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 () and the disk's stellar mass, having measured the spheroid (bulge) stellar mass () and presented the M _BH- relation in Paper I. The relation involving 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 . The scatter from the regression of M _BH on is 0.66 dex; compare 0.56 dex for M _BH on 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 , 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- 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- relation need to be mindful of the galaxy types included at each epoch. We additionally update the -(face-on spiral arm pitch angle) relation.