The Characteristic Momentum of Radiatively Cooling Energy-Driven
  Galactic Winds
  Energy injection by supernovae may drive hot supersonic galactic winds in
rapidly star-forming galaxies, driving metal-enriched gas into the
circumgalactic medium and potentially accelerating cool gas. If sufficiently
mass-loaded, such flows become radiative within the wind-driving region,
reducing the overall mass outflow rate from the host galaxy. We show that this
sets a maximum on the total outflow momentum for hot energy-driven winds. For a
spherical wind of Solar metallicity driven by continuous star formation, $\dot
p_\rm{max} \simeq 1.9\times10^4\ M_\odot/\rm{yr\ km/s}\
(\alpha/0.9)^{0.86}(R_\star/300\ \rm{pc})^{0.14}(\dot M_\star/20\
M_\odot/\rm{yr})^{0.86}$, where $\alpha$ is the fraction of supernova energy
that thermalizes the wind, and $\dot M_\star$ and $R_\star$ are the star
formation rate and radius of the wind-driving region. This maximum momentum for
hot winds can also apply to cool, ionized outflows that are typically observed
in starburst galaxies, if the hot wind undergoes bulk radiative cooling or if
the hot wind transfers mass and momentum to cool clouds within the flow. We
show that requiring the hot wind to undergo single-phase cooling on large
scales sets a minimum on the total outflow momentum rate. These maximum and
minimum outflow momenta have similar values, setting a characteristic momentum
rate of hot galactic winds that can become radiative on large scales. We find
that most observations of photoionized outflow wind momentum fall below the
theoretical maximum and thus may be signatures of cooling hot flows. On the
other hand, many systems fall below the minimum momentum required for bulk
cooling, indicating that perhaps the cool material observed has instead been
entrained in or mixed with the hot flow.
