A Spiral Galaxy's Central Black Hole's Mass Is Accurately Measured By Its Spiral Arms

After carefully analysing a larger sample of galaxies, imaged by an array of space telescopes, the researchers observed an unexpectedly strong relationship, and one which predicts lower mass black holes in galaxies with open spiral arms (types Sc and Sd). 

"The strength of the correlation is competitive with, if not better than, all our other methods used to predict black hole masses," says Dr Davis. "Anyone can now look at an image of a spiral galaxy and immediately gauge how massive its black hole should be."

From here.  The source and its abstract are as follows:

We have conducted an image analysis of the (current) full sample of 44 spiral galaxies with directly measured supermassive black hole (SMBH) masses, MBH, to determine each galaxy’s logarithmic spiral arm pitch angle, ϕ. For predicting black hole masses, we have derived the relation: log (MBH/M⊙) = (7.01 ± 0.07) − (0.171 ± 0.017)[|ϕ| − 15°]. The total root mean square scatter associated with this relation is 0.43 dex in the log MBH direction, with an intrinsic scatter of 0.30 ± 0.08 dex. The MBH–ϕ relation is therefore at least as accurate at predicting SMBH masses in spiral galaxies as the other known relations. By definition, the existence of an MBH–ϕ relation demands that the SMBH mass must correlate with the galaxy discs in some manner. Moreover, with the majority of our sample (37 of 44) classified in the literature as having a pseudobulge morphology, we additionally reveal that the SMBH mass correlates with the large-scale spiral pattern and thus the discs of galaxies hosting pseudobulges. Furthermore, given that the MBH–ϕ relation is capable of estimating black hole masses in bulge-less spiral galaxies, it therefore has great promise for predicting which galaxies may harbour intermediate-mass black holes (IMBHs, MBH < 105 M⊙). Extrapolating from the current relation, we predict that galaxies with |ϕ| ≥ 26°7' should possess IMBHs.

Benjamin L. Davis, Alister W. Graham, Marc S. Seigar. "Updating the (Supermassive Black Hole Mass) – (Spiral Arm Pitch Angle) Relation: A Strong Correlation for Galaxies with Pseudobulges." Monthly Notices of the Royal Astronomical Society (July 19, 2017) DOI: 10.1093/mnras/stx1794