Giant Black Hole Caught in the Act of Preventing Future Stars

May 5, 2015

It has long been suspected that the processes at the center of active galaxies prevent the gas from forming stars. Now, for the first time, a KIPAC team has seen that happening before our eyes.

One of the seeming paradoxes of astrophysics is that stars form because something got colder.  Only cold gas can sink into a gravitational potential and coalesce to form a ball as dense as a star, while warmer gas has too much thermal energy and keeps moving.  Over time, dense hot gas will cool through the radiation of photons, eventually becoming cold enough to lead to star formation.  Therefore, where there is dense hot gas one would expect stars to eventually arise given enough time. 
 
The biggest reservoirs of dense hot gas in the universe are in of galaxy clusters, massive aggregations of galaxies, intergalactic gas, and dark matter.  In clusters, the gas, which contains several times the total mass of all the stars, is heated to temperatures such that is glows in X-rays.  At the clusters' centers, where the densities are high enough, the gas can give off its heat through the emission of photons as above, giving rise to the phenomenon known as 'cooling core clusters', where the photon emission from the cooling centers is seen.  It has long been speculated that the reason that the cooling gas at the center of a cluster does not get cold enough to form stars is that it is periodically driven away or reheated by activity from the center of the cluster.
 
One of the best clusters for study and also the closest is the Virgo cluster, which has a giant active galaxy, known as M87, in the middle.  In the center of M87, the interaction of a supermassive black hole and the accretion disk of matter orbiting it give rise to two spectacular jets of particles and radiation, which can be seen from their radio and optical emission.  The region around M87, then, provides a beautiful laboratory for studying the interaction of the jets arising from the supermassive black hole of an active galaxy with the gas of a cluster environment.
 
KIPAC researchers Norbert Werner, Evan Million, Aurora Simionescu, Steve Allen, and Anja von der Linden, leading a team with colleagues from several other institutions, have analyzed data of the region surrounding M87 taken with NASA's Chandra X-ray observatory, along with the XMM-Newton satellite, and optical and radio images of the same field.  The Chandra data provide high resolution imaging in several X-ray bands, while the XMM-Newton data allow the observation of spectral lines originating from the region.  With the combination of the two, the researchers determined the properties and precise spatial distribution of different temperature phases within the X-ray emitting gas surrounding M87.
 
Werner, Million, and colleagues find that the coolest phase of the X-ray gas traces roughly the radio emitting structure, while the warmer phases are present outside that region.  The cooler gas is clearly being expelled by the force of the jets.  The researchers' analysis shows that the total mass of the gas being ejected is as large as the total mass of gas remaining in the center of M87.  Furthermore, they also see the central engine of  M87 heating the ambient gas, with two shocks of intense heat transfer seen at different radii from the center.
 
With this analysis, the main processes hypothesized to explain the lack of star formation in the centers of clusters have been seen in action.  The intense jets resulting from the supermassive black hole in M87 are both driving away the potential star forming gas in the center of the galaxy, and heating the ambient gas farther out, quenching star formation in both regions. 
 
This work is based in part on a paper to appear in Monthly Notices of the Royal Astronomical Society which is available from astro-ph at http://arxiv.org/abs/1003.5334
 
Science Contact
Dr. Norbert Werner
Stanford/KIPAC
email: norbertw@stanford.edu