Black hole mass estimation for Active Galactic Nuclei from a new angle
  The scaling relations between supermassive black holes and their host galaxy
properties are of fundamental importance in the context black hole-host galaxy
co-evolution throughout cosmic time. In this work, we use a novel algorithm
that identifies smooth trends in complex datasets and apply it to a sample of
2,000 type I active galactic nuclei (AGN) spectra. We detect a sequence in
emission line shapes and strengths which reveals a correlation between the
narrow L([OIII])/L(H$\beta$) line ratio and the width of the broad H$\alpha$.
This scaling relation ties the kinematics of the gas clouds in the broad line
region to the ionisation state of the narrow line region, connecting the
properties of gas clouds kiloparsecs away from the black hole to material
gravitationally bound to it on sub-parsec scales. This relation can be used to
estimate black hole masses from narrow emission lines only. It therefore
enables black hole mass estimation for obscured type II AGN and allows us to
explore the connection between black holes and host galaxy properties for
thousands of objects, well beyond the local Universe. Using this technique, we
present the $M_{\mathrm{BH}} - \sigma$ and $M_{\mathrm{BH}} - M_{*}$ scaling
relations for a sample of about 10,000 type II AGN from SDSS. These relations
are remarkably consistent with those observed for type I AGN, suggesting that
this new method may perform as reliably as the classical estimate used in
non-obscured type I AGN. These findings open a new window for studies of black
hole-host galaxy co-evolution throughout cosmic time.
