Double-peaked profiles : ubiquitous signatures of disks in the broad emission lines of active galactic nuclei

Abstract

Broad (10,000 km s−1), 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 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 nondisk 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 in type 1 AGN, MBH = f (RBLRDV G) 2 , is f = 1 sin2 i for the disk-dominated sources. Our median i=27° 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.