Cosmic Mergers & Acquisitions with Dr. Jenny Greene

Dr. Jenny E. Greene. Photo: Princeton University Astronomy Dept. website.

What are black holes?  How do they evolve?  How do we find them?  And what happens to them when their host galaxies merge?  These are the questions addressed by Dr. Jenny Greene on October 27 in her talk entitled Cosmic Mergers & Acquisitions.

Dr. Greene is a professor at Princeton University’s astronomy department and specializes in the demographics of black holes and their relationship with their host galaxies, as well as black hole binaries and mergers.  Dr. Greene’s M&A talk on October 27 at Columbia University was one of a series of public lectures, which are offered by the university each month, followed by observing sessions at the rooftop observatory of Pupin Hall, where the astronomy department is housed.

Dr. Greene began her lecture by explaining how to define black holes and how to find them. To transform the sun into a black hole, for example, you would have to shrink its volume to a radius of just 3km, at which point the escape velocity from its surface, given its mass, would equal 300,000 km/second, which is the speed of light. At that point, neither light nor anything else could travel fast enough to escape the sun’s gravity, which is the defining characteristic of a black hole. (But please don’t worry, it’s not about to happen, now or even in the distant future.)

Until very recently, we have found black holes either through observing their accretion disks when the black hole is in an “eating phase”;  or via their gravitational influence on neighboring stars which we can observe.  Within the past two years, we have added the detection of gravitational waves via LIGO as an additional black hole detection method, which does not have to rely on our seeing electromagnetic radiation such as visible light or radio waves.

One example of black hole detection via their gravitational influence on stars is Sagittarius A*, the 4-million solar mass black hole at the center of the Milky Way.  One fact we have discovered in our lifetimes is that black holes are ubiquitous in massive galaxies, with millions or even billions of solar masses in the central black hole.

There is considerable evidence for a direct correlation between the mass of such supermassive black holes and the central bulge of their host galaxy.  This implies some process of interaction between the central black holes and their galaxies.

Photo by: Dr. Jenny Greene

But what happens to the central black holes when galaxies merge?  We know from observation that galactic mergers and cannibalism are a common phenomenon, which accounts for the growth of large galaxies.  Dr. Greene discussed the processes involved in such a merger:  a feedback loop whereby the merger-driven growth of the black hole expels gas from the merged galaxy, thereby eventually limiting the growth of the black hole.  This results in the relationship we observe between the black hole mass and the bulge mass.

Photo by: Dr. Jenny Greene


As Dr. Greene explained, this raises the question of whether the central black holes from two merging galaxies merge themselves.  Or are they doomed to simply orbit each other forever, never to touch?

The fact is that of the black hole binaries we observe, fewer than one percent of them have a separation of less than 0.1 light year, whereas our models for galactic mergers would suggest an order of magnitude higher of very close binaries.  The fact that we are just not finding sub-light year supermassive binaries implies one of three conclusions:

  • The merger “stalls” at a larger radius;
  • Such binaries are shorter lived than expected;
  • Or as she puts it, “Our method is nonsense.”
Photo by: Dr. Jenny Greene

At present this problem remains unsolved, along with the issue of by what process were supermassive black holes able to grow so large in the first place, in the time they had since the Big Bang?  At present we are hoping that the James Webb Space Telescope, currently scheduled to be launched in 2021,  may help us to solve these mysteries.