Filaments and Swirls in a Massive Galaxy

Dec 07, 2016

While most galaxies harbor a supermassive black hole at their center, only a minority of these black holes - which may be tens to hundreds of million solar masses - are active at any one time. In their inactive state, they are detectable only though their influence on the motions and positions of nearby stars. In their active state they  accrete gas which becomes superheated, emitting x-rays and magnetically induced synchrotron radiation before disappearing into the black hole. An important and still unsolved cosmic mystery is how the activity/inactivity cycle, which controls the growth of the black hole and the environment of the galaxy and its surroundings, is regulated. The UW Astronomy Department has contributed significantly to ths problem over the years through the work of Professors Amy Barger, Jay Gallagher, Sebastian Heinz, and Ellen Zweibel and their sudents and postdocs.

Some years ago, it was discovered that black holes launch narrowly collimated jets which inflate to form giant magnetized "bubbles" filled with magnetic fields and plasma particles moving at nearly the speed of light. This led to the proposal of an activity/inactivity regulation mechanism according to which gas accumulates at the center of a galaxy until enough is accreted onto the black hole to launch a jet/bubble system. In escaping the gravitational field of the galaxy, the bubble drags the gas along, turning off accretion and ending the active phase. Then, as more gas accumulates, the cycle begins anew.

Now, a team led by Prof. Andrew Fabian of Univ. Cambridge and including Profs. Gallagher and Zweibel, have obtained Hubble Space Telescope images of the active galaxy NGC 4696 in the Centaurus galaxy cluster. The images show spectacular long thin filaments of gas and dust together with swirling motion near the center of the galaxy that are the expected signatures of magnetized gas accreting onto a supermassive black hole. One puzzle concerns the nature of this dusty gas. Under normal galaxy conditions, dense, dusty material should collapse under its own weight, forming stars. No star formation is seen in these filaments, however. This may be further evidence for a strong magnetic field that prevents gravitational collapse.

Image: Dust and gas filaments in NGC4696 as seen by the Hubble Space Telescop (NASA)

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