|Annu. Rev. Astron. Astrophys. 2005. 43:
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1.1. Fundamental role
On the largest physical scales, the Universe is shaped by gravity and the dark energy. On galactic scales, gravity is responsible for only part of what we see through telescopes. This has become clear from the most comprehensive numerical simulations involving up to 1010 particles, which purport to track the evolution of individual galaxies to the present day. Although Cold Dark Matter (CDM) simulations account well for the observed large-scale structure, problems emerge in regions of high density and high density contrast. Even the most detailed simulations do not explain adequately what we know of galaxies.
Contemporary astrophysics is entering a new era where a proper understanding of galaxy formation and evolution forces us to confront details of physical processes involved in feedback, including star formation and evolution, energetic and chemical recycling in the interstellar and intergalactic media (ISM, IGM), gas dynamics, and ultimately plasma magnetohydrodynamics. Progress is complicated by: (i) the need for comprehensive data over the full electromagnetic spectrum at comparable sensitivity and spatial resolution; and (ii) a lack of detailed theoretical and numerical models that can accommodate the multi-wavelength observations.
In this review, we confront arguably the dominant feedback in galaxy formation and evolution - galactic winds (GWs). Also important is radiative feedback. But even here, outflows likely play a key role in clearing a path for the escaping radiation (e.g., Dove, Shull, & Ferrara 2000).
1.2. Early History
More than forty years ago, Lynds & Sandage (1963) announced `evidence for an explosion in the center of the galaxy M82.' In the following year, Burbidge, Burbidge, & Rubin (1964) remarked that `the activity in M82 is yet another manifestation of the generation of vast fluxes of energy by processes which are not yet properly understood.' This comment echoed the discovery of the first quasar in 1962, and the slow realization that many extragalactic radio sources must be enormously energetic, powered by processes that could only be guessed at (Hoyle & Fowler 1963). However, these seminal papers note important similarities between M82 and the Crab nebula when comparing their optical and radio properties, and they imply a possible role for nuclear star clusters in driving the central explosion. By the end of the decade, black holes (BHs) were identified as the driving mechanism in quasars and powerful radio sources (Lynden-Bell 1969).
Before the discovery of a central explosion in M82, few workers discussed galactic outflows. The observed corona and radio halo in the Galaxy (Spitzer 1956; Baldwin 1955) motivated Burbidge & Hoyle (1963) to consider whether such a halo was in fact bound to the Galaxy. In 1957, van Woerden, Rougoor & Oort provided the first evidence of outflowing gas from the Galactic Center, and identified an `expanding arm.' Starting with Moore & Spiegel (1968), many authors went on to consider the possibility of a central explosion to power the radial outflow (e.g., van der Kruit 1971; Sanders & Prenderghast 1974; Oort 1977).
In a parallel development, after the detection of broad [O II] 3727 emission in some elliptical galaxies by Osterbrock (1960), Burke (1968) suggested the presence of a galaxy-scale wind. Interest in GWs continued and models grew in sophistication, motivated largely by the observation that ellipticals have very little ISM and therefore may have been swept clean by galaxy-scale outflows (Johnson & Axford 1971; Mathews & Baker 1971).
Over the past 40 years almost the entire electromagnetic spectrum has become accessible, and we have become aware of the ubiquity of GWs. Their study is now a key area of research, and advances in ground-based and spaceborne instrumentation have produced a flurry of high-precision data on spatially resolved winds in nearby galaxies. At low redshift, winds in gas-rich galaxies are studied because the dense ISM accentuates the outflow. The most recent development is the discovery of powerful winds in high-redshift galaxies. These new observations strongly implicate wind-related feedback processes as key to the chemical and thermal evolution of galaxies and the IGM.
1.3. Review structure
Despite the importance of this topic, there has never been a comprehensive review of GWs published in the Annual Reviews of Astronomy and Astrophysics, although articles on closely related topics were written by Tenorio-Tagle & Bodenheimer (1988) on superbubbles and Spitzer (1990) on the hot ISM.
It is only recently that astronomers have been able to compile multiwavelength data on a significant sample of galactic winds. We recognize the need for these observations to be discussed within a universal framework, where hydrodynamical CDM simulations are able to predict the onset of galactic winds and track their evolution in different environments. However, numerical models that give repeatable results at different resolution scales (e.g. mass) have only recently become possible and the published results are limited at the present time (Springel & Hernquist 2003).
The principal goals of our review are to use both observational constraints and theoretical predictions to discuss critically the properties of GWs in the local and distant universe, and to evaluate the importance of these winds for the formation and evolution of galaxies and the IGM. We focus on material published within the past 15 years. Our discussion centers on three fundamental issues:
Section 2 outlines the basic physics behind the GW phenomenon, and presents several useful formulae and prescriptions to help with data interpretation. Sections 3, 4 and 5 provide an observational summary of wind-blown events in local star-forming and active galaxies. Section 6 summarizes the results from surveys at high redshifts, and Section 7 discusses the impact of winds on the galactic and intergalactic environments. In the last section (Section 8), we raise some important, unanswered questions and suggest future directions for research.