Galactic winds are the primary mechanism by which energy and metals are recycled in galaxies and are deposited into the intergalactic medium New observations are revealing the ubiquity of this process, particularly at high redshift We describe the physics behind these winds, discuss the observational evidence for them in nearby star-forming and active galaxies and in the high-redshift universe, and consider the implications of energetic winds for the formation and evolution of galaxies and the intergalactic medium To inspire future research, we conclude with a set of observational and theoretical challenges

Galactic Winds

Starburst-driven galactic winds are responsible for the transport of mass, in particular metal-enriched gas, and energy out of galaxies and into the intergalactic medium. These outflows directly affect the chemical evolution of galaxies, and heat and enrich the intergalactic and intercluster medium.



Currently, several basic problems preclude quantitative measurements of the impact of galactic winds: the unknown filling factors of, in particular, the soft X-ray-emitting gas prevent accurate measurements of densities, masses and energy content; multiphase temperature distributions of unknown complexity bias X-ray-determined abundances; unknown amounts of energy and mass may reside in hard to observe T∼105 K and T∼107.5 K phases; and the relative balance of thermal versus kinetic energy in galactic winds is not known.



In an effort to address these problems, we perform an extensive hydrodynamical parameter study of starburst-driven galactic winds, motivated by the latest observation data on the best-studied starburst galaxy M82. We study how the wind dynamics, morphology and X-ray emission depend on the ISM distribution of the host galaxy, the starburst star formation history and strength, and the presence and distribution of mass-loading by dense clouds. We also investigate and discuss the influence of finite numerical resolution on the results of these simulations.



We find that the soft X-ray emission from galactic winds comes from low filling factor (η≲2 per cent) gas, which contains only a small fraction (≲10 per cent) of the mass and energy of the wind, irrespective of whether the wind models are strongly mass-loaded or not. X-ray observations of galactic winds do not directly probe the gas that contains the majority of the energy, mass or metal-enriched gas in the outflow.



X-ray emission comes from a complex phase-continuum of gas, covering a wide range of different temperatures and densities. No distinct phases, as are commonly assumed when fitting X-ray spectra, are seen in our models. Estimates of the properties of the hot gas in starburst galaxies based on fitting simple spectral models to existing X-ray spectra should be treated with extreme suspicion.



The majority of the thermal and kinetic energy of these winds is in a volume-filling hot, T∼107 K, component which is extremely difficult to probe observationally because of its low density and hence low emissivity. Most of the total energy is in the kinetic energy of this hot gas, a factor that must be taken into account when attempting to constrain wind energetics observationally. We also find that galactic winds are efficient at transporting large amounts of energy out of the host galaxy, in contrast to their inefficiency at transporting mass out of star-forming galaxies.

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Starburst-driven galactic winds — I. Energetics and intrinsic X-ray emission

We present arcsecond resolution Chandra X-ray and ground-based optical Hα imaging of a sample of 10 edge-on star-forming disk galaxies (seven starburst and three "normal" spiral galaxies), a sample that covers the full range of star formation intensity found in disk galaxies. The X-ray observations make use of the unprecedented spatial resolution of the Chandra X-ray observatory to more robustly than before remove X-ray emission from point sources and hence obtain the X-ray properties of the diffuse thermal emission alone. We have combined the X-ray observations with existing, comparable-resolution, ground-based Hα and R-band imaging and present a mini-atlas of images on a common spatial and surface brightness scale to aid cross-comparison. In general, the morphology of the extraplanar diffuse X-ray emission is very similar to the extraplanar Hα filaments and arcs, on both small and large scales (scales of tens of parsecs and kiloparsecs, respectively). The most spectacular cases of this are found in NGC 1482 (for which we provide the first published X-ray observation) and NGC 3079. We provide a variety of quantitative measures of how the spectral hardness and surface brightness of the diffuse X-ray emission varies with increasing height z above the plane of each galaxy. Of the eight galaxies in which diffuse X-ray emitting halos are found (the starbursts and the normal spiral NGC 891), significant spatial variation in the spectral properties of the extraplanar emission (|z| ≥ 2 kpc) is only found in two cases: NGC 3628 and NGC 4631. In general, the vertical distribution of the halo-region X-ray surface brightness is best described as an exponential, with the observed scale heights of the sample galaxies lying in the range Heff ~ 2-4 kpc. The presence of extraplanar X-ray emission is always associated with the presence of extraplanar optical line emission of similar vertical extent. No X-ray emission was detected from the halos of the two low-mass normal spiral galaxies NGC 6503 and NGC 4244. Active galactic nuclei, where present, appear to play no role in powering or shaping the outflows from the starburst galaxies in this sample. The Chandra ACIS X-ray spectra of extraplanar emission from all these galaxies can be fitted with a common two-temperature spectral model with an enhanced α-to-iron element ratio. This is consistent with the origin of the X-ray emitting gas being either metal-enriched merged SN ejecta or shock-heated ambient halo or disk material with moderate levels of metal depletion onto dust. Our favored model is that SN feedback in the disks of star-forming galaxies create, via blow-out and venting of hot gas from the disk, tenuous exponential atmospheres of density scale height Hg ~ 4-8 kpc. The soft thermal X-ray emission observed in the halos of the starburst galaxies is either this preexisting halo medium, which has been swept up and shock-heated by the starburst-driven wind, or wind material compressed near the walls of the outflow by reverse shocks within the wind. In either case, the X-ray emission provides us with a powerful probe of the properties of gaseous halos around star-forming disk galaxies.

