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Galaxy formation

When the wind blows

Nature volume 519, pages 423424 (26 March 2015) | Download Citation

Astronomical observations of a luminous galaxy that has a central, mass-accreting supermassive black hole reveal how such entities launch and propel gas through galaxies at high speeds. See Letter p.436

Models of how galaxies form require regulatory feedback mechanisms that control the growth of the galaxies' stellar mass. Without them, galaxies in simulated Universes would grow too big, too quickly1. Feedback can take many guises, but, for massive galaxies, the most important and perhaps also the most dramatic one is thought to be associated with the growth of supermassive black holes (SMBHs), which are ubiquitous at the centres of large galaxies. But no one really understands the details of how SMBH feedback works. Part of the problem is relative scale: the immediate sphere of influence of an SMBH is minute compared with the host galaxy that it inhabits. How does a single, tiny galactic component affect the fate of an entire galaxy? The main observational challenge is to link the astrophysical chain connecting galaxy-wide processes, such as large-scale outflows of gas, right back to their source, deep within the compact, complex and crowded environment of a galaxy's core. On page 436 of this issue, Tombesi et al.2 provide just such a link.

SMBHs grow by swallowing (accreting) matter, releasing copious amounts of energy in the process3(Fig. 1). During such episodes, they are referred to as active galactic nuclei (AGNs), and energy is deposited in the interstellar medium either through powerful, radio-bright jets of magnetic fields and high-energy particles or through intense radiation from a hot accretion disk around the SMBH. These feedback channels are called the radio and quasar modes4, respectively, with most AGNs falling into the latter class. There is convincing circumstantial evidence that both types of AGN can drive large-scale (hundreds of parsecs) outflows of molecular gas5 — the dense component of the interstellar medium from which stars form — and can thus potentially quench star formation and regulate the growth of the SMBH. But how such winds are launched and propelled is unclear.

Figure 1: A nearby quasar driving a molecular wind.
Figure 1

Markarian 231, seen here in an image from the Hubble Space Telescope, is arguably the best example of a quasar in the local Universe: a galaxy harbouring a central, actively mass-accreting supermassive black hole. Like galaxy IRAS F11119+3257, the subject of Tombesi and colleagues' study2, this quasar is driving an outflow of molecular gas, moving at 1,000 kilometres per second, that is potentially quenching star formation on galactic scales. Image: NASA, ESA, Hubble Heritage (STSCI/AURA) — ESA/Hubble Collaboration & A. Evans (Univ. Virginia, Charlottesville/NRAO/Stony Brook Univ.)

Hot disks form around SMBHs as surrounding gas, laden with angular momentum, is gravitationally accreted. Gravitational energy is converted to heat, causing the disk to blaze with ultraviolet and X-ray radiation. This radiation affects the immediate ambient, gaseous medium through processes including photoionization and radiation pressure. Both of these can launch high-velocity outflows of material near the disk, either by Compton heating, as photons scatter off free electrons in ionized gas, or through momentum transfer from the photons to interstellar dust and gas. High-velocity winds launched from the disk can drive a shock wave out through the galaxy that can heat, sweep up and possibly eject gas that would otherwise form stars. Unfortunately, the mechanics of the interface between the fast winds emanating from the accretion disk and bulk motion of the interstellar medium is poorly understood, because this environment is buried deep inside the galaxy and is heavily obscured.

Tombesi et al. have delved into the heart of a galaxy called IRAS F11119 + 3257 — a luminous galaxy with a central AGN. By detecting the telltale signature of the absorption of X-ray light by highly ionized iron atoms, they have revealed the 'inner wind' of gas within a few hundred astronomical units (Earth–Sun distances) of the black hole, travelling at about 25% of the speed of light. Such winds have been seen before, but what makes this study remarkable is that the authors have linked this inner wind with a large-scale (a few hundred parsecs) outflow of cold molecular gas detected in the same galaxy. This provides insights into the mechanism by which the central black hole is affecting its host galaxy.

A key question for astrophysicists is whether such outflows are momentum-conserving or energy-conserving. Such a description lies at the heart of theoretical frameworks of quasar-mode feedback6. In a momentum-conserving outflow, thermal energy in the shocked wind is rapidly radiated away, whereas if such radiative losses are low, then the outflow is described as energy-conserving. Determining which regime is in operation is crucial, because it can strongly affect the efficacy of star-formation quenching in the galaxy.

Energy-conserving outflows are thought to be more effective because they can entrain more material than their momentum-conserving counterparts, and their momentum flux increases as the shock front propagates through the interstellar medium. This seems to be the scenario for IRAS F11119 + 3257, with the momentum-conserving case being convincingly disfavoured. Another advance obtained from this study is a reasonable handle on the efficiency of the coupling between the inner wind and the molecular outflow: approximately 20% of the power of the inner wind is transferred to the larger-scale outflow.

Encouragingly, these observations support current models of quasar-mode feedback, suggesting that theory is at least on the right track. This work is a step forward because it provides empirically motivated input for implementations of AGN feedback in galaxy-formation models7, and it highlights the symbiotic relationship that exists between theoretical and observational galaxy-evolution studies. The picture is far from complete; these observations are limited in their ability to constrain the geometries of the inner wind and the large-scale outflow, and so assumptions — albeit reasonable ones — have been made. These introduce inevitable uncertainties in the results, but the picture that Tombesi et al. present is a convincing one. The next steps will be to study the link between inner winds and bulk neutral- and ionized-gas outflows in a large sample of galaxies, both near and far away.

The outlook is bright: facilities such as the Atacama Large Millimeter/submillimeter Array (ALMA) image molecular outflows in galaxies with increasing clarity8; the planned European Extremely Large Telescope (E-ELT) will probe the dynamics of ionized-gas components with exquisite sensitivity; and future X-ray missions such as the planned Advanced Telescope for High-Energy Astrophysics (ATHENA) will offer revolutionary views of the high-energy physics associated with the front line of AGN feedback. As Tombesi et al. have shown, uniting the multiscale and multiphase astrophysics involved in feedback processes is the key to unlocking the secrets of galaxy growth.



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  1. James E. Geach is at the Centre for Astrophysics Research (CAR), University of Hertfordshire, Hatfield AL10 9AB, UK.

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Correspondence to James E. Geach.

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