|Annu. Rev. Astron. Astrophys. 2005. 43:
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Although great strides have been made over the past 25 years in understanding the physics and impact of GWs in the local and distant universe, much work remains to be done to quantify the role of these winds on the formation and evolution of galaxy-sized structures and the intergalactic environment. We now outline observational and theoretical issues that we feel deserve urgent attention.
8.1. Observational Challenges
8.1.1. UNBIASED CENSUS OF LOCAL WINDS As noted in Section 4.1, our current sample of GWs detects outflows that are sufficiently large and/or powerful but not so energetic as to expel all gas from their hosts. For example, the Galaxy's wind (Section 3.2) would be undetectable beyond the Local Group. While it will be difficult to detect blown-away relics, there is a clear need to search the local volume systematically for winds. At optical wavelengths, the advent of tunable filters on 8-meter class telescopes will improve tenfold the sensitivity of optical wind surveys. These instruments will be ideal for searching for galaxies with starburst-driven winds through the contrast in gaseous excitation between wind and star-forming disk (Section 4.8). An IF spectrometer equipped with adaptive optics would complement tunable filters by providing densely sampled data on kinematics, filling factor, and excitation processes. CXO and XMM-Newton will continue to harvest high-quality data on the hot medium in GWs. New long-wavelength radio telescopes (e.g., GMRT, GBT, EVLA, and SKA) can better search for the relativistic component of GWs. Particularly important will be to determine the relative importance of GWs in dwarf and massive galaxies (Section 4.5).
8.1.2. WIND FLUID This component drives starburst-driven winds, yet has been detected in very few objects. Metal abundances suggest enrichment by SNe II, but the measurements are highly uncertain. Both sensitivity and high spatial resolution are needed to isolate the hot wind fluid from X-ray stellar binaries and the rest of the X-ray-emitting gas. But, no such instrument is planned for the foreseeable future. Indirect methods that rely on the properties of gas in the energy injection zone to constrain the wind pressure may be necessary. Current measurements of the pressure profiles in wind galaxies are certainly contaminated by the foreground/background disk ISM. Measurements in the mid- or far-IR with the Spitzer Space Telescope (SST) and Herschel Space Observatory will reduce the effects of dust obscuration.
8.1.3. ENTRAINED MOLECULAR GAS & DUST Despite the important role of the molecular component in GWs, high-quality mm-wave data exist only for M82. This is due to the limited sensitivity and spatial resolution of current instruments, but this will change soon. New mm-wave arrays (e.g., CARMA, and especially ALMA) will map the molecular gas in a large sample of nearby galaxies with excellent resolution (< 1"). Sub-mm and mid-IR data from the ground (e.g., SMA, JCMT, CSO) and from space (e.g., SST and Herschel) will constrain the amount and location of dust in the winds.
8.1.4. ZONE OF INFLUENCE & ESCAPE EFFICIENCY The environmental impact of GWs depends on the size of their zone of influence and on the fraction of wind fluid and entrained ISM that can vent from their hosts. Very deep emission-line, X-ray, and radio data on large scale would help tremendously to constrain wind extent. Tunable filters on 8-meter class telescopes may be particularly useful here. Absorption-line studies of bright background galaxies (e.g., high-z quasars, LBGs) have proven to be a very powerful tool to constrain the zone of influence of GWs at large redshifts. The Cosmic Origins Spectrograph (COS) on HST could extend the sample to a larger set of wind galaxies. Deep 21-cm maps of GW hosts on scales of up to ~ 100 kpc would help to quantify the effects of halo drag. The escape efficiency of winds may also be constrained indirectly by measuring the stellar metallicities of galaxies suspected to have experienced GWs (e.g., largely gas-free dwarf spheroids in the Local Group) and then comparing these values with the predictions of leaky-box models (e.g., Lanfrancini & Matteucci 2004).
8.1.5. THERMALIZATION EFFICIENCY Observational constraints on the thermalization efficiency of GWs are rare because of an incomplete accounting of the various sources of thermal energy and KE in the wind. A multiwavelength approach that considers all gas phases is needed.
8.1.6. WIND/ISM INTERFACE & MAGNETIC FIELDS Constraints on microphysics at the interface between the wind and galaxy ISM are available in only a handful of galaxies. High-resolution ( parsec scale) imaging and spectra of the entrained disk material in a sizable sample of local objects are required. The large-scale morphology of the magnetic field lines has been mapped in a few winds, but the strength of the field on pc scale is unknown. This information is crucial in estimating the conductivity between the hot and cold fluids.
