## Pressure Relations and Vertical Equilibrium in the Turbulent, Multiphase ISM

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Koyama, H.

Ostriker, E. C.

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We use numerical simulations of turbulent, multiphase, self-gravitating gas
orbiting in model disk galaxies to study the relationships among pressure, the
vertical gas distribution, and the ratio of dense to diffuse gas. We show that
the disk height and mean midplane pressure are consistent with effective
hydrostatic equilibrium, provided that the turbulent vertical velocity
dispersion and gas self-gravity are included. Mass-weighted pressures are an
order of magnitude higher than the midplane pressure because self-gravity
concentrates gas and increases the pressure in clouds. We also investigate the
ratio Rmol=M(H2)/M(HI) for our simulations. Blitz and Rosolowsky (2006) showed
that Rmol is proportional to the estimated midplane pressure. For model series
in which the epicyclic frequency, kappa, and gas surface density, Sigma, are
proportional, we recover the empirical relation. For other model series in
which kappa and Sigma are independent, the midplane pressure and Rmol are not
well correlated. We conclude that the molecular fraction -- and star formation
rate -- of a galactic disk inherently depends on its rotational state, not just
the local values of Sigma and the stellar density rho*. The empirical
correlation between Rmol and midplane pressure implies that the "environmental
parameters" kappa, Sigma, and rho* are interdependent in real galaxies,
presumably as a consequence of evolution toward states with Toomre Q near
unity. We note that Rmol in static models far exceeds both the values in our
turbulent simulations and observed values, implying that turbulence is crucial
to obtaining a realistic molecular fraction in the ISM.

Comment: 31 pages including 9 Figures; Accepted for Publication in ApJ

Comment: 31 pages including 9 Figures; Accepted for Publication in ApJ

##### Keywords

Astrophysics