and have been propagating through space since the Universe was only a few hundred thousand years old. Studying the tiny fluctuations in the CMB spectrum observed at different locations on the sky can reveal what the Universe was like at these early times, which provides vital information to cosmologists seeking to understand how the Universe began and how it evolved to the state in which we find it today. Dust in the ISM can scatter or absorb CMB photons, so these effects must be taken into account before the CMB can be used to provide meaningful cosmological constraints. For more information about the role that the ISM plays as a foreground to CMB measurements, see the module by Kirit Karkare [link].
  • Turbulence (green slice): Much of the ISM is not smooth but rather full of turbulent motions on a wide range of scales. Turbulence may play an important role in star formation.

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