MadSci Network: Astronomy
Query:

Re: How many stellar BlackHoles observed so far?

Date: Fri May 28 13:33:05 1999
Posted By: Denise Kaisler, Grad student, Astronomy, UCLA, Division of Astronomy
Area of science: Astronomy
ID: 927267390.As
Message:

Based on what information I've been able to gather, I think you should forget the name "pink black hole" and substitute "pink quasar". Also, you might want to view the "discovery" of these objects with skepticism until more information is availible.

I put "discovery" in quotations because I'm not convinced that Drs. Paul Francis, Rachel Webster, and Michael Drinkwater have found anything new. What they have found is a quasar that appears pinkish, rather than the usual blue. But what does that mean?

First, a bit of explanation:

A quasar is a distant galaxy with a massive, central black hole. The hole typically has the mass of a billion suns. It is surrounded by dust and gas that accretes onto a disk as it falls inward. Material within the disk is heated by friction and gives off tremendous amounts of radiation. Since this matter is not actually within the event horizon of a black hole, it can still escape and hence be seen by us. The luminosity (energy per unit time) of a quasar can be as much as a thousand times that of the Milky Way galaxy.

There is ample evidence for massive black holes in the centers of many galaxies including our own. In fact, a popular modern theory is that all galaxies have massive central black holes and that the major difference between quasars and nearby galaxies is that the black holes in nearby galaxies are no longer getting much "food" (gas and dust) which makes them unable to pump out large amounts of radiation.

The color of most quasars is bluish-white. This simply means that the quasar simply emits more light at blue than at red wavelengths. In order for a quasar to be pink, it would have to

  1. be much cooler than a normal quasar
  2. be shrouded in a cloud of dust that absorbs and then re-emits the radiation at longer wavelengths
  3. exist in some exotic region of the universe where physics is strange.
Occam's Razor dictates that we ignore the last idea in favor of the first two.

Of those, the second one seems far more likely. Unfortunately, this is where I really suffer from a lack of information. No technical papers about this research have been published yet and so I don't understand why the discovery team has ruled out that explanation.

Some quasars are embedded in dust clouds. The dust changes their color from blue-white to yellow. A pink cloud would, to me, suggest a superabundance of dust. But then again, quasars are not my area of expertise.

Lynn Cominsky of California State University suggests that we may be looking at a related type of active galaxy (a blazar) from the side. Currenly, the unified theory of active galaxies states that blazars, quasars, Seyfert galaxies, and radio galaxies are all the same type of object seen from different angles. Radio galaxies are currently assigned to the edge-on view that Cominsky ascribed to the new pink objects. But the unified theory has its problems and is by no means etched in stone. Much more work will have to be done before this is resolved.

Another explanation, proposed by Francis and colleagues, is that the swirling gas and dust is "acting as a vast natural radio transmitter". Again, the physics of accretion disks is a very new and challenging field, and one that I don't know that much about.

However, one thing is quite clear. The terms "pink black hole" and (heaven help us) "pink hole" are really quite misleading. By definition, no light can escape from a black hole. So we can never actually see one, much less tell what color it is. The only way that a black hole can be detected is through indirect means, such as observations of accretion disks or the gravitational influence of a hole on neighbouring stars. So if people are going to think pink, they should think "pink quasar" instead.

Thanks for asking this interesting question!


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