The nature of astrophysical shocks

Energetic phenomena are common in the Universe: Stars inject matter into the interstellar medium via high-velocity winds at their birth and during their lifetime, and massive stars explode as supernovae when they die. Observations of active galaxies reveal rapidly expanding radio sources and jets which appear to be blasting into the ambient medium. On yet larger scales, over-dense regions of the Universe draw matter in at high velocity by gravitational attraction, whereas under-dense regions can expand at high velocity into their surroundings. In many of these cases, the sound speed of the ambient medium is less than the velocity of the gas expanding into it, especially if radiative cooling is efficient. In this case there is no way for the ambient medium to respond to the energy injection in a smooth way. Instead a near discontinuity is produced, a shock, which suddenly accelerates, heats, and compresses the ambient gas. Shocks often govern the dynamics of astrophysical plasmas: they transmit energy from stars to the interstellar medium, they can compress gas past the point of gravitational instability so that stars form, they may terminate the growth of protostars by driving the ambient gas away, they efficiently destroy dust grains and thereby determine interstellar gas phase abundances, and they are efficient accelerators of highenergy particles. Shocks are particularly important in astronomy because they heat gas and cause it to radiate, providing valuable diagnostics for the underlying energetic phenomena and for the ambient medium%.