Doubled-peaked emission lines in active galactic nuclei

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Abstract

We present the results of a comprehensive study of the broad Hα emission lines of radio galaxies and radio-loud quasars. The data consist of 74 new spectra plus information drawn from 20 spectra in the literature. The principal result is the discovery of eight new double- peaked Hα profiles that can be fitted by a simple relativistic disk model, bringing the total number of such profiles to 12. Disklike emitters comprise 10% of radio-loud objects in our representative sample and arguably constitute a new "class" on the basis of the following additional properties which they possess: (1) average Hα full width at half-maximum twice as large as the remainder of the sample (12,500 km s^-1^ vs. 5700 km s^-1^), (2) optical continuum around Hα consisting of 20%- 100% starlight, versus less than 20% for the other objects, (3) large equivalent widths of low-ionization forbidden lines [O I] and [S II]), and (4) large [O I]/[O III] ratios. The properties of the disklike emitters are argued to be intrinsic, since they are not extensions of any of the known correlations which are thought to represent viewing angle effects in several unifying schemes for radio- loud objects. Neither are there any radio-quiet counterparts of this class. The Chen & Halpern model, in which the inner disk is an ion- supported torus which illuminates the outer disk with a hard ionizing continuum, goes a long way toward explaining the properties of disklike emitters, and why they are best recognized in Hα. The applicability of other models, such as supermassive binary black holes, jets, and other radial flows, are also evaluated for disklike profiles and other objects in our sample. For the entire sample of radio-loud objects, the broad Hα lines at half-maximum are preferentially redshifted with respect to the narrow lines by <{DELTA}λ/λ> = (6 +/- 2) x 10^-4^ on average, which corresponds to Keplerian motion at a radius of ~2500 GM/c^2^ if interpreted as gravitational plus transverse redshift. This result is entirely consistent with previous studies of Hβ, which find more redshifts in radio-loud samples, but equal numbers of blueshifts and redshifts in optically selected samples. Thus, in radio-loud objects, the Balmer-line emitting gas is more likely to be dominated by gravitational motion close to the black hole than in radio-quiet objects. Although we neither support nor contradict unifying models based on viewing direction, it appears that the disklike profiles are better fitted by small inclination angles. Although contrary to the conventional interpretation of the broadest emission lines in radio-loud AGNs, this is to be expected for an accretion disk origin. A number of reasons are suggested for this possible selection effect, with the consequence that the accretion disk could contribute at least partially to the Balmer lines of significantly more objects than just the 10% which are obvious. We obtained new (secure) redshifts for five objects which had incorrect values in the literature.

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