746 research outputs found
Feedback Regulated Turbulence, Magnetic Fields, and Star Formation Rates in Galactic Disks
We use three-dimensional magnetohydrodynamic (MHD) simulations to investigate
the quasi-equilibrium states of galactic disks regulated by star formation
feedback. We incorporate effects from massive-star feedback via time-varying
heating rates and supernova (SN) explosions. We find that the disks in our
simulations rapidly approach a quasi-steady state that satisfies vertical
dynamical equilibrium. The star formation rate (SFR) surface density
self-adjusts to provide the total momentum flux (pressure) in the vertical
direction that matches the weight of the gas. We quantify feedback efficiency
by measuring feedback yields, \eta_c\equiv P_c/\Sigma_SFR (in suitable units),
for each pressure component. The turbulent and thermal feedback yields are the
same for HD and MHD simulations, \eta_th~1 and \eta_ turb~4, consistent with
the theoretical expectations. In MHD simulations, turbulent magnetic fields are
rapidly generated by turbulence, and saturate at a level corresponding to
\eta_mag,t~1. The presence of magnetic fields enhances the total feedback yield
and therefore reduces the SFR, since the same vertical support can be supplied
at a smaller SFR. We suggest further numerical calibrations and observational
tests in terms of the feedback yields.Comment: To appear in Proceedings of IAU Symposium 315, From interstellar
clouds to star-forming galaxies: universal processes?, P. Jablonka, P. Andre,
and F.. van der Tak, ed
Vertical Equilibrium, Energetics, and Star Formation Rates in Magnetized Galactic Disks Regulated by Momentum Feedback from Supernovae
Recent hydrodynamic (HD) simulations have shown that galactic disks evolve to
reach well-defined statistical equilibrium states. The star formation rate
(SFR) self-regulates until energy injection by star formation feedback balances
dissipation and cooling in the interstellar medium (ISM), and provides vertical
pressure support to balance gravity. In this paper, we extend our previous
models to allow for a range of initial magnetic field strengths and
configurations, utilizing three-dimensional, magnetohydrodynamic (MHD)
simulations. We show that a quasi-steady equilibrium state is established as
rapidly for MHD as for HD models unless the initial magnetic field is very
strong or very weak, which requires more time to reach saturation. Remarkably,
models with initial magnetic energy varying by two orders of magnitude approach
the same asymptotic state. In the fully saturated state of the fiducial model,
the integrated energy proportions E_kin:E_th:E_mag,t:E_mag,o are
0.35:0.39:0.15:0.11, while the proportions of midplane support
P_turb:P_th:\Pi_mag,t:\Pi_mag,o are 0.49:0.18:0.18:0.15. Vertical profiles of
total effective pressure satisfy vertical dynamical equilibrium with the total
gas weight at all heights. We measure the "feedback yields"
\eta_c=P_c/\Sigma_SFR (in suitable units) for each pressure component, finding
that \eta_turb~4 and \eta_th~1 are the same for MHD as in previous HD
simulations, and \eta_mag,t~1. These yields can be used to predict the
equilibrium SFR for a local region in a galaxy based on its observed gas and
stellar surface densities and velocity dispersions. As the ISM weight (or
dynamical equilibrium pressure) is fixed, an increase in from turbulent
magnetic fields reduces the predicted \Sigma_SFR by ~25% relative to the HD
case.Comment: To appear in Ap
Momentum Injection by Supernovae in the Interstellar Medium
Supernova (SN) explosions deposit prodigious energy and momentum in their
environments, with the former regulating multiphase thermal structure and the
latter regulating turbulence and star formation rates in the interstellar
medium (ISM). However, systematic studies quantifying the impact of SNe in
realistic inhomogeneous ISM conditions have been lacking. Using
three-dimensional hydrodynamic simulations, we investigate the dependence of
radial momentum injection on both physical conditions (considering a range of
mean density n=0.1-100) and numerical parameters. Our inhomogeneous simulations
adopt two-phase background states that result from thermal instability in
atomic gas. Although the SNR morphology becomes highly complex for
inhomogeneous backgrounds, the radial momentum injection is remarkably
insensitive to environmental details. For our two-phase simulations, the final
momentum produced by a single SN is given by 2.8*10^5 M_sun*km/s n^{-0.17}.
This is only 5% less than the momentum injection for a homogeneous environment
with the same mean density, and only 30% greater than the momentum at the time
of shell formation. The maximum mass in hot gas is quite insensitive to
environmental inhomogeneity. Strong magnetic fields alter the hot gas mass at
very late times, but the momentum injection remains the same. Initial
experiments with multiple spatially-correlated SNe show a momentum per event
nearly as large as single-SN cases. We also present a full numerical parameter
study to assess convergence requirements. For convergence in the momentum and
other quantities, we find that the numerical resolution dx and the initial size
of the SNR r_init must satisfy dx, r_init<r_sf/3, where the shell formation
radius is given by r_sf = 30 pc n^{-0.46} for two-phase models (or 30% smaller
for a homogeneous medium).Comment: 51 pages, 16 figures, accepted for publication in Ap
Three Dimensional Hydrodynamic Simulations of Multiphase Galactic Disks with Star Formation Feedback: II. Synthetic HI 21 cm Line Observations
We use three-dimensional numerical hydrodynamic simulations of the turbulent,
multiphase atomic interstellar medium (ISM) to construct and analyze synthetic
HI 21 cm emission and absorption lines. Our analysis provides detailed tests of
21 cm observables as physical diagnostics of the atomic ISM. In particular, we
construct (1) the "observed" spin temperature, , and its optical-depth weighted mean
T_s,obs; (2) the absorption-corrected "observed" column density,
; and (3) the "observed"
fraction of cold neutral medium (CNM), for T_c
the CNM temperature; we compare each observed parameter with true values
obtained from line-of-sight (LOS) averages in the simulation. Within individual
velocity channels, T_s,obs(v_ch) is within a factor 1.5 of the true value up to
. As a consequence, N_H,obs and T_s,obs are
respectively within 5% and 12% of the true values for 90% and 99% of LOSs. The
optically thin approximation significantly underestimates N_H for .
