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 Σ gas as in the Kennicutt-Schmidt (KS) relation, we find the mass loading factor, η m , defined as the mass outflow flux divided by the star formation surface density, decreases with increasing Σ gas as η m ∝ Σ −0.61 gas . Approximately Σ gas 50M / pc 2 marks when η m 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 > 3×10 5 K), which fills 60-80% of the volume at mid-plane, 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.

Galactic outflows produced by stellar feedback are known to be multiphase in nature. Observations and simulations indicate that the material within several kiloparsecs of galactic disk midplanes consists of warm clouds embedded within a hot wind. A theoretical understanding of the outflow phenomenon, including both winds and fountain flows, requires study of the interactions among thermal phases. We develop a method to quantify these interactions via measurements of mass, momentum, and energy flux exchanges using temporally and spatially averaged quantities and conservation laws. We apply this method to a star-forming interstellar medium simulation based on the TIGRESS framework, for solar neighborhood conditions. To evaluate the extent of interactions among the phases, we examine the validity of the “ballistic model,” which predicts the trajectories of the warm phase (5050 K < T < 2 × 104 K) treated as non-interacting clouds. This model is successful at intermediate vertical velocities ( ), but at higher velocities, we observe an excess in simulated warm outflow compared to the ballistic model. This discrepancy cannot be fully accounted for by cooling of high-velocity, intermediate-temperature (2 × 104 K < T < 5 × 105 K) gas. We examine the fluxes of mass, momentum, and energy and conclude that the warm phase gains mass via cooling of the intermediate phase and momentum from the hot (T > 5 × 105 K) phase. The large energy flux from the hot outflow, transferred to the warm and intermediate phases, is quickly radiated away. A simple interaction model implies an effective warm cloud size in the fountain flow of a few 100 pc, showing that warm–hot flux exchange mainly involves a few large clouds rather than many small ones.

Supernovae (SNe) drive multiphase galactic outflows, impacting galaxy formation; however, cosmological simulations mostly use ad hoc feedback models for outflows, making outflow-related predictions from first principles problematic. Recent small-box simulations resolve individual SNe remnants in the interstellar medium (ISM), naturally driving outflows and permitting a determination of the wind loading factors of energy η E , mass η m , and metals η Z . In this Letter, we compile small-box results, and find consensus that the hot outflows are much more powerful than the cool outflows: (i) their energy flux is 2-20 times greater, and (ii) their specific energy e s,h is 10-1000 times higher. Moreover, the properties of hot outflows are remarkably simple: e s,h ∝ η E,h /η m,h is almost invariant over four orders of magnitude of star formation surface density. Also, we find tentatively that η E,h /η Z,h ∼ 0.5. If corroborated by more simulation data, these correlations reduce the three hot phase loading factors into one. Finally, this one parameter is closely related to whether the ISM has a "breakout" condition. The narrow range of e s,h indicates that hot outflows cannot escape dark matter halos with log M halo [M ] 12. This mass is also where the galaxy mass-metallicity relation reaches its plateau, implying a deep connection between hot outflows and galaxy formation. We argue that hot outflows should be included explicitly in cosmological simulations and (semi-)analytic modeling of galaxy formation.

We obtained new optical observations of the X-ray source XMMU J083850.38−282756.8, the previously proposed counterpart of the γ-ray source 3FGL J0838.8−2829. Time-series photometry in the r ′ band reveals periodic modulation of ≈ 1 magnitude that is characteristic of the heating of the photosphere of a low-mass companion star by a compact object. The measured orbital period is 5.14817±0.00012 hr. The shape of the light curve is variable, evidently due to the effects of flaring and asymmetric heating. Spectroscopy reveals a companion of type M1 or later, having a radial velocity amplitude of 315 ± 17 km s −1 , with period and phasing consistent with the heating interpretation. The mass function of the compact object is 0.69 ± 0.11 M ⊙ , which allows a neutron star in a highinclination orbit. Variable, broad Hα emission is seen, which is probably associated with a wind from the companion. These properties, as well as the X-ray and γ-ray luminosities at the inferred distance of < 1.7 kpc, are consistent with a redback millisecond pulsar in its non-accreting state. A search for radio pulsations is needed to confirm this interpretation and derive complete system parameters for modeling, although absorption by the ionized wind could hinder such detection.

Feedback is indispensable in galaxy formation. However, lacking resolutions, cosmological simulations often use ad hoc feedback parameters. Conversely, small-box simulations, while they better resolve the feedback, cannot capture gas evolution beyond the simulation domain. We aim to bridge the gap by implementing small-box results of supernovae-driven outflows into dark matter halo-scale simulations and studying their impact on large scales. Galactic outflows are multiphase, but small-box simulations show that the hot phase (T ≈ 106–7 K) carries the majority of energy and metals. We implement hot outflows in idealized simulations of the Milky Way halo, and examine how they impact the circumgalactic medium. In this paper, we discuss the case when the star formation surface density is low and therefore the emerging hot outflows are gravitationally bound by the halo. We find that outflows form a large-scale, metal-enriched atmosphere with fountain motions. As hot gas accumulates, the inner atmosphere becomes “saturated.” Cool gas condenses, with a rate balancing the injection of the hot outflows. This balance leads to a universal density profile of the hot atmosphere, independent of mass outflow rate. The atmosphere has a radially decreasing temperature, naturally producing the observed X-ray luminosity and column densities of O vi, O vii, and O viii. The self-regulated atmosphere has a baryon and a metal mass of (0.5–1.2) × 1010 M ⊙ and (0.6–1.4) × 108 M ⊙, respectively, small compared to the “missing” baryons and metals from the halo. We conjecture that the missing materials reside at even larger radii, ejected by more powerful outflows in the past.