A Composite Broad-Line Region in SDSS J1609+4902: a Double-Peaked Disk component and a Gaussian Component

TL;DR

This paper reports the discovery of a unique AGN with a composite broad-line region featuring both a Gaussian and a double-peaked disk component, revealing insights into BLR structure and stability over a decade.

Contribution

It presents the first detailed observation of a dual-component BLR in an AGN, combining stable double-peaked and Gaussian emission features over 10 years.

Findings

The Gaussian component size aligns with reverberation mapping expectations.

The double-peaked component suggests a disk-like BLR structure.

The BLR components remain stable over approximately 10 years.

Abstract

The profiles of broad emission lines in active galactic nuclei (AGNs) provide critical insights into the geometry and kinematics of the broad-line region (BLR), which in turn influence the uncertainties in estimating the masses of central supermassive black holes. In this study, we report the discovery of a low-luminosity AGN, SDSS J1609+4902, in which the H$\alpha$ line exhibits two distinct BLR components: a Gaussian component and an extremely broad double-peaked component. Follow-up observations conducted at the Lijiang Observatory in 2025 reveal that the line profile remains roughly unchanged, suggesting that this BLR structure may remain stable over a timescale of $\sim$10 years. We find that the size of the central Gaussian (Full Width at Half Maximum, FWHM$\sim 3000\,{\rm km\, s^{-1}}$) component is consistent with the classical reverberation mapping correlation. In contrast, the asymmetric double-peaked wing (FWHM$\sim 23,000\,{\rm km\, s^{-1}}$) likely originates from a disk-like BLR with an inner radius of approximately 70 gravitational radii. These results provide new constraints on the structure and dynamics of BLRs in AGNs and highlight the potential for long-term stability in such systems.