Mississauga Astronomical Society
Thirtyeight Meeting
Speaker’s’ Night
Day: Friday, April 15, 2005
Guest Speaker: Michael De Robertis
Supermassive Black Holes
Dr. Michael De Robertis, Professor of Astronomy and Associate Dean of Science at York University received his PhD at the University of Victoria, and did postdoctoral studies at the University of California at Lick Observatory. He spoke about supermassive black holes (SBHs) .
One of the most interesting discoveries in the past decades is the presence of supermassive black holes in galaxies.
In 1796, Pierre LaPlace first considered objects so heavy that not even light could escape from them. Einstein, in 1915, formulated his theory of general relativity which helped describe black holes (BHs). In 1916, Carl Schwarzschild described their spherical rotation whose mathematical solution was solved in 1963 by Kerr. And in 1967, A. Wheeler coined the term “black hole”. Lyndon postulated in 1969 that BHs can shine by consuming matter – giving an explanation to quasars and active galactic nuclei. Currently, stellar mass ( less than 100,000 Mo, where Mo is a solar mass) and supermassive ( 108 – 1010 Mo ) black holes have been found.
Since light cannot escape from a BH, detection is based on the effect it has on its surroundings. In general relativity theory, geometry tells space how to curve and matter how to move. With sufficient mass, not even light can escape. Dr. De Robertis described the structure of the black hole as a central singularity with a Schwarzschild radius (Rs) of 2GM/c2 = 3(M/Mo)km, , and a surrounding event horizon where the escape velocity is exactly to the speed of light. The event horizon acts as a one-way membrane where things can go in but not come out. For an object the mass of the sun, the Rs would be 3 km; for a billion Mo it would be 1 astronomical unit. Therefore 3 zones exist: outside the horizon where we are, at the event horizon, and in the singularity. Black holes have only 3 characteristics: mass, angular rotation, and charge. Because charge is insignificant, BHs are the simplest macroscopic objects in the universe.
A black hole can be detected by an object rotating around it and its mass determined. By the theory of relativity, we can calculate the precession of the orbit and frame dragging, or warping of space-time lines. Black holes lose mass from evaporation through Hawking radiation. In addition, gravitational waves can theoretically be detected (by LIGO) as BHs form or merge. Black holes can also be detected through microlensing when a BH of a few solar masses moves in front of a star causing characteristic brightening. In a binary system a star of greater than 8 Mo explodes within 20 million years as a Type 2 supernova forming a BH which subsequently draws in an accretion disc from the companion star. The tremendous amount of energy radiated by the accretion disc can be detected.
Active galactic nuclei (AGN) are SBHs greater than 106 Mo in the interiors of galaxies. The brightest ones are brighter than the host galaxies. The BH grows from the accretion of material. Because the light varies by 40 to 50% on a time scale of a few days, we know that the maximum dimension is 3 light days. In addition, the energy spectrum is unlike the shape or characteristic of stars or gas. The accretion disc gets brighter and speed increases (to 1/3 the speed of light) towards the centre of the AGN as shown by broadening of spectral lines and sometimes demonstrating superluminal motion.
In a recent study of the Milky Way galaxy, the HST was able to resolve a small volume at the centre of the galaxy wherein motions of individual stars can be resolved. The degree of randomness of star motion within the effective radius of the BH can be measured.
Dr. De Robertis then described a number of galaxies with AGN’s. M87 contains an optical jet; and the doppler shift of the gas motion around the accretion disc implies a central mass is 5 billion Mo. Gas motion in the dust ring of NGC 7052 implies a central mass of 300 million Mo, and in NGC 4261, 1.2 billion. There is no doubt that black holes lie in the centres of these galaxies. NGC 4268 is a water maser galaxy emitting radio waves at a specific frequency and yielding a central mass of 40 million Mo. Infra-red images of the central light year of our Milky Way Galaxy reveals the orbits of stars and shows a 2 to 3 million Mo object. The Andromeda Galaxy, M31 has 2 blobs in the center which may be 2 black holes.
Random motion inside galaxies correlates with the mass of the central black hole in the relation as BH mass / mass of oldest stars = 0.02%, a constant. The fact that this is a constant number comes as a great surprise and is a very recent discovery. The good resolution of the Hubble Space Telescope was necessary to collect the required data.
We still don’t know whether all large galaxies hosted AGN’s, nor how they got their BH’s. Did they grow over time? If so, how did they “know” that the ratio should be 0.02%? If they came formed, then how? Answers to these questions can tell us how galaxies formed.
In conclusion to his very interesting talk, Dr. De Robertis stated that all relatively massive galaxies contain SBH’s where the mass is related to the total mass of old stars in the galaxy. We have yet to understand this relationship. Many questions then ensued.
Submitted by Chris Malicki, Secretary Chris Malicki, Secretary
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