Estimating the mass of a black hole at the center of a galaxy is a difficult task. Only 44 spiral galaxies have been “weighed” by observing the rate at which stars or gas orbit them, while others have been estimated through alternative, but probably less reliable, methods.
However, a new technique relates the mass of the black hole to the shape of the galaxy’s spiral arms, meaning that anyone can calculate the mass using just an image of the galaxy and a formula. It’s easy enough that the authors suggest it could be a problem that primary school children tackle to get them enthusiastic about science (although we'd be more optimistic about high school).
Much effort has been made over the years to relate the mass of black holes in the heart of galaxies to the size of the bulge in which they sit. But even galactic bulges can be difficult to measure. In 2008, Dr Marc Seigar of the University of Minnesota Duluth proposed to extend this to the disk, claiming that heavier holes produced more tightly wound spiral arms.
Dr Ben Davis of Swinburne University, Melbourne, told IFLScience that Seigar’s work drew on a sample of just 27 galaxies, of which only five had been directly weighed, while the others were based on various extrapolations. Consequently, the reliability of the technique was open to question. Now, Davis has co-authored a paper with Seigar in the Monthly Notices of the Royal Astronomical Society that makes use of extra galactic measurements that have been done since then.
Although the reason for the connection remains uncertain, the result is a more precise formula and increased confidence in its validity. “The strength of the correlation is competitive with, if not better than, all our other methods used to predict black hole masses,” Davis said in a statement. “Anyone can now look at an image of a spiral galaxy and immediately gauge how massive its black hole should be.”
An extra feature of the refinement is a greater capacity to extrapolate the relationship to galaxies with loosely wound arms. This is important because astronomers have put a lot of work into the search for intermediate-mass black holes, which fall between those left behind by the collapse of large stars and the giant ones at the heart of galaxies like our own. Davis thinks we could find these by searching for galaxies with extremely loose spirals.
One of the reasons astronomers are so keen to find intermediate-mass black holes is the expectation that these are more likely to be undergoing mergers that could produce gravitational waves. Since the capacity of the Laser Interferometer Gravitational-Wave Observatory (LIGO) to detect waves was confirmed last year, the idea of knowing where to look in the future is exciting and this work provides helpful suggestions.