Kinematic Evidence for a Relativistic Keplerian Disk: ARP 102B

Abstract

The broad emission lines of the elliptical galaxy Arp 102B have peaks which are significantly displaced in velocity with respect to the host galaxy of the active galactic nucleus (AGN). We calculated line profiles for a Keplerian disk, including relativistic effects which were not treated rigorously in previous papers. We found an excellent fit of the resulting double peaked, asymmetric profiles to the Hα line of Arp 102B, yielding an accurate determination of several parameters of the disk. The inner and outer radii are ~250 and ~1000 in units of GM/c^2^. The inclination angle is 33.5^deg^ +/- 5.5^deg^. The Hα emissivity peaks at a radius of ~450GM/c^2^. Both the models and the data show that relativistic effects of Doppler boosting and gravitational and transverse redshift cause observable asymmetries when the velocity is of order 0.02c, typical of the broad-line velocities in AGNs. We conclude that the line profile of Arp 102B shows the most convincing direct kinematic evidence for rotation in any AGN. The gravitational energy available in a standard accretion disk is marginally sufficient to power the line emission locally, so an extra source of heating, possibly photoionization by radiation from the inner disk or central nonthermal continuum, may be required. We speculate that ion-supported tori might account for the unusual properties of Arp 102B and other broad-line radio galaxies, including a far-infrared peak at 25 microns.