How heavy is a black hole?
It depends what it had for lunch! Astronomers are fairly sure that black holes exist in the centres of most galaxies, not just large, luminous galaxies. But what are they? When a very massive star runs out of fuel and collapses, the nuclear reactions at the centre stop. These reactions produce photons which we see as heat and light, and the pressure keeps the star from collapsing under it's own gravity so when they stop, the star begins to collapse. If the star was heavy enough, the collapse continues until the core is so dense that the escape velocity becomes greater than the speed of light. As nothing can travel faster than this, nothing can escape a black hole, not even light, which is why they are known as "black" holes. The black holes in the centres of galaxies are much more massive than those formed by the collapse of stars and are known as supermassive black holes.
One method of estimating the mass of a galactic black hole is to look at the stars in orbit around it. The more massive the black hole, the faster the stars will be moving in their orbits. The speed can be measured using redshift measurements, an effect similar to the change in pitch of a siren as an abulance speeds past. In this case we are interested in the colour of the light rather than the pitch of the sound, but the idea is the same. The faster the stars go, the bigger the shift in the colour of the light from the stars. This shift can be seen as a broadening of the emission lines from the stars: the broader the line, the greater the velocity and the heavier the black hole. This only works for relatively nearby galaxies though, it gets trickier when the galaxy in whiuch you are interested is more distant.
The paper that grabbed my attention on astro-ph today described a method for estimating the mass of a black hole in the middle of a galaxy by observing emission caused by hydrogen atoms. Each atom emits at particular wavelengths (colours) depending on the energy levels of electons orbiting the nuclei. Each kind of atom has slightly different levels so by looking at the colours in a spectrum you can work out what atoms are present in a particular cloud of gas. The standard method of determining the mass of a galactic black hole is to look at the overall emission (the continuum) and the width of one particular line known as Hβ. The trouble is that Hβ can be difficult to measure. What these researchers have found is that the width of another hydrogen line, Hα, has a strong correlation with the width of Hβ. As Hα is often easier to measure, this correlation makes determining the masses of many black holes much easier.