Pink Black Holes: Frequently Asked Questions

[Mt Stromlo Observatory] [Department of Physics] [Astrophysical Theory Centre]

This page is intended to answer most the the questions I've been asked since this news story went out.

Could they be pink because of redshift?

The light from very distant objects is stretched when it reaches us, due to the expansion of space. We have measured this "redshift" for the pink black holes, and allowed for it in our calculations. It is not the explanation of their colours: they would look pink regardless of their redshift.

A varient on this explanation used the Doppler effect: light emitted from stuff moving towards us will appear to have a shorter wavelength, and light moving away from us will have a longer wavelength. We see this effect in the emission from gas around these pink black holes, but it is far too small an effect to account for the pinkness. Thus the swirling motions of gas around or into the black holes cannot produc the colour.

Yet another varient is gravitational redshift: light escaping from close to a black hole has a lot of gravity to fight, which can exhaust it and make it's wavelength increase. This is seen in X-ray emission from some black holes, but is a negligible effect at the wavelengths we are using.

The 100 or so pink black holes we've studied to date have redshifts between 0.1 and 3. (ie. the light from them is shifted to the red, due to the expanding universe, by between 10% and 300%).

How do we know that these things really are black holes?

In two ways. Firstly, when we examine the light from them in detail (taking a spectrum) we see signatures of gas swirling around them at up to 30,000 km/s: so called "broad emission lines". This swirling motion is only found around black holes.

Secondly, these sources emit a particular type of radio emission (high surface brightness, flat radio spectrum emission) which comes only from beams of relativistic particles squirted out from spinning disks of material around black holes.

What are our theories for why they are pink?

We've been going through theories at the rate of several a month! At first we thought that these things were normal blue black holes hidden behind clouds of interstellar dust, that absorbed the blue light and made them appear pink. It turns out that this is only true for a few of them: most have the wrong colours to be expained by this.

Then we thought that perhaps these were strange galaxies rather than black holes (quasars): but when we measured their spectra we rapidly learned that they were indeed black holes, as described above.

What other colours do Black Holes come in?

We never see black holes themselves: all we see is gas falling in to them. This gas gets so hot that it normally emits either X-rays, ultra-violet light or blue light. Thus most black holes that we could see with the human eye would appear bright blue-white. A few appear yellow, but this isn't their true colour: the really are blue, but are obscured by interstellar dust.

Where in the Sky are the Pink Holes?

We have now found over 100 of these strange things, scattered all over the southern sky. We suspect that many more are scattered throughout the northern sky too: we just haven't looked up there yet.

How do we know the distance to the Pink Black Holes?

If a black hole wasn't eating something we could never tell how far away it was. Luckily, we can measure the light given off from the gas beeing eaten by these things, and measure its redshift (how much it has been stretched by the explanding universe). This tells us how long the light has been flying for, and hence the distance to the objects.

Could these things be some form of White Hole, Wormhole, or the place where matter comes into the universe?

Some astronomers have proposed all sorts of bizarre objects such as white holes, mini-big-bangs and wormholes in space. There is no evidence that these things exist, or even that they are possible. It is pure unsupported speculation at this point, so we are tying hard to find more prosaic explanations of what is going on. If this fails, we may be forced to use one of these bizarre theories.

What is the Difference between Normal and Pink Black Holes?

We don't know!

How do we know they appear pink?

We obtained electronic (CCD) images of the black holes through blue, green and red filters that roughly approximate the sensitivtity of the human eye to colours. We also measured stars of known spectrl type frequently during the night. This allowed us to calibrate precisely the sensitivity of our telescope/detector system in the three filters.

I took the advice of famous astro-photographer David Malin in converting these flux values into colours. His technique is to treat mid-day sunlight as pure white (ie. the light from a G-type star), and to scale everything from there. I thus calculated how much brighter or fainter my sources were through each filter as compared with a G-type star (I actually used Alpha Centauri), scaled the images appropriately and plotted them in colour.

Basically, these objects are bright in the red compared to the Sun, faint in green and very slightly bright in the blue: hence the purplish pink colour.

Last updated 12th May 1999.
Maintainer: Paul Francis,