Broad Hbeta Emission-Line Variability in a Sample of 102 Local Active Galaxies

TL;DR

This study investigates the variability of broad Hbeta emission lines in 102 local Seyfert galaxies over several years, revealing that most exhibit some form of variability which can lead to changes in galaxy classification.

Contribution

It provides the first statistical analysis of Hbeta line variability frequency in low-redshift Seyfert galaxies, highlighting the prevalence of changing-look phenomena.

Findings

66% of galaxies show broad Hbeta variability

38% experience Seyfert-type changes

3% show disappearance of broad Hbeta line

Abstract

A sample of 102 local (0.02 < z < 0.1) Seyfert galaxies with black hole masses MBH > 10^7 M_sun was selected from the Sloan Digital Sky Survey (SDSS) and observed using the Keck 10-m telescope to study the scaling relations between MBH and host galaxy properties. We study profile changes of the broad Hbeta emission line within the ~3-9 year time-frame between the two sets of spectra. The variability of the broad Hbeta emission line is of particular interest, not only since it is used to estimate MBH, but also since its strength and width is used to classify Seyfert galaxies into different types. At least some form of broad-line variability (in either width or flux) is observed in the majority (~66%) of the objects, resulting in a Seyfert-type change for ~38% of the objects, likely driven by variable accretion and/or obscuration. The broad Hbeta line virtually disappears in 3/102 (~3%) extreme cases. We discuss potential causes for these changing-look AGNs. While similar dramatic transitions have previously been reported in the literature, either on a case-by-case basis or in larger samples focusing on quasars at higher redshifts, our study provides statistical information on the frequency of H$\beta$ line variability in a sample of low-redshift Seyfert galaxies.