M31, whose central black hole is thought to weigh about 170 million Suns, winds tightly with pitch angles of about 7°. "M31 has the tightest-wound spiral structure of any galaxy we looked at, and it also has the most massive black hole," Seigar says. M33, with looser spiral arms angling at 42°, hosts a black hole with a mass of just 1,500 Suns. The Milky Way, with a central black hole of nearly 4 million Suns and a pitch angle of 22°, falls in between.
Seigar next hopes to apply the technique to very distant galaxies. "We can detect spiral structure in galaxies 8 billion light-years away, maybe a little farther," he says. "Using this simple relationship, we can establish the average black-hole mass at different distances and watch them grow as the universe gets older."
Such a study would reveal how supermassive black holes evolved through time, at least in a statistical sense. The team notes that Hubble images of some 700 distant spiral galaxies are now available as part of the Great Observatories Origins Deep Survey, and suggests the time is ripe for such a study.
Seigar presented the results today at the American Astronomical Society meeting in St. Louis. The full paper
appeared in the May 10 Astrophysical Journal Letters