Black Hole Mass Scaling Relations for Spiral Galaxies. II. M BH –M ,tot and M BH –M ,disk

Black hole mass ( M BH ) scaling relations are typically derived using the properties of a galaxy’s bulge and samples dominated by (high-mass) early-type galaxies. Studying late-type galaxies should provide greater insight into the mutual growth of black holes and galaxies in more gas-rich environments. We have used 40 spiral galaxies to establish how M BH scales with both the total stellar mass ( M * , tot ) and the disk’s stellar mass, having measured the spheroid (bulge) stellar mass ( M * , sph ) and presented the M BH – M * , sph relation in Paper I. The relation involving M * , tot may be beneficial for estimating M BH either from pipeline data or at higher redshift, conditions that are not ideal for the accurate isolation of the bulge. A symmetric Bayesian analysis finds log ( M BH / M ⊙ ) = ( 3.05 − 0.49 + 0.57 ) log { M * , tot / [ υ ( 6.37 × 10 10 M ⊙ ) ] } + ( 7.25 − 0.14 + 0.13 ) . The scatter from the regression of M BH on M * , tot is 0.66 dex; compare 0.56 dex for M BH on M * , sph and 0.57 dex for M BH on σ * . The slope is >2 times that obtained using core-Sérsic early-type galaxies, echoing a similar result involving M * , sph , and supporting a varied growth mechanism among different morphological types. This steeper relation has consequences for galaxy/black hole formation theories, simulations, and predicting black hole masses. We caution that (i) an M BH – M * , tot relation built from a mixture of early- and late-type galaxies will find an arbitrary slope of approximately 1–3, with no physical meaning beyond one’s sample selection, and (ii) evolutionary studies of the M BH – M * , tot relation need to be mindful of the galaxy types included at each epoch. We additionally update the M * , tot –( face-on spiral arm pitch angle) relation.