10-Star Room Service: Supermassive Shut-Down

Research led by astronomers at Michigan State University has provided evidence to support the theory that black hole feedback and type 1A (supernova) heating work together to limit the growth of elliptical galaxies. The group used the Chandra X-Ray Observatory to take the radial profiles of gas entropy for 10 massive elliptical galaxies, half of which are thought to be in the process of star formation and half of which are inactive. Elliptical galaxies in our universe stopped generating new stars billions of years ago, despite having plentiful sources of hot gas – a fact that science cannot yet thoroughly explain.

Black holes, found at the cores of galaxies, launch such violent streams of charged particles into their surroundings that the gas within the galaxy is redistributed. This process does limit star formation to a certain extent, but the effect is only temporary and black hole outbursts don’t fully account for the quenching that we know must occur. For galactic models to match observations the gas that gets expelled into the galaxy needs to be continually removed; otherwise over time it would cool and star creation would continue as before.

The researchers have hypothesised that, in the process of expelling jet streams, black hole outbursts might heat the gaseous contents of the galaxy sufficiently to allow supernovae to carry out a process called ‘supernova-sweeping’, by which supernovae provide the mechanism for clearing the gaseous contents of galaxies. According to the paper, ‘particularly strong black hole outbursts can shut off star formation in massive elliptical galaxies by boosting the entropy of the hot gas and flipping the system into the SN-sweeping state.’

There may be further developments in related areas of astronomy as a result of this paper. For example, one of the 10 ellipticals examined was found to emit jets that were about 50 times more powerful than those of the other galaxies, a result that is consistent with related models ‘indicating that precipitation near the black hole should switch its fueling mode from Bondi-like accretion to cold chaotic accretion’.

http://iopscience.iop.org/2041-8205/803/2/L21