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Understanding the origin of the wind from black holes

Contrary to common belief, black holes don’t swallow everything that comes nearby. In fact, they expel a good part of the gas of the centre of galaxies. This happens when a wind of ionized gas is formed in the vicinity of the black hole. In the case of supermassive black holes that occur at the centre of many galaxies, they produce a wind that can interact with the galaxy itself shaping its evolution through time. We may say that this wind could come in two “flavours”: in form of radiation emitted from a disc before falling onto the black hole or a jet of particles launched in opposite directions perpendicular to the same disc. We know, for instance, that they keep the intergalactic gas hot and prevent the galaxy from growing bigger, suppressing star formation in most of them.

The tricky part here is how these winds efficiently couples its energy to the gas around the galactic nucleus: radiative winds basically pass through the gas and jets are too collimated to push the gas away. Since the power of these winds depends on the amount of gas that falls onto the black hole, this is an even more delicate problem when we are talking about nearby galaxies, which have much less fuel to feed the black hole. In short, we are able to explain the transfer of energy from the winds to the surrounding gas only in very specific cases, although we see its effect in a broad range of galaxies. The lack of a consistent link between the winds and the interstellar gas tells us that something is missing.

Therefore, one of the mysteries that has never been fully explained is where, why, and how these winds are formed. Because of this, we sought to answer these questions by exploring the canonical galaxy NGC 1068, the first active nucleus discovered in the sky.

By looking at NGC 1068 using archival data obtained with adaptive optics in the infrared and eight-metre telescopes of the Very Large Telescope (ESO) and Gemini North, we found that the wind is formed in two stages. In the first stage, the strong radiative wind that comes from the disc around the black hole (primary wind) impacts a torus of dust and gas, located three light-years from the black hole. This impact evaporates the torus, accelerating it outwards and ionizing part of the gas it contains.

An artist’s impression showing the surroundings of a supermassive black hole, typical of that found at the heart of many galaxies. Image by ESO/L. Calçada. CC BY 4.0 via Wikimedia Commons.

The second stage of the wind formation is more dramatic and occurs when a powerful jet of particles, collimated by a strong central magnetic field, hits a large molecular cloud (made mainly of atoms of hydrogen), located at about a hundred light-years from the black hole. Without a confining surface, the jet would dissipate its energy in an almost useless way out of the galaxy, but with the energy injected in this cloud, it could be spread by means of a thermal wind (called the secondary wind), capable of blowing out the gas to even more distant regions from the nucleus.

We can compare this mechanism to partially covering a high pressure garden hose with a finger: the water would escape with a high velocity jet-like beam. If this beam hits a hard spot in the way, the water will be inevitably spread in all directions. Therefore, this second stage enables the huge amount of kinetic energy concentrated only in a narrow open angle of the jet to be re-distributed in a wider way, coming out from a region far away from the black hole. The heated gas and the thermal radiation would be blown away, accelerating the interstellar material outwards in a way that the radiative wind from the central disc would not be able to, since at these distances it would be very rarefied.

The wind formed with these high velocities is confined to the interior of an hour-glass-like geometry. The walls of this hour-glass, seen through iron emission lines, glow in the radiation emitted by low-velocity gas. However, both structures (distinguished by their velocities) have distinct reasons to present similar orientations: the walls of the hour-glass are excited by the radiation that escapes from the central disc (without accelerating them) and the high-velocity gas comes in the preferred direction from where the jet is bent and spread by the cloud. Although we may not see this specific configuration in other galaxies, the same mechanism could play a similar role in expelling the nuclear gas. This would happen when a jet hits a dense molecular material, known to be very common in the centre of all active galaxies, such as the dusty torus around black holes.

An image showing the location of the secondary wind (RGB of the iron lambda 1.64 microns emission line). Author’s own image, used with permission.

The discovery of these new mechanisms fills an important gap in the understanding of galaxy formation because it shows that black holes are capable of blowing out the surrounding gas more efficiently than we knew before. Without this, virtually all the existing matter would be trapped in stars in a kind of “super-galaxy”. The Universe would be darker and colder, since stars have a limited luminous lifetime. Such black holes keep the intergalactic gas hot, which will eventually form new stars, as if we have living-galaxies slowly breathing through the cosmos.

Featured image credit: Hubble Space Telescope image of Messier 77 spiral galaxy by NASA, ESA & A. van der Hoeven. Public domain via Wikimedia Commons.

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