Tracking Matter Around a Black Hole

How are you going to answer the questions of what is falling into the black hole and how fast does it fall? It would be great if you could see the accretion disk directly; then, you could track the accretion by watching matter fall into the black hole. However, the black hole is too far away, and the accretion disk too small, to see it directly. Instead, you are going to look for emission or absorption lines in the spectrum.

Emission and absorption lines

What is an emission or absorption line?

When atoms are excited, they emit and/or absorb light at certain fixed energies. These "excitations" show up in spectra as a spike or dip at a very specific energy based on the element that emitted or absorbed the light. By precisely measuring the energy of these features in a spectrum, astronomers can determine what elements are present. The features in the spectrum are called "lines", either "emission line" if the feature comes from atoms emitting the light, or "absorption line", if the feature comes from atoms absorbing the light.

See the examples below:

Two different spectra showing emission and absorption lines. The one on the left shows emission lines (labeled silicon, sulfur, argon, calcium and iron) in the supernova remnant called Cassiopeia A. The spectrum on the right shows absorption lines (labeled Ca K and Ca H) in the optical spectrum of galaxy M 31.

(Click images for larger versions.)

Doppler shift

Have you ever listened to a police car as it drives by? The pitch of its siren appears to change as it passes by. The siren itself doesn't actually change pitch – it stays constant. The thing that changes is that the police car first is first moving toward you, then is moving away. This change in pitch is due to the Doppler effect. Essentially, the peaks in the sound waves "bunch up" as the car is approaching you, so the frequency is higher. Similarly, the peaks in the sound waves spread out as the car is moving away from you, so the frequency of the siren is lower. A similar thing happens with light.

When atoms that produce an emission line are moving toward or away from you, the energy at which you see the line changes. Hopefully you remember that light is made of waves, just as sound (though different kinds of waves). Just as the pitch of the sound you hear is shifted depending on whether the source of the sound is moving or stationary, the energy of the light will also change if the light source is moving.

If the source is moving away from you, the light you see will have lower energy than the emitted light. If the source is moving toward you, the light you see will have higher energy than the emitted light. Astronomers refer to these as redshift (source moving away) or blueshift (source moving toward).

The figure below shows redshift and blueshift for an optical spectrum:

The three spectra above show how spectral lines shift if the source is moving relative to the observer. The top spectrum is produced by a source that is stationary with respect to the observer.

In the middle spectrum, the absorption lines (the dark lines) are shifted to the right (toward the red), and are thus redshifted and produced by a source that is moving away from the observer.

In the bottom spectrum, the absorption lines are shifted to the left of where they lie in the stationary spectrum. In this cast the source is moving toward the observer.