(updated for 2013)
The goal of the next several sections is to build an understanding of how photons are produced by, are absorbed by, and interact with the ISM. We consider a system in which one or more constituents are excited under certain physical conditions to produce photons, then the photons pass through other constituents under other conditions, before finally being observed (and thus affected by the limitations and biases of the observational conditions and instruments) on Earth. Local thermodynamic equilibrium is often used to describe the conditions, but this does not always hold. Remember that our overall goal is to turn observations of the ISM into physics, and vice-versa.
The following contribute to an observed Spectral Energy Distribution:
- gas: spontaneous emission, stimulated emission (e.g. masers), absorption, scattering processes involving photons + electrons or bound atoms/molecules
- dust: absorption; scattering (the sum of these two -> extinction); emission (blackbody modified by wavelength-dependent emissivity)
- other: synchrotron, brehmsstrahlung, etc.
The processes taking place in our “system” depend sensitively on the specific conditions of the ISM in question, but the following “rules of thumb” are worth remembering:
- Very rarely is a system actually in a true equilibrium state.
- Except in HII regions, transitions in the ISM are usually not electronic.
- The terms Upper Level and Lower Level refer to any two quantum mechanical states of an atom or molecule where . We will use k to index the upper state, and j for the lower state.
- Transitions can be induced by photons, cosmic rays, collisions with atoms and molecules, and interactions with free electrons.
- Levels can refer to electronic, rotational, vibrational, spin, and magnetic states.
- To understand radiative processes in the ISM, we will generally need to know the chemical composition, ambient radiation field, and velocity distribution of each ISM component. We will almost always have to make simplifying assumptions about these conditions.