Updating the (supermassive black hole mass)-(spiral arm pitch angle) relation: A strong correlation for galaxies with pseudobulges

We have conducted an image analysis of the (current) full sample of 44 spiral galaxies with directly measured supermassive black hole (SMBH) masses, M_BH, to determine each galaxy's logarithmic spiral arm pitch angle, ϕ. For predicting black hole masses, we have derived the relation: log (M_BH/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 logM_BH direction, with an intrinsic scatter of 0.30 ± 0.08 dex. The M_BH-ϕ relation is therefore at least as accurate at predicting SM_BH masses in spiral galaxies as the other known relations. By definition, the existence of an M_BH-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 M_BH-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,M_BH < 10⁵ M_⊙). Extrapolating from the current relation, we predict that galaxies with |ϕ| ≥ 26. ° 7 should possess IMBHs.