How can supersonic accretion flows exist in astrophysical scenarios? I often hear about supersonic accretion flows in astrophysical scenarios: for example, the inner parts of the accretion disk around a supermassive black hole or cold filaments accreting into galaxies from the intergalactic medium. In these scenarios, how can the supersonic accretion persist over long timescales? Wouldn't there be shocks that would slow down the flows from supersonic to subsonic (or just heat and destroy the flows altogether)?
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In these scenarios, how can the supersonic accretion persist over long timescales? Wouldn't there be shocks that would slow down the flows from supersonic to subsonic (or just heat and destroy the flows altogether)?

No, not necessarily.  The solar wind is a supersonic flow and the only time it generates a shock wave is if it encounters a magnetic obstacle like the Earth's magnetosphere.  The bulk flow of a fluid or plasma can be supersonic without causing any shocks.  A shock wave occurs when a piston/driver moves faster than the communication speed of the medium.  In the absence of a piston/driver, there will be no shock.
Note that a slower moving parcel of fluid can act as a piston/driver.  So in the case of the acretion disks, the bulk of the dusty plasma is likely all co-moving (at each radial segment) thus no shocks.  There will, however, most likely be things like Kelvin-Helmholtz instabilities.

How can supersonic accretion flows exist in astrophysical scenarios?

So long as there is sufficient energy (and appropriate mechanisms) to accelerate a fluid, a supersonic flow can exist in most mediums for extended time periods in the absence of a piston/driver.
