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

Blueshifted absorption lines in the UV and X-ray spectra of active galaxies reveal the presence of massive outflows of ionized gas from their nuclei. The intrinsic UV and X-ray absorbers show large global covering factors of the central continuum source, and the inferred mass loss rates are comparable to the mass accretion rates. Many absorbers show variable ionic column densities which are attributed to a combination of variable ionizing flux and motion of gas into and out of the line of sight . Detailed studies of the intrinsic absorbers. with the assistance of monitoring observations and photoionization models. provide constraints on their kinematics] physical conditions. and locations relative to the central continuum source. which range from the inner nucleus (approx.0.01 pc) to the galactic disk or halo (approx.10 kpc) . Dynamical models that make use of thermal winds. radiation pressure. and/or hydromagnetic flows have reached a level of sophistication that permits comparisons with the observational constraints .

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Mass Loss from the Nuclei of Active Galaxies

Using a simple description of feedback from black hole growth, we develop an analytic model for the fueling of Seyferts (low-luminosity active galactic nuclei [AGNs]) and their relation to their host galaxies, Eddington ratio distributions, and cosmological evolution. We derive a solution for the time evolution of accretion rates in a feedback-driven blast wave, applicable to large-scale outflows from bright quasars in galaxy mergers, low-luminosity AGNs, and black holes or neutron stars in supernova remnants. Under the assumption that cold gas stochastically accretes onto a central supermassive black hole at a rate set by the dynamics of that gas, our solution determines the evolution of Seyfert light curves. Using this model, we predict the Seyfert luminosity function, duty cycles and AGN "lifetimes," and the distribution of host morphologies, Eddington ratios, and obscuration as a function of AGN luminosity and black hole mass, and we find agreement with observations at z = 0. We consider the breakdown of the contribution from this mechanism and from stellar wind and virialized hot gas accretion and merger-driven activity. We also make specific predictions for the weak evolution of the Seyfert luminosity function; i.e., luminosity function of quiescent as opposed to merger-driven activity, as a function of redshift, and for changes in both the slope and scatter of the MBH-σ relation at low MBH. Our modeling provides a quantitative and physical distinction between local, low-luminosity quiescent AGN activity and violent, merger-driven bright quasars. In our picture, the quiescent mode of fueling dominates over a wide range of luminosities (-15 MB -22) at z = 0, where most black hole growth occurs in objects with MBH 107 M☉, in S0 and Sa/b galaxies. However, quasar activity from gas-rich mergers evolves more rapidly with redshift, and by z = 1, quiescent fueling is important only at luminosities an order of magnitude or more below the "break" in the luminosity function. Consequently, although non-merger-driven fueling is important for black hole growth and the MBH-σ relation at low MBH, it does not significantly contribute to the black hole mass density of the universe or to cosmological backgrounds.

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Fueling Low-Level AGN Activity through Stochastic Accretion of Cold Gas*

Although soft X-ray absorption features in AGNs were discovered almost ten years ago, the nature and location of the gas creating them has remained controversial Guided by the results of recent high-resolution X-ray spectroscopy, we argue that these features are created in a multitemperature wind whose source of matter is photoionized evaporation from the inner edge of the obscuring torus often found surrounding an AGN Photoionized evaporation in the presence of a copious mass source locks the ratio of ionizing intensity to pressure to a critical value However, a broad range of temperatures can coexist in equilibrium for this value of the ratio of ionizing intensity to pressure Consequently, the flow is expected to be strongly inhomogeneous in temperature The inferred distance of this material from the source of ionizing radiation depends on how much matter exists at the highest obtainable temperature This distance can be measured by monitoring how ionic column densities respond to changes in the ionizing continuum on timescales of days to years

Warm Absorbers in Active Galactic Nuclei: A Multitemperature Wind

We present the first three-dimensional magnetohydrodynamic (MHD) simulations of a circumbinary disk surrounding an equal-mass binary. The binary maintains a fixed circular orbit of separation a. As in previous hydrodynamic simulations, strong torques by the binary can maintain a gap of radius {approx_equal} 2a. Streams curve inward from r {approx_equal} 2a toward the binary; some of their mass passes through the inner boundary, while the remainder swings back out to the disk. However, we also find that near its inner edge the disk develops both a strong m = 1 asymmetry and growing orbital eccentricity. Because the MHD stresses introduce more matter into the gap, the total torque per unit disk mass is {approx_equal} 14 times larger than found previously. The inner boundary accretion rate per unit disk mass is {approx_equal} 40 times greater than found from previous hydrodynamic calculations. The implied binary shrinkage rate is determined by a balance between the rate at which the binary gains angular momentum by accretion and loses it by gravitational torque. The large accretion rate brings these two rates nearly into balance, but in net, we find that a-dot /a < 0, and its magnitude is about 2.7 times larger than predicted bymore » the earlier hydrodynamic simulations. If the binary comprises two massive black holes, the accretion rate may be great enough for one or both to be an active galactic nucleus, with consequences for the physical state of the gas both in the disk body and in its inner gap.« less

Three-dimensional magnetohydrodynamic simulations of circumbinary accretion disks: disk structures and angular momentum transport

Numerical hydrodynamics simulations are presented of X-ray heated winds believed to form the 'mirrors' which reflect light from obscured Seyfert galaxy nuclei into our line of sight. Above a likely range of parameters, it is shown that steady state flows are created when a cool torus surrounds an AGN. Some of the mass evaporated off the inner edge of the torus is captured by the central gravitational well, while the rest is driven off in a warm wind. When the central gravity is strong relative to the radiative heating rate, i.e., L/L(E) is small, nearly all the injected matter is captured. When the gravity is weaker, a given combination of heating rate and injected pressure leads to a maximum mass loss rate; when the rate at which mass is injected exceeds this amount, the excess is captured. The dividing line between these two cases comes at L/L(E) = about 0.08.

Dinshaw S. Balsara

Papers

Numerical simulation of X-ray-heated winds in Seyfert galaxies. I - The case of zero angular momentum