  We present a new relationship between central black hole (BH) mass and host
galaxy stellar bulge mass extending to the lowest BH masses known in dwarf
galaxies ($M_{\rm BH} \lesssim 10^{5} M_{\odot}$; $M_{\star} \sim 10^{9}
M_{\odot}$). We have obtained visible and near-infrared $Hubble \ Space \
Telescope \ (HST)$ imaging of seven dwarf galaxies with optically-selected
broad-line active galactic nuclei (AGN) and BH mass estimates from single epoch
spectroscopy. We perform 2D photometric modeling with GALFIT to decompose the
structure of these galaxies and find that the majority have an inner
bulge/pseudobulge component with an exponential disk that dominates the total
stellar mass. Using the modeling results and color-dependent mass-to-light
ratios, we determine the stellar mass of each photometric component in each
galaxy. We determine the $M_{\rm BH} - M_{\rm bulge}$ relation using a total of
12 dwarf galaxies hosting broad-line AGNs, along with a comparison sample of 88
galaxies with dynamical BH masses and 37 reverberation-mapped AGNs. We find a
strong correlation between BH mass and bulge mass with ${\rm log}(M_{\rm
BH}/M_\odot) = (1.24\pm 0.08)~{\rm log}(M_{\rm bulge}/10^{11} M_\odot) +
(8.80\pm 0.09)$. The near-linear slope and normalization are in good agreement
with correlations found previously when only considering higher mass systems.
This work has quadrupled the number of dwarf galaxies on the BH-bulge mass
relation, with implications for BH seeding and predictions for gravitational
wave detections of merging BHs at higher redshifts with $LISA$.
