Supermassive black holes and galaxies evolved together

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 03 February, 2009

Using ground- and space-based telescopes, astronomers have uncovered new evidence to show that the most massive galaxies in the Universe and the supermassive black holes at their centres evolved together.

"They evolved in lockstep," says John Kormendy of the University of Texas at Austin.

It is well known that dense populations of stars – galaxies – grow into sprawling metropolises through collision and merging events. The largest galaxies in the Universe contain as many as a thousand billion stars and form elliptical galaxies with an infinitely dense black hole at their centres. A current leading theory says that when galaxies collide, their black holes end up revolving around each other, stirring up the galactic centre with their incredibly strong gravity, and flinging stars out into the galaxy’s suburbs. As the black hole pair sinks to the centre of the new merger remnant, this supergalaxy's core is depleted of the stars that were flung away.

Two giant elliptical galaxies, NGC 4621 and NGC 4472, look similar from a distance, as seen on the right in images from the Sloan Digital Sky Survey. But zooming into these galaxies' cores with Hubble reveals their differences (left, black and white images). NGC 4621 shows a bright core, while NGC 4472 is much dimmer. The core of this galaxy is populated with fewer stars. Many stars have been slung out of the core when the galaxy collided and merged with another. Their two supermassive black holes orbited each other, and their great gravity sent stars careening out of the galaxy's core. Image:
NASA/AURA/STScI and WikiSky/SDSS.

Kormendy joined forces with Ralf Bender of Germany's Max-Planck-Institute for Extraterrestrial Physics and Ludwig Maximilians University Observatory to measure the resulting dimming of such galaxies' cores, their so-called ‘light deficits’. Previous work on light deficits showed that the biggest elliptical galaxies contain the most massive black holes at their centres, some of which exceed a billion times the mass of our Sun. With such strong gravitational forces, it seemed logical that these monsters would force the galaxy’s stars into a tiny, dense cluster at the centre. But observations revealed the opposite: the biggest galaxies have large, ‘fluffy’, low-density centres.

Astronomers were left scratching their heads wondering why these giant black holes are not surrounded by a dense cluster of stars and where the missing stars went. One popular theory says that black hole binaries gravitationally slingshot the stars out of galactic centres, but this has remained an unproved explanation.

That is, until now. Using the wide field view of the Prime Focus Camera on McDonald Observatory's 0.8 metre Telescope, and the Hubble Space Telescope, Kormendy and Bender made detailed studies of 11 such galaxies in the Virgo Cluster. Many other telescopes were used to connect the central data from Hubble with the outer data from the McDonald telescope, the results of which are reported in this week's issue of Astrophysical Journal Letters. The results on 27 elliptical galaxies in the Virgo Cluster measured by Kormendy, Bender, and their University of Texas colleagues David Fisher and Mark Cornell are scheduled for publication in a forthcoming issue of the Astrophysical Journal Supplement Series.

"Our new observations are a strong and direct link between black holes and galaxy central properties," says Kormendy. "They are a 'smoking gun' that connects black holes with the formation of the surprisingly fluffy centres of giant elliptical galaxies."

The astronomers made precise measurements of the number of stars at various distances from the galaxy’s centres to determine the brightness of each elliptical, which in turn enabled them to calculate the so-called ‘missing’ stellar mass in the galaxy’s centres. What they found was a precise correlation between the missing mass and the measured mass of the central black holes. The missing mass was also found to increase in lockstep with another galaxy property that is known to be tied directly to black holes, namely the speeds at which stars move far out in the galaxy where they cannot feel the black hole's gravity. “The new observations give us much stronger evidence that black holes control galaxy formation, at least at their centres," says Bender.

Kormendy and Bender’s observations finally reveal the footprint of merging pairs of black holes, and lends credence to the theory that the largest galaxies form as smaller systems collide; the black holes in the smaller galaxies then merge, forming a single more massive black hole at the centre of the combined galaxy. Thus the black hole subsequently grows along with the galaxy.

"We have long believed that black holes power quasars in galactic nuclei - they are the brightest objects in the Universe," adds Kormendy. "And we have come to suspect that putting giant black holes at the centres of young galaxies and shining so much quasar light on them affects galaxy formation. In other words, we suspect that the study of quasars and the study of galaxies are really one subject. We can't understand one without understanding the other. We think we have helped to merge these subjects by connecting black holes directly to galaxy structure. John Muir famously said that everything is hitched to everything else in the world. As we find that different subjects are hitched together, we build a theory of galaxy formation that we confidently believe."

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