Abstract
Galactic outflows are ubiquitously observed in star-forming disk galaxies and
are critical for galaxy formation. Supernovae (SNe) play the key role in
driving the outflows, but there is no consensus as to how much energy, mass and
metal they can launch out of the disk. We perform 3D, high-resolution
hydrodynamic simulations to study SNe-driven outflows from stratified media.
Assuming SN rate scales with gas surface density \$\Sigma\_gas\$ as in the
Kennicutt-Schmidt (KS) relation, we find the mass loading factor, defined as
the mass outflow flux divided by the star formation surface density, decreases
with increasing \$\Sigma\_gas\$ as \$\Sigma^-0.61\_gas\$.
Approximately \$\Sigma\_gas łesssim\$ 50 \$M\_ødot/pc^2\$ marks when the
mass loading factor \$\gtrsim\$1. About 10-50\% of the energy and 40-80\% of the
metals produced by SNe end up in the outflows. The tenuous hot phase
(\$T>310^5\$ K) carries the majority of the energy and metals in outflows.
We discuss how various physical processes, including vertical distribution of
SNe, photoelectric heating, external gravitational field and SN rate, affect
the loading efficiencies. The relative scale height of gas and SNe is a very
important factor in determining the loading efficiencies.
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