# Black hole mass estimation for Active Galactic Nuclei from a new angle

## TL;DR

This paper introduces a novel method to estimate black hole masses in active galactic nuclei using narrow emission lines, enabling studies of obscured AGN and black hole-galaxy relations across cosmic time.

## Contribution

A new algorithm linking emission line properties to black hole mass, applicable to obscured AGN, expanding the scope of black hole and galaxy co-evolution studies.

## Key findings

- Established a correlation between emission line ratios and broad Hα width.
- Derived black hole mass scaling relations for type II AGN consistent with type I AGN.
- Demonstrated the method's potential for studying black hole growth beyond the local universe.

## Abstract

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.