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A Bonsai Black Hole in Our Own Backyard

Radio image of Fornax A

Radio image of Fornax A, an iconic radio galaxy with extended lobes (orange). The grey region between the lobes is stellar light from the much smaller host galaxy. Fomalont et al. 1989, Astrophysical Journal Letters, 346, 17.

By Robert Soria

The discovery of powerful jets from a nearby black hole reveals new clues about the behaviour of massive quasars in the early universe.

Black holes are popularly portrayed as a place of darkness and gloom, but to astronomers they are the cleanest and most efficient source of energy in the universe. The recent discovery of a very large glowing bubble of ionised gas inflated and heated by a black hole in the nearby galaxy NGC 7793 helps us to understand their role as cosmic powerhouses.

While stars use nuclear fusion to extract energy from their gas, black holes extract gravitational energy from the infalling matter before it disappears into the hole – a process known as accretion. Indeed hydrogen bombs and hydro power plants are based on the same physical principles as stars and black holes, respectively.

Hydro power may seem less efficient than nuclear fusion. While it takes a lot of falling water to produce the same energy as we would get from nuclear fusion, this is only because the gravitational field of the Earth is so weak. But gravity near a black hole is much stronger, and accretion power can be up to 50 times more efficient than nuclear fusion.

This is why black holes – or, more exactly, the gas in the region immediately outside the black hole horizon – can be the most luminous and powerful objects in the universe when enough matter is falling towards them. The end result is that black holes get bigger over time as long as there is matter to fuel them.

The general energetics of black hole accretion is known from fundamental physical principles, but there are still many important things we do not know about how that energy is released. The two main channels are radiation of photons and the kinetic energy of a jet.

The photons are emitted from the surface of an accretion disk that is formed by gas that is slowly spiralling towards the event horizon – and is getting very hot in the process. When photon output dominates, a black hole appears as a very luminous source, especially in the X-ray and ultraviolet bands. A supermassive black hole in the centre of a galaxy, with a mass that is many millions of times the mass of the Sun, can be more luminous than all the stars in that galaxy put together.

Alternatively, accretion power can be carried out by a collimated jet of charged particles – either electrons and positrons, or electrons and protons – moving at almost the speed of light.

A third ingredient may also be present, especially in the most active black holes: a dense wind launched from the surface of the accretion disk can carry away mass, energy and angular momentum.

How does a black hole decide whether to release its accretion power via photons, jets or winds? What switches the jet on and off? These are still unsolved mysteries of black hole astrophysics, but this is where our recent discovery of the most powerful stellar-mass black hole jet 12 million light years away in NGC 7793 will help to provide some answers.

Radio Galaxies and Microquasars
One of the best things about black holes is that their fundamental physical properties are the same at all scales. Supermassive black holes in distant quasars work just like stellar-mass black holes in our galaxy, apart from a simple rescaling in the equations. What changes is the environment around them, such as whether their fuel comes from galaxy-scale gas inflows or a companion star.

From an astronomer’s point of view, a crucial advantage of nearby stellar-mass black holes over their more distant and massive cousins is that they evolve over shorter timescales and can switch between jet, disk and wind states within the timescale of a university research grant. Supermassive black holes may be bigger and brighter, but they switch states over tens of millions of years.

Over the past two decades, much work has been done to understand the connections between astrophysical objects that once seemed totally unrelated but are all powered by active black holes. Radio galaxies and microquasars are among the most spectacular examples.

“Radio galaxy” is, in fact, a poorly chosen name. The “radio” label comes from their initial discovery from radio observations in the 1950s: it was not known at the time that the radio emission is produced by relativistic electrons in a jet. And the “galaxy” label is misleading: it is not the galaxy that produces the jet, it is the supermassive black hole in its centre. Today, we have independent evidence of black holes with jets from infrared, optical and X-ray studies, but radio observations still provide the best spatial resolution and are not affected by interstellar absorption.

Powerful black hole jets share a common structure. They drill through the surrounding gas until they lose enough energy or hit denser interstellar material. A shock front forms at the slowly-advancing head of the jet and is often visible as a bright “hot spot” in several energy bands. When the relativistic jet particles reach the shock front, their orderly bulk motion is converted into random motion and the collimation of the jet stream is lost.

After going through the shock front, electrons backflow and disperse into a large, fluffy lobe around the end of the jet, and sometimes form a full cocoon around the black hole. As they swirl around magnetic field lines, the electrons emit synchrotron radiation – mostly in the radio bands but sometimes extending also to the optical and X-ray band. The rest of the jet power is transferred to the ambient gas as thermal energy and used to inflate the hot cocoon.

