Turbulent Dissipation in the Interstellar Medium: The Coexistence of Forced and Decaying Regimes and Implications for Galaxy Formation and Evolution
V. Avila-Reese, Vá, E. Zquez-Semadeni;
ApJ, 2001, 553, 645
ABSTRACT:We discuss the dissipation of turbulent kinetic energy Ek in the global interstellar medium (ISM) by means of two-dimensional,
MHD, nonisothermal simulations in the presence of model radiative heating
and cooling.
We argue that dissipation in two dimensions is
representative of that in three dimensions as long as it is dominated by shocks rather
than by a turbulent cascade.
Contrary to previous treatments of
dissipation in the ISM, this work considers realistic, stellar-like forcing:
energy is injected at a few isolated sites in space, over relatively small
scales, and over short time periods.
This leads to the coexistence of forced
and decaying regimes in the same flow, to a net propagation of turbulent
kinetic energy from the injection sites to the decaying regions, and to
different characteristic dissipation rates and times in the forced sites and in
the global flow.
We find that the ISM-like flow dissipates its turbulent
energy rapidly.
In simulations with forcing, the input parameters are the
radius lf of the forcing region, the total kinetic energy
ek each source deposits into the flow, and the rate of formation of those
regions, Σ&d2;OB.
The global dissipation time
td depends mainly on lf.
We find that for most of our
simulations td is well described by a combination of parameters of the
forcing and global parameters of the flow:
td~u2rms/(&epsis;&d2;kf), where urms is the rms velocity dispersion,
&epsis;&d2;k is the specific power of each forcing region, and f is the filling factor
of all these regions.
In terms of measurable properties of the ISM,
td>~<Σg>u2rms/ (ekΣ&d2;OB), where
<Σg> is the average gas surface density; for the solar neighborhood,
td>~1.5×107 yr.
The global dissipation time is consistently smaller than the
crossing time of the largest energy-containing scales, suggesting that the
local dissipation time near the sources must be significantly smaller than
what would be estimated from large-scale quantities alone.
In decaying
simulations, we find that the kinetic energy decreases with time as
Ek(t)~t-α, where α~0.8-0.9.
This result can be translated into a decay
with distance l when applied to the mixed forced-plus-decaying case,
giving Ek~l-2α/(2-α) at large
distances from the sources.
Our results, if applicable in the direction
perpendicular to galactic disks, support models of galaxy evolution in which stellar
energy injection provides significant support for the gas disk thickness but
do not support models in which this energy injection is supposed to reheat
an intrahalo medium at distances of up to 10-20 times the optical galaxy
size, as the dissipation occurs on distances comparable to the disk
height.
However, this conclusion is not definitive until the effects of stratification
on our results are tested.
KEYWORDS: galaxies: evolution, galaxies: ism, ism: kinematics and dynamics, magnetohydrodynamics: mhd, stars: formation, turbulence
CODE: avila-reese2001