https://iopscience.iop.org/article/10.1086/382214/pdf

A High Spatial Resolution X-Ray and Hα Study of Hot Gas in the Halos of Star-forming Disk Galaxies. I. Spatial and Spectral Properties of the Diffuse X-Ray Emission

We measure the net energy efficiency of supernova (SN) and stellar wind feedback in the starburst galaxy M82 and the degree of mass loading of the hot gas piston driving its superwind by comparing a large suite of one and two-dimensional hydrodynamical models to a set of observational constraints derived from hard X-ray observations of the starburst region (the fluxes of the Heα and Lyα-like lines of S, Ar, Ca, and Fe, along with the total diffuse E = 2-8 keV X-ray luminosity). These are the first direct measurements of the feedback efficiency and mass loading of SN-heated and enriched plasma in a starburst galaxy. We consider a broad range of plausible parameters for the M82 starburst, varying the age and mode of star formation, the starburst region size and geometry, and SN metal yields. Over all these varied input parameters all the models that satisfy the existing observational constraints have medium to high thermalization efficiencies (30% ≤ ≤ 100%) and the volume-filling wind fluid that flows out of the starburst region is only mildly centrally mass loaded (1.0 ≤ β ≤ 2.8). These results imply a temperature of the plasma within the starburst region in the range 30-80 million Kelvin, a mass-flow rate of the wind fluid out of the starburst region of and a terminal velocity of the wind in the range v ∞ = 1410 – 2240 km s–1 . This velocity is considerably larger than the escape velocity from M82 (v esc 460 km s–1 ) and the velocity of the Hα emitting clumps and filaments within M82's wind (v Hα ~ 600 km s–1 ). Drawing on these results we provide a prescription for implementing starburst-driven superwinds in cosmological models of galaxy formation and evolution that more accurately represents the energetics of the hot metal-enriched phases than the existing recipes do.

https://iopscience.iop.org/article/10.1088/0004-637X/697/2/2030/pdf

Supernova feedback efficiency and mass loading in the starburst and galactic superwind exemplar m82

During a 9 month campaign (1996-1997), the Midcourse Space Experiment (MSX) satellite mapped the Galactic plane at mid-infrared wavelengths (4.3-21.3 μm). The much greater spatial resolution and sensitivity of the MSX 8.3 μm band, compared to IRAS at 12 μm, has revealed diffuse emission not seen before. Here we report evidence of a spectacular, limb-brightened, bipolar structure at the Galactic center extending more than a degree (170 pc at 8.0 kpc) on either side of the plane. The 8.3 μm filamentary emission appears confined to a dome-shaped shell, which extends perpendicular to the plane, and shows a tight correlation with the 3, 6, and 11 cm continuum emission over the same scales. The most likely scenario is that the extraplanar 8.3 μm emission arises from dust entrained in a large-scale, bipolar wind powered by a central starburst. The inferred energy injection at the source is ~1054/κ ergs, for which κ is the covering fraction of the dusty shell (κ 0.1). In accord with the stellar record in the Galactic center, we infer that a powerful nuclear starburst has taken place within the last several million years. The inferred energies require 104/ supernovae, where is the thermalization efficiency of the supernova ejecta. There is observational evidence of a Galactic wind on much larger scales, presumably from the same central source that produced the bipolar shell seen by MSX. Sofue has long argued that the North Polar Spur—a thermal X-ray/radio loop that extends from the Galactic plane to b = +80°—was powered by a nuclear explosion [ × 1055 ergs] roughly 15 Myr ago, although his published models did not explain the projected loop structure. We demonstrate that an open-ended bipolar model, when viewed in projection in the near field, provides the most natural explanation for the observed loop structure. The ROSAT 1.5 keV diffuse X-ray map over the inner 45° provides compelling evidence for this interpretation. Since the faint bipolar emission would be very difficult to detect beyond the Galaxy, the phenomenon of large-scale galactic winds may be far more common than has been observed to date. The derived energetics for the bipolar wind are of the order of 1055 ergs, in line with the energy requirement of the MSX observations. We infer that the Galactic center is driving large-scale winds into the halo every ~10-15 Myr or so.

