How to Fuel an AGN: Mapping Circumnuclear Gas in NGC 6240 with ALMA
  Dynamical black hole mass measurements in some gas-rich galaxy mergers
indicate that they are overmassive relative to their host galaxy properties.
Overmassive black holes in these systems present a conflict with the standard
progression of galaxy merger - quasar evolution; an alternative explanation is
that a nuclear concentration of molecular gas driven inward by the merger is
affecting these dynamical black hole mass estimates. We test for the presence
of such gas near the two black holes in NGC 6240 using long-baseline ALMA Band
6 observations (beam size 0"06 $\times$ 0"03 or 30 pc$\times$15 pc). We find
(4.2-9.8) $\times10^{7}$ M$_{\odot}$ and (1.2-7.7) $\times10^{8}$ M$_{\odot}$
of molecular gas within the resolution limit of the original black hole mass
measurements for the north and south black holes, respectively. In the south
nucleus, this measurement implies that 6-89% of the original black hole mass
measurement actually comes from molecular gas, resolving the tension in the
original black hole scaling relations. For the north, only 5% to 11% is coming
from molecular gas, suggesting the north black hole is actually overmassive.
Our analysis provides the first measurement of significant molecular gas masses
contaminating dynamical black hole mass measurements. These high central
molecular gas densities further present a challenge to theoretical black hole
accretion prescriptions, which often assume accretion proceeds rapidly through
the central 10 pc.