Provided that T_c is constrained, an accurate observational estimate of the CNM
mass fraction can be obtained down to 20%. We show that T_s,obs cannot be used
to distinguish the relative proportions of warm and thermally-unstable atomic
gas, although the presence of thermally-unstable gas can be discerned from 21
cm lines with 200K<<1000K. Our mock observations
successfully reproduce and explain the observed distribution of the brightness
temperature, optical depth, and spin temperature in Roy et al. (2013a). The
threshold column density for CNM seen in observations is also reproduced by our
mock observations. We explain this observed threshold behavior in terms of
vertical equilibrium in the local Milky Way's ISM disk.Comment: 34 pages, 12 figures. Accepted for publication in ApJ. For Paper I,
see http://arxiv.org/abs/1308.323
Three Dimensional Hydrodynamic Simulations of Multiphase Galactic Disks with Star Formation Feedback: I. Regulation of Star Formation Rates
The energy and momentum feedback from young stars has a profound impact on
the interstellar medium (ISM), including heating and driving turbulence in the
neutral gas that fuels future star formation. Recent theory has argued that
this leads to a quasi-equilibrium self-regulated state, and for outer
atomic-dominated disks results in the surface density of star formation
varying approximately linearly with the weight of the ISM (or
midplane turbulent + thermal pressure). We use three-dimensional numerical
hydrodynamic simulations to test the theoretical predictions for thermal,
turbulent, and vertical dynamical equilibrium, and the implied functional
dependence of on local disk properties. Our models demonstrate
that all equilibria are established rapidly, and that the expected
proportionalities between mean thermal and turbulent pressures and
apply. For outer disk regions, this results in , where is the total gas surface
density and is the midplane density of the stellar disk (plus dark
matter). This scaling law arises because sets the vertical
dynamical time in our models (and outer disk regions generally). The
coefficient in the star formation law varies inversely with the specific energy
and momentum yield from massive stars. We find proportions of warm and cold
atomic gas, turbulent-to-thermal pressure, and mean velocity dispersions that
are consistent with Solar-neighborhood and other outer-disk observations. This
study confirms the conclusions of a previous set of simulations, which
incorporated the same physics treatment but was restricted to radial-vertical
slices through the ISM.Comment: 20 pages, 17 figures. Accepted for publication in Ap
Numerical Modeling of Multiphase, Turbulent Galactic Disks with Star Formation Feedback
Star formation is self-regulated by its feedback that drives turbulence and
heats the gas. In equilibrium, the star formation rate (SFR) should be directly
related to the total (thermal plus turbulent) midplane pressure and hence the
total weight of the diffuse gas if energy balance and vertical dynamical
equilibrium hold simultaneously. To investigate this quantitatively, we utilize
numerical hydrodynamic simulations focused on outer-disk regions where diffuse
atomic gas dominates. By analyzing gas properties at saturation, we obtain
relationships between the turbulence driving and dissipation rates, heating and
cooling rates, the total midplane pressure and the total weight of gas, and the
SFR and the total midplane pressure. We find a nearly linear relationship
between the SFR and the midplane pressure consistent with the theoretical
prediction.Comment: 2 pages, 1 figure. To appear in the proceeding of the IAU GA XXVIII,
Special Session 12: Modern Views of the Interstellar Mediu
Numerical Simulations of Multiphase Winds and Fountains from Star-Forming Galactic Disks: I. Solar Neighborhood TIGRESS Model
Gas blown away from galactic disks by supernova (SN) feedback plays a key
role in galaxy evolution. We investigate outflows utilizing the solar
neighborhood model of our high-resolution, local galactic disk simulation
suite, TIGRESS. In our numerical implementation, star formation and SN feedback
are self-consistently treated and well resolved in the multiphase, turbulent,
magnetized interstellar medium. Bursts of star formation produce spatially and
temporally correlated SNe that drive strong outflows, consisting of hot
(T>5x10^5K) winds and warm (5050K < T < 2x10^4K) fountains. The hot gas at
distance d>1kpc from the midplane has mass and energy fluxes nearly constant
with d. The hot flow escapes our local Cartesian box barely affected by gravity
and is expected to accelerate up to the terminal velocity of
v_wind~350-500km/s. The mean mass and energy loading factors of the hot wind
are 0.1 and 0.02, respectively. For warm gas, the mean outward mass flux
through d=1kpc is comparable to the mean star formation rate, but only a small
fraction of this gas is at velocity >50km/s. Thus, the warm outflows eventually
fall back as inflows. The warm fountain flows are created by expanding hot
superbubbles at d< 1kpc; at larger d neither ram pressure acceleration nor
cooling transfers significant momentum or energy flux from the hot wind to the
warm outflow. The velocity distribution at launching near d~1kpc better
represents warm outflows than a single mass loading factor, potentially
enabling development of subgrid models for warm galactic winds in arbitrary
large-scale galactic potentials.Comment: Accepted for publication in Ap
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