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.Comment: Paper to be published in 2005 Annual Review of Astronomy & Astrophysics; revision based on comments from readers and production editor. Figure 1 was replaced to show the proper density scale. A PDF file combining both text and figures is available at http://www.astro.umd.edu/~veilleux/pubs/araa.pd

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The remarkable similarity between emission spectra of narrow line regions (NLR) in Seyfert Galaxies has long presented a mystery. In photoionization models, this similarity implies that the ionization parameter is nearly always the same, about U ∼ 0.01. Here we present dusty, radiation-pressure dominated photoionization models that can provide natural physical insight into this problem. In these models, dust and the radiation pressure acting on it provide the controlling factor in moderating the density, excitation and surface brightness of photoionized NLR structures. Additionally, photoelectric heating by the dust is important in determining the temperature structure of the models. These models can also explain the coexistence of the low-, intermediate-and coronal ionization zones within a single self-consistent physical structure. The radiation pressure acting on dust may also be capable of driving the fast (∼ 3000 km s −1 ) outflows such as are seen in the HST observations of NGC 1068.

HST WFPC2 images are presented that span the inner ∼19 kpc diameter of the edge-on galaxy NGC 3079; they are combined with optical, emission-line imaging spectrophotometry and VLA images of radio polarization vectors and rotation measures. Ionized gas filaments within 9-kpc diameter project up to 3 kpc above the disk, with the brightest forming the ≈ 1 kpc diameter superbubble. They are often resolved into strands ≈ 0. ′′ 3 (25 pc) wide, which emerge from the nuclear CO ring as five distinct streams with large velocities and velocity dispersions (FWHM ≈450 km s −1 ). The brightest stream emits ≈ 10% of the superbubble Hα flux and extends for 250 pc along the axis of the VLBI radio jet to one corner of the base of the superbubble. The other four streams are not connected to the jet, instead curving up to the vertical ≈ 0.6 kpc above the galaxy disk, then dispersing as a spray of droplets each with ≈ 10 3 √ f M of ionized gas (the volume filling factor f > 3 × 10 −3 ). Shredded clumps of disk gas form a similar structure in hydrodynamical models of a galaxy-scale wind. The pattern of magnetic fields and the gaseous kinematics also suggest that a wind of mechanical luminosity L w ≈ 10 43 ergs s −1 has stagnated in the galaxy disk at a radius of ∼ 800 pc, has flared to larger radii with increasing height as the balancing ISM pressure reduces above the disk, and has entrained dense clouds into a "mushroom vortex" above the disk. Hα emissivity of the filaments limits densities to n e > 4.3 f −1/2 cm −3 , hence kinetic energy and momentum to (0.4 − 5) × 10 55 √ f ergs and (1.6 − 6) × 10 47 √ f dyne s, respectively; the ranges result from uncertain space velocities. A prominent star-forming complex elsewhere in the galaxy shows a striking spray of linear filaments that extend for hundreds of parsecs to end in unresolved "bullets."