Microquasars are the stellar-mass equivalent of radio galaxies. They are powered by a black hole formed from the collapse of a massive star. They contain a pair of relativistic jets that interact with the interstellar gas and may produce bright hot spots, radio lobes and an expanding cocoon or cavity.

Radio observations of the inner jet and outer lobes have been a crucial tool for discovering and studying radio galaxies and microquasars, but calculating the total jet power from its radio emission is not simple. The swirling cloud of radio-emitting

electrons carries only a small and poorly-known fraction of the total power. The rest is transferred from the jet to the surrounding gas, which is heated and swept out, forming a hotter, lower-density bubble around the black hole. This is what determines the full impact of black hole’s activity on the surrounding gas, but it is difficult to observe this effect directly.

The Most Powerful Microquasar
This is why an object such as the recently discovered black hole S26 in NGC 7793 is so important. Our team, led by Dr Manfred Pakull at the University of Strasbourg, used optical and X-ray observations to reveal the hot spots at the end of the symmetric jets, and the hot bubble of gas around the system. The bubble is expanding at a speed of 250 km/s. The jets are driving the expansion of the bubble, pushing its shell as they slam against the denser and cooler interstellar medium. Strong radio emissions from the jet lobes and the cocoon were discovered from our observations in 2009 and 2010 at the Australia Telescope Compact Array.

This object contains the largest bubble and the longest jets ever seen from a stellar black hole, with a total length of about 1000 light years. Furthermore, they are emitted from a black hole that is at most a few hundred kilometres in diameter.

We estimate that the active phase of this black hole started around 200,000 years ago – the blink of an eye in the history of the universe. During this short period of time, the black hole has swallowed as much gas as there is in the Sun, and has produced 300 times more energy than will be emitted by our star over its entire lifetime.

Having measured both the radio emission from the relativistic electrons and the heating effect on the surrounding gas, we could then estimate the total jet power. It is a few hundred times the power carried by the radio-emitting electrons alone.

This is more than previously thought. It makes S26 one of the most powerful stellar black holes – including both jets and photon output – by an order of magnitude. It also means that we may have underestimated the total jet power of many other black holes, especially in the distant universe, if we only measured their luminous output.

It was previously thought that radio jets were only associated with moderately weak black holes, while jets could not be launched by black holes that are growing very fast (i.e. when a lot of gas is falling into them). In other words, at low accretion rates, black holes would produce jets, while at high accretion rates they would have a luminous disk.

The discovery of S26, and similar recent discoveries of a few jet-dominated, powerful quasars in the distant universe, have challenged this scenario. It seems now that at least some of the most powerful black holes can have strong jets.

Black Hole Spin and Jets
But a fundamental problem remains unsolved: why do some of the most powerful black holes – both in the stellar-mass and supermassive class – have jets while others do not, even if they are accreting at the same rate and have the same total power? How do black holes switch from one state to the other?

One suggestion is that black hole spin determines the jet power. Rapidly spinning black holes would more likely produce stronger jets or retain their jets at high accretion rates, all other conditions being the same. This is at least in qualitative agreement with what is seen in other astrophysical objects (e.g. protostars, neutron stars), where jets are more often launched by fast-spinning bodies.

Spin is the most elusive property of a black hole, and is even more difficult to measure than its mass. If we could measure both the mass and the spin of an active black hole we could test crucial predictions of general relativity and learn more about black hole formation and growth. Different formation channels predict different spin values, so it would be extremely interesting if the jet power of black holes is indeed an indicator of their spin.

Regardless of the mechanisms for jet formation, the observed existence of very powerful black hole jets has profound implications for the early history of the universe. During the first billion years or so the cosmos was dominated by quasars, which are the most powerful state of supermassive black holes in the nuclei of galaxies. However, the quasar phase ended after most of their gas supply was exhausted.

S26 is a bonsai quasar in our own backyard. It is heating and sweeping the gas around it, much like quasars did on a grander scale billions of years ago. If some quasars can output a significant fraction of their power through jets – instead of, or in addition to, photons – they would be more efficient at heating the ambient gas and would have a stronger effect on the evolution of their host galaxies.

Modelling the interaction between black hole growth and galaxy evolution will be a key science goal of the Australian Square-Kilometre-Array Pathfinder radio telescope being built near Geraldton, WA, in synergy with infrared and X-ray studies.

Roberto Soria is a research fellow at University College London’s Mullard Space Science Laboratory, and will join the Curtin Institute of Radio Astronomy in Perth this year.