The Black Hole-Bulge Mass Relation Including Dwarf Galaxies Hosting Active Galactic Nuclei

Abstract

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 _BH ≲ 10⁵ M _⊙ M _⋆ ∼ 10⁹ M _⊙). We have obtained visible and near-infrared Hubble Space Telescope imaging of seven dwarf galaxies with optically selected broad-line active galactic nuclei (AGNs) 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/pseudo-bulge 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 _BH-M _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 {log}({M}_BH}/{M}_⊙ )=(1.24+/- 0.08) {log}({M}_bulge}/{10}¹¹{M}_⊙ )+(8.80+/- 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.