Copyright © 2015 by Annual Reviews. All rights reserved
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| Annu. Rev. Astron. Astrophys. 2015. 53:115-154
Copyright © 2015 by Annual Reviews. All rights reserved |
Astronomers now generally agree that the center of almost every galaxy but the smallest contains a supermassive black hole (SMBH). In recent years it has become clear that the mass M of the hole correlates strongly with physical properties of the host galaxy. In particular the hole mass M appears always to be a fairly constant fraction of the stellar bulge mass Mb, i.e.
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(1) |
(Häring & Rix, 2004). Even more remarkably, observations give a tight relation of the form
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(2) |
between the SMBH mass and the velocity dispersion σ = 200σ200 km s−1 of the host galaxy's central bulge, with α ≃ 4.4 ± 0.3 (Ferrarese & Merritt 2000, Gebhardt et al. 2000; see Kormendy & Ho 2013 for a recent review). For many practical cases the relation is more conveniently written as
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(3) |
with now σ = 100σ100 km s−1.
Since observationally determining the SMBH mass generally involves resolving its sphere of influence, of radius
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(4) |
with M = 108 M8 M⊙ = 107 M7 M⊙, (2) may represent a maximum SMBH mass for a given velocity dispersion σ (Batcheldor 2010).
At first sight the relations (1, 2) appear surprising. For (4) shows that the black hole's gravity has a completely negligible effect on its host galaxy, which in most ways must be quite unaware of its existence. But we know (Soltan 1982) that the hole grew largely as a result of luminous accretion of gas. This released energy
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(5) |
where η ≃ 0.1 is the accretion efficiency, far larger than the binding energy
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(6) |
of a host bulge of stellar mass Mb ∼ 103 M.
The vast difference in these two numbers suggests that the host must notice the presence of the hole through its energy output, even though it is utterly insignificant in all other ways. We can already see how the black hole mass might correlate with galaxy properties – the hole grows by accreting gas, but in doing this communicates some of its huge binding energy EBH back to the gas reservoir, and so potentially limits its own growth. This suggests that the most relevant quantity to compare with EBH is not Ebulge, but instead the gravitational binding energy of the bulge gas alone, i.e.
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(7) |
where fg < 1 is the gas fraction. In the following we take this as fg ∼ 0.16, the cosmological mean value, giving a typical relation
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(8) |
for a black hole close to the M − σ relation (the rhs has an implicit factor ∼ σ2004 / M8).
This picture requires the black hole to communicate some of its accretion energy to its host. But this process cannot be very efficient, as otherwise the hole could disrupt the host entirely, or at the very least remove a large fraction of its gas. In this sense, the galaxy bulge leads a precarious existence. For much of its life it can ignore the threat that the SMBH poses, but we will see that in the end this is always decisive if accretion continues.
1.3. Communicating the Energy: Feedback
There are two main ways that the SMBH binding energy can potentially interact with its surroundings. By far the larger is direct radiation: after all, this is how all the accretion energy is initially released. But we know from observation that most light escapes relatively freely from active galactic nuclei (AGN). This suggests that radiation is in general not the main way the SMBH affects its host, and we will discuss in detail why this is so in Section 7.4. The second form of coupling SMBH binding energy to a host bulge is mechanical. The huge SMBH accretion luminosity drives powerful gas flows into the host, making collisions and communication inevitable. One form of flow often mentioned is jets – highly collimated flows driven from the immediate vicinity of the SMBH (see Fabian 2012 for a review). To turn these into a way of affecting most of the bulge requires a way of making the interaction relatively isotropic, perhaps with changes of the jet direction over time. Here we will mainly consider another form of mechanical communication which automatically has this property already. This is the observed presence in many AGN of near–isotropic winds carrying large momentum fluxes.
Early X–ray observations of AGN yielded soft X–ray spectra frequently showing the imprint of absorption from ionized gas, the ‘warm absorber’; hereafter WA (Halpern 1984, Reynolds & Fabian 1995). More recent observations have found at least 50% of radio-quiet AGN showing WAs in their soft X–ray (∼ 0.3-2 keV) spectra. The limited spectral resolution of the Einstein Observatory and ASCA observations prevented important parameters of the WAs, in particular the outflow velocity and mass rate, to be determined with useful precision. The higher resolution and high throughput afforded by contemporary X-ray observatories, Chandra, XMM-Newton and Suzaku has transformed that situation over the past decade, with the WA being shown, typically, to be dominated by K-shell ions of the lighter metals (C, N, O, etc) and Fe–L, with outflow velocities of several hundred km s−1 (Blustin et al. 2005, McKernan et al. 2007).
A more dramatic discovery made possible with the new observing capabilities was the detection of blue-shifted X–ray absorption lines in the iron K band, indicating the presence of highly ionized outflows with velocities v ∼ 0.1−0.25c (Chartas et al. 2002; Pounds et al. 2003; Reeves et al. 2003). In addition to adding an important dimension to AGN accretion studies, the mechanical power of such winds, which for a radial flow depends on v3, was quickly recognized to have a wider potential importance in galaxy feedback.
Additional detections of high velocity AGN winds were delayed by the low absorption cross section of such highly ionized gas, combined with strongly blue-shifted lines in low-redshift objects coinciding with falling telescope sensitivity above ∼ 7 keV. However, further extended observations, particularly with XMM-Newton, found evidence in 5 additional AGN for outflow velocities of ∼ 0.1−0.2c (Cappi et al. 2006). Some doubts remained as to how common high velocity outflows were, as the majority of detections were of a single absorption line (with consequent uncertainty of identification), and had moderate statistical significance, raising concerns of ‘publication bias’ (Vaughan and Uttley 2008). In addition, only for PG1211+143 had a wide angle outflow been directly measured, confirming a high mass-rate and mechanical energy in that case (Pounds & Reeves 2007, 2009).
These residual doubts were finally removed following a blind search of extended AGN observations in the XMM-Newton archive (Tombesi et al. 2010), finding compelling evidence in 13 (of 42) radio quiet objects for blue-shifted iron K absorption lines, with implied outflow velocities of ∼ 0.03−0.3c. A later search of the Suzaku data archive yielded a further group of strong detections, with a median outflow velocity again ∼ 0.1c (Gofford et 2013). In addition to confirming that high velocity, highly ionized AGN winds are common, the yield from these archival searches shows the flows must typically have a large covering factor, and therefore be likely to involve substantial mass and energy fluxes.
The observed distributions of velocity, ionization parameter and column density are compatible with Eddington winds launched from close to the black hole, where the optical depth τes ∼ 1, and carrying the local escape velocity (King & Pounds 2003). However, as the mean luminosity in most low-redshift AGN is on average sub-Eddington, such winds are likely to be intermittent, a view supported by repeated observations and by the range of observed column densities.
For the best–quantified high-velocity outflow (the luminous Seyfert PG1211+143), in which a wide–angle flow was directly measured (Pounds & Reeves 2007, 2009), the wind appeared to have more energy than needed to unbind the likely gas mass of the observed stellar bulge. This suggested that the energy coupling of wind to bulge gas must be inefficient, as seen in the discussion following equation (8). Evidence that the fast wind in NGC 4051 is shocked at a distance of ∼ 0.1 pc from the black hole offers an explanation of why such powerful winds do not disrupt the bulge gas: strong Compton cooling by the AGN radiation field removes most of the wind energy before it can be communicated.