How To Weight A Black Hole

Do you know what a black hole is? Well yes, everybody knows by now what this monster is, an astrophysical object that can absorb everything, even light and from the insight of it there is no way out. But do you know how to measure it’s mass?

In a recent article by B. Czerny and M. Nikolajuk (2009), this problem is discussed and different methods to weight a black hole are presented.

There are, in general, three categories of black holes masses, stellar mass black holes, intermediate mass black holes, massive and supermassive black holes with masses that range from a few, to the most supermassive ones that have 10^9 solar masses(Msol) ( Msol = a mass unit with a value of the mass of our Sun).

There are over 40 black holes that have a measured mass in our Galaxy and the best one is the microquasar “GRO J1655-40” with a mass of 6.3 Msol (Greene, Bailyn & Orosz, 2001).

The second category, intermediate mass black holes (IMBH), is the most controversial where we don’t know exactly even the range, if the existence of the black holes with masses between 10^5 to 10^6 is quite certain, we don’t know much about the ones with masses of hundreds to thousands Msol, and this all should be in the range of the IMBH’s.

Above 10^6 Msol, where the domain of the massive black hole starts, the best example is found in the center of our Galaxy, Sagittarius A*, with a mass of 4.1×10^6 Msol. Even above this, are the most massive black holes, the so called super massive black holes, gigantic monster which reside at the center of galaxies and are surrounded by a massive disk of matter and in most cases eject long and powerful jets perpendicular on this disk. The number of super massive black holes is of the order of thousands and growing with most of them living at the center of the so called AGN (active galactic nucleus), extremely luminous center of active galaxies.

The methods for evaluating the mass of this objects are dynamical methods, spectra fitting methods and scaling ones.

The first ones are based on a simple idea, you have a test particle which moves gravitationally around a black hole and by observing it’s velocity and the radius of the orbit, one can determine the mass of the black hole. Even if the procedure looks simple in practice there are lots of observational difficulties like for example the inclination of the orbit. As test particle we have companion stars, companion black holes, nearby star or stars (as in the case of Sgr A*), broad line region clouds or H2O masers, with the last one being one of the most accurate.

The second procedure is based on modeling the physical processes of the emission coming from around a black hole, like the emission from the accretion disk around the black hole, such modeling depend on many parameters and is not very reliable.

Scaling methods imply evaluating the black hole masses for large surveys, when one can find the mass of a some black holes using different methods and then using correlation between some characteristics of the host galaxies or environment to infer the mass for the rest of the candidates. One of the best example is the correlation between the stellar velocity dispersion in the bulge and the black hole mass.