Towards Precision Supermassive Black Hole Masses using Megamaser Disks
  Megamaser disks provide the most precise and accurate extragalactic
supermassive black hole masses. Here we describe a search for megamasers in
nearby galaxies using the Green Bank Telescope (GBT). We focus on galaxies
where we believe that we can resolve the gravitational sphere of influence of
the black hole and derive a stellar or gas dynamical measurement with optical
or NIR observations. Since there are only a handful of super massive black
holes (SMBH) that have direct black hole mass measurements from more than one
method, even a single galaxy with a megamaser disk and a stellar dynamical
black hole mass would provide necessary checks on the stellar dynamical
methods. We targeted 87 objects from the Hobby-Eberly Telescope Massive Galaxy
Survey, and detected no new maser disks. Most of the targeted objects are
elliptical galaxies with typical stellar velocity dispersions of 250 km/s and
distances within 130 Mpc. We discuss the implications of our non-detections,
whether they imply a threshold X-ray luminosity required for masing, or
possibly reflect the difficulty of maintaining a masing disk around much more
massive (>10^8 Msun) black holes at low Eddington ratio. Given the power of
maser disks at probing black hole accretion and demographics, we suggest that
future maser searches should endeavour to remove remaining sample biases, in
order to sort out the importance of these covariant effects.
