Supermassive Black Hole and Broad-line Region in NGC~5548: Results from Five-season Reverberation Mapping

TL;DR

This study presents five-season reverberation mapping of NGC 5548, measuring SMBH mass, BLR structure, and their evolution, revealing dynamic changes in emission line lags and BLR size-luminosity relations.

Contribution

It provides a comprehensive five-year dataset of reverberation mapping for NGC 5548, improving SMBH mass estimates and revealing BLR structural changes over time.

Findings

Mean time lags for emission lines measured across five seasons.

Disappearance of the 'M-shaped' velocity-resolved lag profile after 2018.

Estimated SMBH mass of approximately 1.4×10^8 solar masses.

Abstract

NGC 5548 is one of the active galactic nuclei (AGN) selected for our long-term spectroscopic monitoring with the Lijiang 2.4~m telescope, aiming at investigating the origin and evolution of the broad-line regions (BLRs), accurately measuring the mass of the supermassive black holes (SMBHs), and understanding structure and evolution of the AGN. We have performed five-season observations for NGC~5548 with the median sampling interval ranging from 1.25 to 3 days. The light curves of the 5100~\AA\ continuum and broad emission lines are measured after subtracting contamination of the host galaxy starlight. The time lags of the broad He~{\sc ii}, He~{\sc i}, H$\gamma$, and H$\beta$ lines with respect to the 5100~\AA\ continuum are obtained for each season and their mean time lags over the five seasons are 0.69, 4.66, 4.60, 8.43 days, respectively. The H$\gamma$ and H$\beta$ velocity-resolved lag profiles in the seasons of 2015, 2018, 2019, and 2021 are constructed, from which an ``M-shaped'' structure is found in 2015 but disappears after 2018. Our five-season reverberation mapping (RM) yields an averaged virial SMBH mass of $M_\bullet/10^7M_\odot=14.22$, with a small standard deviation of $1.89$. By combining the previous 18 RM campaigns and our five-season campaign for NGC~5548, we find that there exists a time lag of 3.5~years between the changes in the BLR size and optical luminosity. In addition, we also construct the BLR radius$-$luminosity relation and the virial relation for NGC~5548.