Double-peaked profiles: ubiquitous signatures of disks in the Broad Emission Lines of Active Galactic Nuclei

TL;DR

This paper demonstrates that double-peaked emission lines in AGN are common and originate from accretion disks, providing insights into black hole mass estimation and the structure of the broad-line region.

Contribution

It shows that broad double-peaked profiles are prevalent in various AGN types and links their geometry to black hole mass calculations, refining the factor used in mass estimates.

Findings

Double-peaked profiles are common in AGN spectra.

The inclination angle influences black hole mass estimates.

A new relation between FWHM and the factor f improves SMBH mass accuracy.

Abstract

Broad (~10,000 km/s), double-peaked emission-line profiles of Balmer lines emitted by active galactic nuclei (AGN) are thought to originate in the outer parts of an accretion disk surrounding a nuclear supermassive black hole (SMBH), at R~1000 gravitational radii and are most frequently observed in the nuclear spectra of low-luminosity AGN (LLAGN) and radio-galaxies. In the present paper we argue that broad double-peaked profiles are present also in the spectra of other Type 1 AGN, such as Seyfert 1 galaxies, suggesting that the inner part of the broad-line region (BLR) is also the outer part of the accretion disk. We use the Palomar spectral survey of nearby galaxies to show that the only difference between Seyfert 1 BLR line profiles and "bona fide" double peakers is that, in most cases, besides a disk component, we need an additional Gaussian component attributed to non-disk clouds. The recognition that the inner and most variable part of the BLR has a disk geometry suggests that the factor 'f ' in the expression to obtain the SMBH mass (M) in Type 1 AGN M(SMBH)=f [R(BLR) (Delta V)^2]/G is f = 1/[sin^2(i)] for the disk dominated sources. Our median inclination i=27 degrees implies f=4.5, very close to the most recent value of f=4.3+/-1.05, obtained from independent studies. We derive a relation between f and the FWHM of the broad profile that may help to reduce the uncertainties in the SMBH mass determinations of AGN.