The Large-Scale Bipolar Wind in the Galactic Center

We have obtained detailed Fabry-Perot imaging observations of the nearby galaxy M82 in order to understand the physical association between the high-velocity outflow and the starburst nucleus. The high spatial and kinematic resolution of our observations has allowed us to perform photometric analyses of Hα, [N II], and [O III] spectral lines at roughly 100,000 positions across the extent of the galaxy. The observed velocities of the emitting gas in M82 reveal a bipolar outflow of material, originating from the bright starburst regions in the galaxy's inner disk but misaligned with respect to the galaxy spin axis. The deprojected outflow velocity indicated by the optical filaments increases with radius from 525 to 655 km s-1. All three spectral lines show double components in the centers of the outflowing lobes, with the Hα line split by ~300 km s-1 over a region almost 1 kpc in size. The filamentary lobes lie along an axis tilted by 15° with respect to the spin axis, a finding confirmed by the regions of line splitting and by the ionization pattern over the outflow. The filaments are not simple surfaces of revolution, nor is the emission distributed evenly over the surfaces. We model these lobes as a composite of cylindrical and conical structures, collimated in the inner ~500 pc but expanding at a larger opening angle of ~25° beyond that radius. We compare our kinematic model with simulations of starburst-driven winds in which disk material surrounding the source is entrained by the wind. There is some evidence for rotation of the wind filaments about the outflow axis in support of entrainment, and we find strong similarities between the observed and predicted structures. The data reveal a remarkably low [N II]/Hα ratio in the region of the outflow, indicating that photoionization by the nuclear starburst may play a significant role in the excitation of the optical filament gas, particularly near the nucleus. An increase in the [O III]/Hα ratio along the outflow is observed. At larger radii, the line diagnostics and a strong spatial correlation between Hα and soft X-ray filaments are consistent with shock ionization. A smooth spherical halo around M82, extending to at least 2 kpc, is observed in emission lines. We propose that the dusty halo is the primary source of the linearly polarized optical emission. A diffuse ionized medium with enhanced [N II]/Hα emission pervades the stellar disk. We discuss likely sources of ionization and heating.

http://iopscience.iop.org/article/10.1086/305108/pdf

The Asymmetric Wind in M82

We have obtained detailed imaging Fabry-Perot observations of the nearby galaxy M82, in order to understand the physical association between the high-velocity outflow and the starburst nucleus. The observed velocities of the emitting gas in M82 reveal a bipolar outflow of material, originating from the bright starburst regions in the galaxy's inner disk, but misaligned with respect to the galaxy spin axis. The deprojected outflow velocity increases with radius from 525 to 655 km/s. Spectral lines show double components in the centers of the outflowing lobes, with the H-alpha line split by ~300 km/s over a region almost a kiloparsec in size. The filaments are not simple surfaces of revolution, nor is the emission distributed evenly over the surfaces. We model these lobes as a composite of cylindrical and conical structures, collimated in the inner ~500 pc but expanding at a larger opening angle of ~25 degrees beyond that radius. We compare our kinematic model with simulations of starburst-driven winds in which disk material surrounding the source is entrained by the wind. The data also reveal a remarkably low [NII]/H-alpha ratio in the region of the outflow, indicating that photoionization by the nuclear starburst may play a significant role in the excitation of the optical filament gas, particularly near the nucleus.

P. L. Shopbell

Papers

The Asymmetric Wind in M82

The Asymmetric Wind in M82