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

We present an ISO SWS and ISOPHOT-S, mid-infrared spectroscopic survey of 15 ultraluminous IRAS galaxies (LIR ≥ 1012 L☉). We combine the survey results with a detailed case study, based on arcsecond resolution, near-IR, and millimeter imaging spectroscopy, of one of the sample galaxies (UGC 5101). We compare the near- and mid-IR characteristics of these ultraluminous galaxies to ISO and literature data of 30 starburst and active galactic nuclei (AGN) template galaxies. We find the following: 1. Of the ultraluminous IRAS galaxies in our sample, 70%-80% are predominantly powered by recently formed massive stars, and 20%-30% are powered by a central AGN. These conclusions are based on a new infrared diagnostic diagram involving the ratio of high- to low-excitation mid-IR emission lines on the one hand, and the strength of the 7.7 μm PAH feature on the other hand. 2. At least half of the sources probably have simultaneously an active nucleus and starburst activity in a 1-2 kpc diameter circumnuclear disk/ring. 3. The mid-IR emitting regions are highly obscured [Av(screen) ~ 5-50 or Av(mixed) ~ 50-1000]. In a model where star-forming regions and dense molecular clouds are fully mixed, the ISO-derived, V-band dust extinctions approach the dust column densities inferred from CO millimeter measurements. After correction for these extinctions, we estimate that the star-forming regions in ultraluminous infrared galaxies have ages between 107 and 108 yr, similar to but somewhat larger than those found in lower luminosity starburst galaxies. 4. In the sample we have studied there is no obvious trend for the AGN component to dominate in the most compact, and thus most advanced mergers. Instead, at any given time during the merger evolution, the time-dependent compression of the circumnuclear interstellar gas, the accretion rate onto the central black hole, and the associated radiation efficiency may determine whether star formation or AGN activity dominates the luminosity of the system.

https://iopscience.iop.org/article/10.1086/305576/pdf

What Powers Ultraluminous IRAS Galaxies

Clumpy torus models with N{sub 0} {approx} 5-15 dusty clouds along radial equatorial rays successfully explain AGN infrared observations. The dust has standard Galactic composition, with individual cloud optical depth {tau}{sub V} {approx} 30-100 at visual. The models naturally explain the observed behavior of the 10 {mu}m silicate feature, in particular the lack of deep absorption features in AGNs of any type, and can reproduce the weak emission feature tentatively detected in type 2 QSOs. The clouds' angular distribution must have a soft edge, e.g., Gaussian, and the radial distribution should decrease as 1/r or 1/r{sup 2}. In line with recent interferometry, the ratio of the torus outer to inner radius can be as small as {approx}5-10. The models can produce nearly isotropic IR emission together with highly anisotropic obscuration, as required by observations. Clumpiness implies that the viewing angle determines an AGN classification only probabilistically; a source can display type 1 properties even from directions close to the equatorial plane. The fraction of obscured sources depends not only on the torus angular thickness but also on the cloud number N{sub 0}, and this fraction's observed decrease with luminosity can be explained with a decrease of either parameter. X-ray obscuration,more » too, is probabilistic; resulting from both dusty and dust-free clouds, it might be dominated by the latter, giving rise to the observed QSOs that are X-ray obscured. Observations indicate that the torus and broad-line-emitting clouds form a seamless distribution, with the transition between the two caused by dust sublimation. Torus clouds may have been detected in the outflow component of H{sub 2}O maser emission from two AGNs. Proper-motion measurements of outflow masers, especially in Circinus, are a promising method for probing the morphology and kinematics of torus clouds.« less

https://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1474&context=physastron_facpub

AGN Dusty Tori. II. Observational Implications of Clumpiness

This paper proposes a simple, empirically derived, unifying structure for the inner regions of quasars. This structure is constructed to explain the broad absorption line regions (BALRs) and the narrow "associated" ultraviolet and X-ray "ionized" absorbers (NALs) and is also found to explain the broad emission line regions (BELRs) and several scattering features, including a substantial fraction of the broad X-ray Fe-K emission line and the biconical extended narrow emission line region (ENLR) structures seen on large kiloparsec scales in Seyfert images. The model proposes that a funnel-shaped thin shell outflow creates all of these features. The wind arises vertically from a narrow range of radii on a disk at BELR velocities. Radiation force then accelerates the flow radially, so that it bends outward to a cone angle of ~60° and has a divergence angle of ~6°, to give a covering factor of ~10%. When the central continuum is viewed from the side, through this wind, narrow high-ionization "associated" ultraviolet absorption lines and the X-ray "ionized absorbers" are seen, as in many low-luminosity active galactic nuclei (AGNs). When viewed end-on, the full range of velocities is seen in absorption with a large total column density, giving rise to the broad absorption line systems seen in a minority of quasars, the BAL QSOs. The wind is both warm (~106 K) and highly ionized. This warm highly ionized medium (WHIM) has a density of ~109 cm-3, putting it in pressure equilibrium with the BELR clouds; the BELR is then a cool phase embedded in the overall outflow, avoiding cloud destruction through shear. The wind has the correct ionization parameter and filling factor for this. The high- and low-ionization zones of the BELR correspond to the cylindrical and conical regions of the wind, since the former is exposed to the full continuum while the latter receives only the continuum filtered by the former. The warm wind is significantly Thomson thick along the radial flow direction, producing the polarized optical continuum found in BALs, but is only partially ionized, creating a broad fluorescent 6.4 keV Fe-K emission line and greater than 10 keV Compton hump. The conical shell outflow can produce a biconical matter-bounded NELR. Luminosity-dependent changes in the structure, reducing the cylindrical part of the flow or increasing the mean angle to the disk axis and decreasing the wind opening angle, may explain the UV and X-ray Baldwin effects and the greater prevalence of obscuration in low-luminosity AGNs.

