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When a jet hits a cloud

Submitter: Tom Oosterloo & Raffaella Morganti
Description: It is thought that every fairly massive galaxy has a super-massive black hole (SMBH) sitting in its centre. Sometimes, such a SMBH is 'active', meaning it ejects ultra-fast plasma jets, almost at the speed of light. These jets plays an important role in the evolution of galaxies because they interact with the gas in the galaxy, and the energy released through this interaction dramatically changes the properties of the gas. The main effect is that fewer stars can form from the gas, because for the gas to form stars again, it first has to settle back to normal conditions. So the galaxy does not grow so big as it should.

The physics of these jet-gas interactions is not yet well understood. One would naively expect that, because of the enormous amounts of energy dumped into the gas reservoir by the jet, the gas affected would become very hot. However, it turns out that this is not the case. One of the big surprises which came out of observations we did some years ago (including some with the WSRT) is that, in fact, the jet interaction makes the gas very cold...

To try resolving this puzzle, we used ALMA to observe the galaxy IC 5063, which harbours one of the nicest cases of jet-gas interactions. In this galaxy, this interaction manifests itself as a very fast outflow of very cold gas, driven by the jet. Such cold gas usually is in the form of molecular which can be best observed with ALMA.

Using observations of several emission lines of the cold, molecular gas, we were able, for the first time, to map in detail the physical properties (kinematics, density, temperature and pressure) of the jet-affected gas and how these relate to the properties of the jet plasma. This provides important new information for models of jet-gas interactions that are being developed. The picture shows the image of the ratio of the intensities of two different emission lines of the molecular gas. The contours show the radio emission coming from the SMBH (the central blob) and the plasma jets (the things sticking out left and right of the SMBH). This ratio gives indications about the density and temperature of the gas. The colours clearly show that very different ratios are found in the region coinciding with the jet compared what is seen for gas outside the jet. From this information one can determine how the gas is affected by the jet and learn more about the processes involved.

All this is in a paper by Tom Oosterloo, J. B. Raymond Oonk, Raffaella Morganti, Francoise Combes, Kalliopi Dasyra, Philippe Salome', Nektarios Vlahakis, and Clive Tadhunter recently accepted by Astronomy & Astrophysics: "Properties of the molecular gas in the fast outflow in the Seyfert galaxy IC 5063"
Copyright: Astron
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