Comparisons of pitch angle data with measurements of core stellar velocity dispersion (σ) for galaxies from Ferrarese (2002). Left: σ _c compared with our measured pitch angles. Right: masses taken from the M–σ relation compared to spiral arm pitch angle. Both figures illustrate a strong correlation between (σ) and spiral arm pitch angle, as one would expect from our argument that both measure the mass of the galaxy's central bulge (see the Appendix). The fit to the SMBH mass–pitch angle relation is log ( M _BH/ M _☉) = (8.47 ± 0.24) − (0.089 ± 0.013) P with a χ ² = 4.86 and a scatter of 0.48 dex.