8.1.7. GALACTIC WINDS IN THE DISTANT UNIVERSE Absorption-line studies of high-z galaxies and QSOs will remain a powerful tool to search for distant GWs and to constrain their environmental impact. Future large ground and space telescopes will extend such studies to the reionization epoch. These galaxies are very faint, but gravitational lensing by foreground clusters can make them detectable and even spatially resolved. Cross-correlation analyses of wind galaxy surveys with detailed maps of the cosmic background radiation (CBR) (e.g., from the Planck mission) may also help to constrain the extent of the hot medium in winds by means of the Sunyaev-Zel'dovich effect (e.g., Voit 1994; Scannapieco & Broadhurst 2001), although one will need to consider all other foreground sources that affect the CBR (Hernàndez-Monteagudo, Genova-Santos, & Atrio-Barandela 2004 and references therein).
8.1.8. POSITIVE FEEDBACK BY WINDS Star-forming radio jet/gas interactions have been found in a few nearby systems (e.g., Minkowski's Object: van Breugel et al. 1985; Cen A: Oosterloo & Morganti 2005) and are suspected to be responsible for the "alignment effect" between the radio and UV continua in distant radio galaxies (e.g., van Breugel et al. 2004 and references therein). The same physics may also provide positive feedback in wind galaxies. Convincing evidence for superbubble-induced star formation has recently been found in the disk of our own Galaxy (Oey et al. 2005). In Section 4.3 we noted that shocked H2 gas and circumnuclear rings of H II regions in a few wind galaxies may represent wind-induced star formation at the contact discontinuity/ISM shock associated with lateral stagnation of the wind in the galaxy disk. This region is also a gas reservoir from which to fuel the starburst. We do not know how often such rings form. Excess free-free emission on the inner edge of the outflow near the disk in M82 has been interpreted as a wind-induced starburst (Matsushita et al. 2004). A galaxy companion within the zone of influence of a GW may also be searched for wind-induced starburst activity (e.g., Irwin et al. 1987). This effect may have triggered the starburst in NGC 3073, a companion to NGC 3079 (e.g., Filippenko & Sargent 1992).
8.2. Theoretical Challenges
8.2.1. MODELING THE ENERGY SOURCE Current simulations do a poor job of modeling the energy source itself, especially in AGN-driven winds where energy and momentum injection rates are virtually unknown. Deeper understanding of AGN jets and winds is needed before simulating the impact of AGN-driven outflows. The situation for starburst-driven winds is much better, but the input energetics are still highly uncertain because the thermalization efficiency is constrained poorly by observation and theory. Simulations by Thornton et al. (1998) have shown that radiative losses of SN remnants expanding into uniform media of ~ 0.02 - 10 cm-3 are ~ 0% - 90%. But, it would be useful to run more realistic simulations with a range of molecular filling factors for young star clusters that evolve within a multiphase ISM.
8.2.2. MODELING THE HOST ISM The work of Sutherland et al. (2003b) noted in Section 2.4 is the first of a new generation of simulations able to handle a multiphase ISM with a broad range of densities and temperatures. Such sophistication is crucial to understanding and predicting the mass of gas entrained in winds. As discussed in Section 2.4, simulations show that the initial encounter of clouds with a wind drives a strong shock that may devastate the clouds. Once in ram-pressure equilibrium with the wind, however, clouds may accelerate to a significant fraction of the wind velocity before RT and KH instabilities shred them. To test the survival of entrained gas, these hydro processes should be combined with the effects of conductive evaporation to model the interface between the hot wind fluid and the dense ISM clouds. On the large scale, it will be important to use realistic distributions for the galaxy ISM, accounting for the clumpiness of the halo component and the disk (e.g., Fig. 1), and possible large-scale magnetic fields. These simulations would quantify the drag of the halo gas, the impact of wind on disk ISM, and the feedback from wind-induced star formation.
8.2.3. COUPLING THE RADIATION FIELD TO GAS Current simulations do not account for possible coupling between the wind material and the radiation field emitted by the energy source or the wind itself, and indeed ignore radiation pressure. For instance, thick, dusty ISM clouds entrained in the wind may be photoionized by the hot wind fluid and radiative shocks at the wind/ISM interface, with major impact on their gaseous ionization.
Acknowledgements This article was begun while S.V. was on sabbatical at the California Institute of Technology and the Observatories of the Carnegie Institution of Washington; this author thanks both institutions for their hospitality. G.C. thanks CTIO for its hospitality. We thank the scientific editor, J. Kormendy, for constructive comments on the style and contents of this review, and S. Aalto, P.-A. Duc, R. Braun, D. Forbes, L. Tacconi, D. Calzetti for organizing recent conferences that provided excellent venues to discuss many of the issues reviewed here. S.V. acknowledges partial support of this research by NSF/CAREER grant AST-9874973.