Wednesday, May 2, 2012

From Quasars to Black Holes

Last week Scott Tremaine, a professor at the Institute for Advanced Study in Princeton, visited the University of Zurich & ETH. He is the only person I know who has an asteroid (3806 Tremaine; no worries it's quite small) named after him. He also wrote a standard textbook called "Galactic Dynamics" that is still used by many universities around the world.  I went to two of the three seminars he presented. This post is inspired by his talk on Black Holes in Nearby Galaxies (the slides are available on his website). 

Quasar core. Credit: Hubble Telescope
 What are quasars?
The word Quasar comes from quasi-stellar radio source. Quasars are among the most luminous and most energetic objects in the universe. They are about 10 000 000 000 000 times more luminous than the sun or 100 to 1000 times more luminous than a galaxy. This huge luminosity is produced by gas that heats up due to friction and glows brightly as it gets closer, and closer to the really dense object at the quasar core.

Why Quasars require black holes?
These are the top three reasons:
A. The radio jets that come from quasars are absolutely straight over timescales of 100 000 years. The fact that they can maintain their orientation over such a long timescale suggests that the source could be a spinning black hole - which acts like a gyroscope.  Needless to say, I was glad to hear that the mechanism that helps my toy helicopter run also governs spinning black holes.

B. The velocity of the radio jets is relativistic, i.e., close to the speed of light. These jets actually appear to be superluminal, but this is just an optical illusion due to the jet making a very narrow angle with the line of sight of the observer. This is consistent with the theory that quasars contain black holes because black holes are tiny and can eject mass at relativistic velocities.

C. This enormous amount of light comes from a tiny area in the sky of less than a few mili-parsecs. A mili-parsec is about 200 times the distance between our Earth and the Sun.
On the right we can see what is call "Einstein's cross". Four images of the same quasar appear around a foreground galaxy due to strong lensing. Gravitational lensing occurs because heavy objects bend the space-time, and as a result light rays coming from a background source are bent. When the lensing mass in complex such a galaxy, the observer can see multiple images of the same source. The fact that we see these four images puts a limit on the size of emitting source, which is the quasar - it has to be less than something called the Einstein Radius (Wambsgans 2006).

Of course, they could also be other kinds of compact objects like boson stars that do not contain a singularity, but, so far, no particles that could condense to make up these stars have been detected.

Where are these Quasars today?
The number of observed quasars per volume (or more precisely their co-moving number density) peaks at redshift of 2. The universe was a little more than 3 billion years old at the time. By now 13 billion years have passed since the Big Bang and we don't observe any quasars, but we observe massive black holes in the center of many galaxies and we believe they are 'dead' quasars.

Why are massive black holes at the center of galaxies? and how about multiple black holes?
 Dynamical Friction causes the orbit of massive bodies to spiral towards the center of the galaxy. In fact, more than one black hole could spiral towards the center of galaxy, which causes scientists to think that binary black holes and hopefully black hole mergers are common at galactic centers. I say hopefully because it can take a very long time (sometimes more than the age of the universe; this is called the final parsec problem) for these black holes to get closer and closer together due to gravitational wave emission and eventually merge.  The rate of black hole merges is still unknown. A gravitational wave observatory in space such as LISA would be the only way to get some statistics on these binaries.

How many black holes at galactic centers have been observed so far?
About 40-50 with masses ranging from millions to billions of solar masses.

Which galaxies have black holes?
Roughly speaking, most large galaxies have black holes at their center. More technically, almost all galaxies that have a "hot" component, which is the bulge of elliptical and some types of spiral galaxies, also harbor black holes at their centers. Their masses are correlated with properties of the host galaxy - in particular with velocity dispersion.

Why are black holes at the centers of galaxies important?
They play an important role in galaxy formation.

What do we know about the black hole at the center of the Milky Way, our own galaxy?
The Milky Way's black Hole: Sagittarius A*; Image credit: Chandra

The mass of 'our' black hole is about 4 million solar masses in an area of 0.5 mili-parsecs, which is about 3 x the distance to Neptune or about 100 times the average distance between the Earth and the Sun. The radius of a black hole of this mass would be more than one thousand times smaller. The presence of the black hole is suggested by the fact that the orbits of the stars around the center of the galaxy are closed ellipses (Ghez et al. 2008, Gillessen et al. 2009).  The smallest orbital period is only 16 years.

Since there is still so much to learn about quasars, I end with the poem "Quasar" by George Gamow. I sometimes recite it to my nephew, David and to my son, Edward instead of the standard "Twinkle, Twinkle Little Star" song and even they get confused:

"Twinkle, twinkle quasi-star
Biggest puzzle from afar
How unlike the other ones
Brighter than a billion suns
Twinkle, twinkle, quasi-star
How I wonder what you are."
George Gamow, "Quasar" 1964.

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