https://iopscience.iop.org/article/10.1086/317778/pdf

A Structure for Quasars

Selection of active galactic nuclei (AGNs) in the infrared facilitates the discovery of AGNs whose optical emission is extinguished by dust. In this paper, we use the Spitzer Space Telescope First Look Survey (FLS) to assess the fraction of AGNs with mid-infrared (MIR) luminosities that are comparable to quasars and that are missed in optical quasar surveys because of dust obscuration. We begin by using the Sloan Digital Sky Survey (SDSS) database to identify 54 quasars within the 4 deg^2 extragalactic FLS. These quasars occupy a distinct region in MIR color space by virtue of their strong, red continua. This has allowed us to define an MIR color criterion for selecting AGN candidates. About 2000 FLS objects have colors that are consistent with them being AGNs, but most are much fainter in the MIR than the SDSS quasars, which typically have 8 μm flux densities S_(8.0) ~ 1 mJy. We have investigated the properties of 43 objects with S_(8.0) ≥ 1 mJy that satisfy our AGN color selection. This sample should contain both unobscured quasars as well as AGNs that are absent from the SDSS survey because of extinction in the optical. After removing 16 known quasars, three probable normal quasars, and eight spurious or confused objects from the initial sample of 43, we are left with 16 objects that are likely to be obscured quasars or luminous Seyfert 2 galaxies. This suggests that the numbers of obscured and unobscured AGNs are similar in samples selected in the MIR at S_(8.0) ~ 1 mJy.

/pdf/obscured-and-unobscured-active-galactic-nuclei-in-the-4fc7op61rh.pdf

Obscured and unobscured active galactic nuclei in the Spitzer Space Telescope First Look Survey

Using a new 2D radiative transfer algorithm, we have calculated the thermally reradiated infrared spectra of the compact dust tori which are thought to surround many AGN. These tori radiate anisotropically. Face-on tori may be from one-half to a few orders of magnitude brighter than edge-on tori throughout the infrared. Their spectra at nearly all viewing angles are basically 'bumps' which are about 50 percent wider than blackbodies and peak in the mid-infrared at a wavelength determined mainly by the flux of nuclear radiation on the inner edge of the torus. The infrared color temperatures are hotter for face-on tori than edge-on tori by about 100 K. The 10 micron silicate feature often associated with dust can appear in absorption, emission, or not at all. There is a rough tendency for absorption features to be more prominent for edge-on tori than for face-on tori.

Infrared spectra of obscuring dust tori around active galactic nuclei. I - Calculational method and basic trends

In a previous paper we computed the IR properties predicted for the dusty obscuring tori believed to surround many AGNs. Here we make a detailed comparison of the predictions with published observations. We find good agreement between the model predictions and the observed continuum of NGC 1068, the 3.5-10.5 μm colors of type 2 Seyfert galaxies, the observed range in behavior of the 10 μm silicate feature and 8-13 μm spectral index in both types of Seyfert galaxies, and several examples of anisotropy in the IR emission from AGNs. To explain the IR continua of type 1 Seyfert galaxies and quasars, we suggest that a small amount of dust may exist well inside the inner edge of the torus proper and that the host galaxy often dominates in the far-infrared

Infrared Spectra of Obscuring Dust Tori around Active Galactic Nuclei. II. Comparison with Observations

Radiation pressure acting on dust grains can support the vertical thickness of the obscuring tori believed to exist in active galactic nuclei. Using the results of 2D radiation transfer calculations, we evaluate the radiation force acting on these tori. We find that on the inner edge of the torus the radiation force is about 350 l(E) times the gravitational force of the nucleus, where l(E) is the Eddington ratio. Beyond a few torus heights from the inner edge, the radiation force is negligible with respect to gravity. However, between these two extremes lies a region of considerable size where the ratio of radiation force to gravity is nearly constant and can be of order unity for l(E) about 0.1. If the distribution of material within the torus is sufficiently lumpy, there is a significant time-varying component to the radiation force. This drives the random motions of the constituent clouds, thickening the torus at lower values of l(E).

Edward A. Pier

Papers

Infrared spectra of obscuring dust tori around active galactic nuclei. I - Calculational method and basic trends

Infrared Spectra of Obscuring Dust Tori around Active Galactic Nuclei. II. Comparison with Observations

Radiation-pressure-supported obscuring tori around active galactic nuclei

Photoevaporation of Dusty Clouds near Active Galactic Nuclei

Evaporation of Dusty Molecular Clouds in AGN